As early as 500 B.C.E., Alcmaeon of Croton identified the brain as the physical seat of the mind. Twenty-five hundred years later, modern science has proven this ancient Greek to be absolutely correct. There is no aspect of psychology that is independent of the brain. The very essence of our humanity—our thoughts, our feelings, our beliefs, and our values—all emerge from this three-pound lump of gray tissue. Psychology has no choice but to take the brain into consideration. Moreover, with the remarkable advances of neuroscience in the last few decades, we now know more about the brain and its relationship to the mind than at any other time in human history.
In order to understand brain research, it is important to consider three basic assumptions that neuroscientists make about brain evolution. For one, the brain is believed to carry traces of its evolutionary origins deep within its tissues. Just as we carry traces of our earliest childhood within our adult personalities, the brain carries the history of our whole species within its very anatomy. Secondly, the brain has evolved up and out, so that the lowest and deepest parts of the brain are the oldest. The outermost, uppermost, and the furthest forward brain regions are the youngest on the evolutionary scale. Thirdly, our brains have increased in complexity across evolution. The older structures tend to be simpler and more primitive, both in their anatomy and in the behavioral functions they control. Likewise, the evolutionarily newer structures tend to be more complex.
As brain structures have evolved toward greater complexity, we can ask what benefits complexity may offer. Are there any costs? In general, complexity allows for more flexibility. Complex systems have a broader repertoire of responses with which to adapt to complicated or changing circumstances. However, complexity is expensive. Complex systems take more energy and are more fragile than simpler systems. With more parts involved, it is easier for something to go wrong.
Although our brain only weighs about three pounds (two to three percent of the average person’s body weight), it uses up about 15 percent of the blood that our heart pumps out and about 20 percent of our body’s oxygen and glucose. In other words it uses up to ten times as much of the body’s resources as would be expected for its weight.
Although we will try to stick to plain English in this book, it is useful to know the basic terminology used in the discussion of brain anatomy. As the brain is a three-dimensional structure, specific terms are used to distinguish up from down, back from front, and inside from outside. The terms anterior and posterior are used to refer to front and back, respectively, as are the Latin words rostral and caudal. Superior and inferior refer to top and bottom, respectively, as do the Latin words dorsal and ventral. Lateral refers to outside, while medial refers to the inside.
Lateral view of the brain (iStock).
The English terms are purely directional while the Latin ones are defined in reference to the body. Rostral and caudal are the Latin words for head and tail. Likewise, dorsal and ventral refer to the back and belly of a body (as in the dorsal fin of a shark). Medial means close to the body’s midline while lateral means away from it. Nonetheless, when we speak about the brain, rostral and caudal are generally understood to mean front and back, dorsal and ventral to mean top and bottom, and medial and lateral to mean inside and outside.
The word phylogeny means evolution. To say something is phylogenetically old means that is old in evolutionary terms.
Our understanding of the human brain is heavily indebted to the study of animal brains. Although the use of animals in biological research raises moral questions about animal rights, there is no question that much of our knowledge about the human brain derives from research on animal brains. Legally and ethically, we can perform much more invasive procedures on animal brains than we can on living human brains. Additionally, comparison of the brains of many different kinds of animals gives us critical insight into brain evolution.
The brain is an intricate structure that looks like a boxing glove placed over a spiral of sea creatures. The outer layer of the brain is called the cortex, or the neocortex. It is a wrinkled surface that covers the top and sides of the brain. This is the part that looks like a boxing glove, albeit a wrinkled one. Underneath the cortex are the subcortical regions: the cerebellum and brain stem at the very base of the brain, the thalamus and related regions toward the middle of the brain, and the limbic system, which wraps around the thalamus. The basal ganglia are also in the middle of the brain, close to the thalamus.
The distinction between the cortical and subcortical regions of the brain is an important one. The cortex is a relatively recent evolutionary achievement and the cortical structures are much more developed in humans than they are in more primitive animals. Most of the more complex psychological processes, the ones we think of as uniquely human, such as language, abstract thought, and reading, are controlled by the cortex. The subcortical regions process more fundamental psychological and even physiological functions. The lowest parts of the brain, closest to the spinal cord, are the oldest parts and regulate physiological processes we share with more primitive animals, such as breathing, heartbeat, and digestion.
Why is the cortex so wrinkled?
The surface of the cortex is covered with folds and looks somewhat like a walnut. These folds are referred to as convolutions. The rounded parts of the convolutions are called gyri, gyrus in the singular. The grooves between the gyri are called sulci (sulcus in the singular). These extra folds allow for much greater surface area, which in turn greatly increases the number of neurons that can fit into the relatively small space of the human skull. The more neurons we have, the more powerfully we can process information. To illustrate this efficient use of space, imagine an accordion or pleated paper fan, first folded up and then stretched out from end to end.
The cortex is divided into four lobes, the frontal, temporal, parietal, and occipital lobes. The frontal lobe comprises the front half of the cortex. It extends from the central sulcus forward. The thumb-like segments of the cortex are the temporal lobes. The parietal lobes cover much of the back surface of the cortex, extending from the central sulcus back to the border with the occipital lobe (the parietooccipital sulcus). Finally the occipital lobes are at the lower back end of the cortex.
The frontal lobe is considered the seat of our intellect. It covers about half the human cortex and is the most recently evolved part of the brain. More specifically it mediates our executive functions, a group of psychological functions that serve to control our behavior. These include planning, abstract thought, impulse control, and the control of behavioral sequences. As might be imagined, impairment in these areas can lead to significant problems functioning in the world. The frontal lobe has other functions besides executive functions, though. The most posterior region of the frontal lobe is called the motor strip and is involved with intentional movements. Additionally, Broca’s area, on the left posterior side of the frontal lobe, mediates speech production, or the translation of thought into spoken words.
The three remaining cortical lobes are all involved with some aspect of sensation and perception. The occipital lobe is involved with vision. The parietal lobe processes both touch and taste (in the somatosensory strip) and the temporal lobes are involved with hearing. Additionally, the parietal lobes are involved with attention and visual-spatial information, while the temporal lobes mediate language, memory, and the recognition of familiar objects.
Different areas of the brain control different functions (LifeArt).
In 1909, Korbinian Brodman (1868–1918) tried to standardize the discussion of brain anatomy by creating a map of the cortex. He first divided the cortex into distinct regions based on the way neurons were organized (cytoarchitecture). He then numbered these regions from one to 52. Only 45 Brodmann areas are found in the human brain, however; the other seven are found in the monkey brain. Although brain structure may vary somewhat across individuals, this system has been extremely helpful for neuroscientists, giving them a common language with which to talk about brain anatomy. Nonetheless, there is still variation in the terms used, as many brain structures have several different names.
The two halves of the brain are almost, but not quite, identical. With regard to the cortex, there is considerable difference in the functions they perform. This difference between right and left sides of the brain is called lateralization. The left side of the cortex is involved with language comprehension and speech production. The right side of the brain is involved with spatial and emotional processing and facial recognition. Because of lateralization, the impact of brain damage (for example due to stroke) will differ depending on the side of the brain affected.
What is the limbic system?
The limbic system is the seat of our emotions. The term was first introduced by James Papez in 1937 and refers to a group of brain structures in the middle of the brain. The original Papez circuit included the hippocampus, the fornix, the mammilary bodies, the anterior nucleus of the thalamus, and the cingulate gyrus. Over time the boundaries of the limbic system were expanded, although the exact definition of the limbic system is still not universal. For our purposes, however, we can include the amygdala, hippocampus, hypothalamus, septum, and cingulate gyrus. These regions are all involved with emotional and motivational processing.
The amygdala is a small, almond-shaped structure buried deep in the middle of the brain. Because of its oval shape, the amygdala was named after the Greek word for almond.
The amygdala seems to be an early responder to emotionally significant signals from the environment. It is particularly reactive to fearful stimuli. The amygdala activates the hypothalamus, which in turn activates the autonomic nervous system, in part through control of important hormones. The autonomic nervous system regulates the physiological components of emotion. For example, imagine that a vicious dog has broken free of its leash and is now lunging toward you. Immediately, your amygdala responds to the perception of danger. It sends signals to the hypothalamus, which then activates the autonomic nervous system, resulting in the rapid heartbeat, sweaty palms, and heavy breathing associated with fear.
The hippocampus is a caterpillar-like structure on the medial (inner) side of the temporal lobe and is heavily involved with memory. Early brain anatomists believed it looked more like a seahorse and named it after the Greek word for seahorse (hippo = horse). While the hippocampus does not process emotion, per se, it is located near the other limbic structures and carries memories of emotionally meaningful events. What we interpret as emotionally arousing, therefore, is heavily dependent on our memories of similar experiences.
The septum is a small area that is involved with the experience of pleasure among other functions. The cingulate gyrus is a long structure that wraps around numerous other subcortical regions and has attentional, emotional, and cognitive functions. More specifically, it is involved with decision making.
| Brain Area | Major Function |
| Cortex or Neocortex | Perception, Action, and Cognition |
| Frontal Lobe | Intentional Action and Executive Functions |
| Parietal Lobe | Touch, Taste, Spatial Processing, Attention |
| Temporal Lobe | Hearing, Language, Memory, Object Recognition |
| Occipital Lobe | Visual Processing |
| Limbic System | Emotion and Motivation |
| Amygdala | Emotional Reactions |
| Cingulate Gyrus | Emotion, Attention and Cognitive Functions |
| Hypothalamus | Coordination Mental & Physiological Processes |
| Hippocampus | Memory |
| Basal Ganglia (Globus Pallidus, Putamen, Caudate Nucleus) |
Automatic Behavioral Sequences |
Brain Stem (Pons, Medula Oblongata) |
Basic Physiological Processes: Digestion, Respiration, Cardiac Function |
| Cerebellum | Motor Coordination and Balance |
The basal ganglia are centrally involved with action and motor behavior. The basal ganglia are actually a group of brain regions, including the putamen, globus pallidus, and caudate nucleus. This part of the brain is relatively old, phylogenetically, and handles the more automatic aspects of behavior. When we learn a new behavioral sequence, like riding a bicycle, we initially depend on the frontal lobes while we are concentrating on what we are doing. After the behavior is learned and becomes more automatic, however, it is handled by the basal ganglia. Damage to the basal ganglia can severely disrupt motor behavior, as is found in neurological diseases like Parkinson’s or Huntington’s disease.
The brain stem is the oldest and most primitive part of the brain. It regulates basic physiological processes necessary for life, such as breathing, temperature regulation, sleep-wake cycle, and cardiac function. The brain stem has been relatively conserved across evolution and therefore does not dramatically differ across animal species.
MacLean’s triune system (iStock).
In 1964, Paul D. MacLean (1913–2007) divided the brain into three general regions, the reptilian, palio-mammalian, and neo-mammalian, which he believed to correspond with different periods of evolution. The neo-mammalian region of the brain refers to the neocortex, which includes the frontal lobe and most cortical regions. These parts of the brain are most developed in more complex and evolutionarily younger mammals, such as primates. The paleo-mammalian region incorporates the limbic system, which is found in all mammals. The reptilian area of the brain refers to the brain stem and the cerebellum, phylogenetically ancient regions that are found in some of the most ancient and primitive species (e.g., reptiles). Although MacLean’s triune model has been criticized as overly simplistic, it does provide a useful way for non-specialists to picture the brain.
What is a neuron?
A neuron is a brain cell, the basic building block of the brain. The entire brain is actually a huge network of interlocking, interacting neurons. There are about one hundred billion neurons in the human brain and several times that amount of glial and other smaller cells that support neuronal function. A neuron is composed of a cell body, an axon, and a profusion of branching dendrites.
Brain cells have both input and output sections. Dendrites are the tree-like extensions that reach out from the cell body. They are the input section of the cell and carry electrical information into the cell body from the axons of other neurons. Axons are the output section of the cell and carry electrical information from the cell out to other neurons. Axons can be extremely long, reaching from the brain all the way down to the base of the spine. While some axons branch into two sections, by and large there is only one axon per neuron. At the end of the axon, the cell branches into numerous axon terminals. One axon can have thousands of axon terminals. The majority of axon terminals connect with the dendrites of other cells, resulting in trillions of neuronal connections in the typical brain.
This computer graphic illustrates the basic structure of a neuron. The dendrites branch up away from the cell body. The axon reaches down from the cell body. Note the segmented sheath covering the axon. This is the myelin sheath, a fatty covering that increases the speed that an action potential travels down the axon. Note also the branching at the end of the axon. This is the axon terminal. (iStock)
The contact point between the dendrite of one cell and the axon terminal of another is called the synapse. Neurons communicate across synapses with chemical messengers known as neurotransmitters. The neuron sending the message is called the pre-synaptic neuron while the neuron receiving the message is the post-synaptic neuron. The gap between the neurons is called the synaptic cleft.
Neurotransmitters are the chemical messengers that neurons use to communicate with each other. When an axon meets up with the dendrite of another cell at the synapse, neurotransmitters are released into the synaptic cleft. Some of these neurotransmitters are excitatory and others are inhibitory. Both types of neurotransmitters change the electrical charge of the postsynaptic neuron.
When a neuron fires, it sends an electrical impulse down the length of the axon to the axon terminals. Excitatory neurotransmitters make it easier for the post-synaptic neurons to fire; inhibitory neurotransmitters make it more difficult. Each cell receives excitatory and inhibitory inputs from many synapses. When the sum total of these inputs reaches a certain threshold, the neuron fires, sending an electrical impulse down the axon. This firing is known as an action potential. When the action potential reaches the bottom of the axon, the axon terminals release their own neurotransmitters, stimulating (or inhibiting) action potentials in the next group of neurons. In this way, wide networks of neurons communicate with each other in very little time. Action potentials commonly travel at speeds of 50 meters per second and can occur every five-hundredth of a millisecond.
This graphic illustrates both the pre- and post-synaptic neurons and the synaptic cleft between them. The bright dots are neurotransmitters that have been released into the synaptic cleft (iStock).
What is the difference between white matter and gray matter?
Axons are coated by a fatty sheath known as myelin that speeds up the rate that the action potential travels down the axon. Because myelinated axons are white in appearance, brain tissue made up of these fibers is called white matter. In contrast, grey matter refers to brain tissue made up of dendrites and cell bodies (as well as glial cells and capillaries). The surface of the cortex is composed of grey matter.
Because we see the brain through the lens of evolutionary theory, it is important to consider how the human brain has changed across evolution. We make inferences about how our brain has evolved based on comparisons between human and animal brains. For one, our brain has become much larger relative to our body size. Evolutionarily older and more primitive animals have much smaller brains relative to their body size. Think of the dinosaurs with their tiny brains and huge bodies. In fact, our brains are about three times the size of the brains of our closest relative, the chimpanzee, although our bodies are not much bigger than theirs. Secondly, the brain has become much more complex. It is less the size of an animal’s brain that determines its intelligence than the complexity of its neuronal networks. Billions of neurons create trillions of connections. Brain size and brain complexity, however, do tend to go together.
Another way that the human brain has changed across evolution is the growth of the cortex. The neocortex, the six-layer tissue that forms the outer layer of the human brain, is found only in mammals. Primitive cortices are evident in smaller mammals, such as hares, opossums, and armadillos, and more developed cortices are evident in higher-order mammals, such as elephants, dogs, and dolphins. In humans, the cortex wraps around the entire brain. The cortex allows for much more sophisticated processing of sensory information (e.g. sight, sound, touch). It also allows for more varied and flexible behavioral responses to internal and external stimulation.
In some cases, the cortex and subcortical areas provide redundant or overlapping functions. For example, both the frontal lobe and the basal ganglia regulate motor behavior. But the behavior regulated by the basal ganglia is relatively crude and inflexible. Although fast and efficient, it is not easily adapted to changing conditions. Behavior regulated by the frontal lobe, on the other hand, is much more nuanced, flexible, and responsive to changing conditions. The frontal lobe is often slower, however, and consumes more energy than the basal ganglia. Thus we are happy to rely on our basal ganglia when walking down the sidewalk, but would prefer to employ our frontal lobe when performing surgery or defusing a bomb.
The greatest change in brain structure across human evolution relates to the frontal lobe. Frontal lobes are tiny in many smaller mammals, such as tree shrews and hedgehogs. In higher-order mammals, like cats and dogs, they are still smaller than in humans and also considerably less convoluted. As mentioned above, the convolutions on the cortex provide more surface area for dendrites to expand. Relatedly, our species has the most complex and sophisticated cognitive capacities on earth. That is not to say that other animals do not use some form of thought— chimpanzees use tools to solve problems and gorillas can be taught the rudiments of language. Nonetheless, as far as we know, no other species really comes close to us with regard to intelligence.
One of the most striking differences between the brains of humans and other mammals involves the size of the olfactory bulb, which is the part of the brain involved with smell. In many mammals, the olfactory bulb is a major portion of the entire brain. In fact it is present in even the most primitive vertebrates, such as fish. In humans it is a tiny little orb sandwiched between our limbic system and the bottom of our frontal lobe. This contrast illustrates our reduced reliance on the sense of smell in favor of vision, hearing, and analytic thinking, all functions supported by the cortex.
About four to five million years ago, our ancestors and chimpanzees diverged from a common ancestor. The Australopithecus genus was one of the earliest forms of hominids. The homo genus followed, with several species, such as Homo habilis, Homo erectus, and Homo sapien neanderthalensis (neanderthals) preceding or even overlapping with modern humans. Modern humans (Homo sapien sapiens) evolved between 100,000 to 300,000 years ago.
Because soft tissue decomposes quickly we cannot expect hominid brains to survive over the hundreds of thousands and even millions of years of evolution. Therefore, paleontologists must work with skeletal remains, using skulls and other bone fragments to draw inferences about the biology and behavior of early hominids. However, tools, animal bones, fossilized seeds, and even cave paintings (in the case of early modern humans) have been found alongside hominid skeletal remains, providing intriguing clues about the mental capacity of our predecessors.
A comparison of chimpanzee and human brains and skulls. Chimpanzees are the closest genetic relative to humans (iStock).
Hominid skulls show a steady increase in cubic centimeters across evolution. Estimating from skull size and shape, Homo habilis had a brain size of 600 to 700 cubic centimeters (cc) and Homo erectus about 900 to 1,000 cubic centimeters. Homo sapien sapiens (modern humans) have a brain size of about 1,400 cubic centimeters. Concurrent with the increase in brain size, paleontologists find an increase in the complexity of tools found with hominid remains. Larger brains apparently translated into more sophisticated tool use. Additionally, larger brained hominids were adapted to more varied and/or harsher climates.
Along with the increase in skull size, the shape of the skull also suggests enlargement of the brain and of the frontal lobe in particular. Australopithecine skulls do not look much different from ape skulls. There is a prominent jaw, a small sloping forehead and a relatively small brain casing. Modern humans, in contrast, have flatter faces, very steep foreheads, and small jaws. Our foreheads, which lie just in front of the frontal lobe, cover about 50 percent of our faces. Likewise our brain casing is greatly enlarged relative to the rest of our skull.
Indentations on the inside of Homo habilis skulls (Homo habilis lived about two million years ago) suggest an enlarged area around the location of Broca’s area, a central region for speech production in modern humans. While we cannot know if this area was connected to speech in Homo habilis brains, we can suggest that at least a precursor to modern language regions of the brain was present at a very early point in hominid evolution.
Pedopmorphy refers to an evolutionary process in which adult animals maintain the traits of juveniles. One fairly easy way for genetic mutations to produce physical changes in the animal is to adjust the timing of maturation. No new physical structures or behaviors need to be introduced; the animal simply maintains its youthful traits instead of shedding them when it reaches maturity. There is evidence that many advances in human evolution involve pedomorphy. For example, we are one of the few mammals that retain a high level of playfulness throughout adulthood. Secondly, the shape of our skull mirrors that of juvenile apes. The skulls of juvenile chimpanzees look more like the skulls of adult humans than the skulls of adult chimpanzees. Adult chimpanzees have small sloping foreheads, prominent jaws, and more horizontally-aligned faces. Adult humans, on the other hand, have high foreheads, small jaws, and flat, vertically-aligned faces, similar to juveniles of both species. Of note, juvenile chimpanzees have a larger brain to skull ratio than do adult chimpanzees. In humans, this favorable ratio is retained into adulthood.
This is a nineteenth century idea put forward by a German zoologist named Ernst Haeckel (1834-1919) that development in childhood exactly parallels evolution. Phylogeny refers to evolution and ontogeny to development across the lifespan. Haeckel believed that every stage in the development of a human embryo exactly parallels the stages of human evolution. Besides having a faulty view of human evolution (e.g., there was no cow stage in human evolution), this theory oversimplified the processes of embryonic development. While Haeckel’s specific theory has been discredited, he was correct in pointing out important parallels between evolution and maturation. Careful understanding of our development across the lifespan does offer some clues as to our evolutionary history.
All vertebrates start life the same way. In their earliest stages of embryonic development, a flat plate is formed composed of three layers, the ectoderm, mesoderm, and endoderm. Cells in certain sections divide more quickly than cells in adjacent areas, causing the layers of cells to buckle and fold. In this way, curves and bends are formed and the different parts of the body begin to take shape. The outer layer of the flat plate is called the ectoderm and it is this layer of cells that will curl into the neural tube, out of which the brain and spinal cord will develop.
After an initial period of furious cell division, some cells are created that are post-mitotic, that is, they stop dividing. These cells begin the fascinating process of migrating to their final destination. They do so by means of molecular and cellular signposts that guide their progress. After the cells arrive at their proper place, neuronal connections must be established. Axons are then sent out to travel across broad swaths of territory to create synaptic connections with other cells. Their journey throughout the brain is also directed by chemical signals that point them toward their destination. The establishment of specific synaptic connections between neurons is partially controlled by genetic factors during pregnancy. The refinement of these synaptic connections, however, takes place largely after birth and is highly dependent on experience.
The neural tube is a long tubular structure that develops from the outer layer of the initial plate of embryonic cells. At the head of the neural plate, three bulges form distinct sections. These are called the forebrain, the midbrain and the hindbrain, also known as the prosencephalon, the mesencephalon, and the rhombencephalon. The table below lists the parts of the neural tube.
The Neural Tube and Its Corresponding Brain Regions
| Neural Tube Regions | Final Brain Regions | |
| Hindbrain | Metencephalon | Pons |
| Cerebellum | ||
| Myencephalon | Medulla Oblongata | |
| Midbrain | Tectum | Inferior Colliculus |
| Superior Colliculus | ||
| Cerebral Peduncle | Various Neurotransmitter Cell Bodies | |
| Forebrain | Diencephalon | Hypothalamus |
| Thalamus Other Thalamic Regions |
||
| Telencephalon | Neocortex Limbic System Basal Ganglia Cerebral White Matter |
The hindbrain divides into the metencephalon and the myelencephalon. These two sections subsequently develop into the cerebellum and pons on one hand and the medulla oblongata on the other. The pons and medulla oblongata are both part of the brain stem. Together with the cerebellum, they form the reptilian brain of Paul MacLean’s triune model.
The midbrain divides into the tectum and the cerebral peduncle. These deep brain structures lie just above the brain stem. In primitive vertebrates such as amphibians, fish, and reptiles, the tectum serves as the main visual processing center in the brain. In primates, its function is more restricted as the majority of visual processing is done in the neocortex. The primate tectum helps to control eye movement. In other words, it helps control eye movements. The cerebral peduncle includes several brain areas that contain the neurons that produce important neurotransmitters. For example, the substantia nigra is the origin of a major tract of dopamine neurons.
The forebrain develops into the evolutionarily newest parts of the brain and those most closely involved with mental life per se. The forebrain divides into the diencephalon and the telencephalon. In MacLean’s model, the forebrain includes both the paleo-mammalian and the neo-mammalian brains.
The diencephalon divides into the thalamus, the hypothalamus, and several related regions. The thalamus is involved with sensation. It is the relay station between the sense organs and their corresponding cortical regions. Olfaction (smell) is the only sense modality that does not run through the thalamus as it is processed directly by the olfactory bulb. The hypothalamus links the brain to the autonomic nervous system and serves a critical role in emotion, connecting the mental aspects of emotion with the body’s physical response.
The telencephalon contains the most advanced parts of the brain. Although present in all vertebrates, it is most developed in birds and mammals. In humans the telencephalon develops into the cerebral cortex, the limbic system, the basal ganglia, and important white matter regions. The cerebral cortex includes the four lobes of the neocortex as well as those areas on the inside surface of the cortex that directly contact the subcortical regions. These include the cingulate gyrus, hippocampal and parahippocampal regions, as well as the insula, which is sandwiched between the temporal, frontal, and parietal cortices. The basal ganglia, amygdala, and septum also develop out of the telencephalon. Finally, cerebral white matter is made up of bundles of axons that travel across large sections of the brain. Important cerebral white matter structures that develop out of the telencephalon include the anterior commissure, the internal capsule, and the corpus callosum.
Given the extraordinary journey that brain cells must travel during fetal development it is amazing that so many babies are born without brain damage (iStock).
Because such extraordinarily complex structures develop out of very simple cell groupings, early problems in neuro-development can lead to severe birth defects. For example, spina bifida is linked to defects in the neural tube. In fact, serious problems in the first trimester of pregnancy often lead to miscarriage, such that 80 percent of miscarriages occur in the first trimester. When we consider the enormous lengths the brain must travel in its journey to maturity, it is indeed remarkable that so many human beings are born without brain damage.
Plasticity of the brain refers to the brain’s ability to change with experience. As the very development of our brain is dependent upon our experience, we can say that the human brain is very plastic. In fact, brain development in humans is more experience-dependent than in any other species, reflecting the central role our capacity to learn has played in human evolution.
How does the brain change across childhood?
The brain weighs about 350 grams at birth and about 1,450 grams by adulthood. The increase in weight is mainly due to growth in dendritic branches. This is because the basic structures of the brain are in place at birth, but the connections between neurons are still under developed.
The synaptic connections between neurons are highly dependent on experience. In other words, the firing of neurons greatly influences the creation and strengthening of synaptic connections. When our brain responds to its environment—be it a sensory, perceptual, emotional, or motor response—we are activating all the neurons in the relevant brain circuitry and causing them to fire. This activation then strengthens the synaptic connections between them. As the saying goes, “Neurons that fire together, wire together.”
Synaptogenesis (the creation of synapses) involves multiple steps. New dendrites are formed, thickening the dendritic branches of the neuron. New synapses are made when these dendritic branches make contact with other neurons’ axon terminals. Additionally, an existing synapse can be strengthened by the creation of new receptor sites on the post synaptic neuron (the dendrite). These new receptor sites increase sensitivity to the neurotransmitters released into the synaptic cleft.
Agrowing body of research has yielded clues as to the best way to maintain a healthy brain. This becomes especially important as the U.S. population is living longer. In fact, the average lifespan in the United States has increased 32 years over the course of the twentieth century. In the coming decades, therefore, we can expect far more people to live into their seventies, eighties, and nineties than ever before. There are many ways to promote the brain’s health. General approaches to healthy living reduce cardiovascular disease, which is one of the main culprits in dementia. Nutritious meals, regular exercise, and avoidance of excessive alcohol, weight gain, and smoking are all important. Exercise, in particular, has been shown to protect cognition in older people, probably by promoting blood flow into the brain.
Good mental health is also important, as depression and excessive stress put strain on the brain. Mental stimulation is helpful, as well. Keep in mind that these factors all work together. Mentally active people with good social support are more likely to be happy and physically more active. Furthermore, you should not wait until you retire to start healthy behavior. It is important to instill good habits while still young. At any given time, our brain reflects our entire lifetime of experience.
Experience affects brain development in two major ways. Activation of synaptic connections strengthens the connections but lack of activation causes these connections to die off. The atrophy (or dying off) of unused connections is known as pruning. In short, the brain has a “use it or lose it” policy. For example, a baby is born with the capacity to recognize all sounds of all languages on earth. With exposure to the child’s native language, however, the synapses activated by those sounds are strengthened, but the neural networks related to other sounds weaken. Eventually the child’s brain has been wired to respond only to its own native language. Although a strong capacity to learn new languages is retained throughout childhood, receptivity to new languages decreases with age.
The brain does not remain equally plastic throughout the lifespan. There are critical periods where the most growth takes place. The peak of synaptic growth occurs within the first two years of life but synaptogenesis continues at a rapid pace for the first ten years of life. If we think of how much learning takes place within the first two years of life—a child learns to walk, talk, manipulate objects, and begins to understand the social world—it is not surprising that this is a peak period of brain growth. Children also remain extremely open to learning throughout the first decade; they learn to ride bicycles, follow societal rules, and read and write. If these tasks are not learned in childhood, it becomes much more difficult to learn them later on.
Learning does not stop after the age of ten and the ability to form new memories continues throughout the lifespan. However, the brain networks laid down during critical periods are quite conservative and difficult to change. Consider how easy it is for children under ten to learn a new language and how comparatively difficult it is for their parents to do the same thing. This is often the case in immigrant families, where the children’s ability to learn the new language far outstrips that of their parents.
As the rest of the body ages, so does the brain. The good news is that there are things everyone can do to keep both your mind and body healthier for longer periods of time (iStock).
Myelin is a fatty sheath that covers the surface of axon fibers and acts as a kind of insulation. Myelination increases the speed at which the action potential travels down the axon. The myelination of axon fibers is not well developed at birth. Myelination continues throughout childhood and myelination of the frontal lobe is not complete until the third decade of life.
The frontal lobe is one of the last areas of the brain to reach full maturity. In fact the frontal lobe does not complete synaptic formation and myelination until the mid-twenties. In this regard, ontogeny does recapitulate phylogeny; in both development and evolution the frontal lobe is a late comer. This is entirely consistent with our observations about the intellectual abilities and social judgment of children and adolescents. While many brain functions, such as physical coordination and language abilities, are fully mature by adolescence, social judgment and abstract thought take considerably longer to mature.
As new synapses are formed, the brain becomes more tightly networked. The mature brain has trillions of synapses forming neural networks of extraordinary complexity. This complexity allows for much greater intellectual power and sophistication. Although children have an advantage over adults in their ability to take in and retain new information, adults maintain a profound advantage over children in their ability to process complex information. Thus, while children’s brains may be “little sponges”, adult brains permit much greater understanding of the world around them.
Experience continues to shape the brain throughout the life span, though the changes tend to be more finely tuned later in life than in childhood and infancy. Nonetheless, the more you perform an action, think a thought, or feel a feeling, the more those circuits are reinforced. Circuits that are not reinforced fall off. As such, the “use it or lose it” adage holds across the lifespan. Of course, core circuitry that is laid down in childhood is conservative and difficult to change. That is why early learning and early emotional experience have such a profound effect on adult functioning.
How does the brain age?
An unfortunate aspect of aging involves the gradual degeneration of most organs. The brain is no exception. There is an overall decrease in the speed, flexibility and efficiency of neuronal networks. There is atrophy and shrinking of many parts of the brain. However, the news is not all grim. Much can be done to preserve brain health in aging.
Cortical atrophy refers to the shrinking of the neocortex that occurs with age. The gyri (convolutions) shrink, the sulci (grooves between gyri) and the ventricles (vessels filled with cerebral spinal fluid) expand, and there is about a 17 percent decrease in brain weight in both sexes by age eighty.
Brain cells seem to have a preprogrammed life span and cell death occurs throughout the life span. However neuronal death is intensified in aging and neurogenesis, the production of new neurons, is slowed down.
Another finding of aging involves the thinning of dendritic branching. This may account for some of the atrophy of cerebral gray matter. Fewer dendrites mean fewer synapses for neurons to communicate with each other, which in turn reduces the speed and efficiency of brain functions.
The reduction in brain volume and density definitely has an impact on the aging adult’s cognitive functions. These changes correspond with decreased processing speed, working memory, and psychomotor speed, and a reduced ability to commit new information to memory. However, many critical cognitive functions remain intact well into late life. Recognition memory, verbal skills, conceptual abilities, and general IQ stay stable for a long time despite decreases in speed and raw processing power.
Psychologists make a distinction between crystallized intelligence, which remains stable into old age, and fluid intelligence, which reduces with age. Crystallized intelligence involves verbal skills, conceptual skills, and fund of knowledge. Fluid intelligence involves immediate information processing skills, such as the speed at which information is processed, the amount of information that can be processed at a time, and the ability to commit new information to memory.
Although reductions in processing speed and memory efficiency are evident by middle age, there are some ways the brain improves with age. By the seventh or eighth decade of life, a lifetime of experience is encoded in the brain’s neural networks. A lifetime of synaptic strengthening suggests better connectivity across the brain, better integration of different brain areas. This translates into a more comprehensive understanding of the world in which we live. Moreover, enhanced cortical control of limbic responses supports a more thoughtful response to emotion. We know that impulsivity, violence, and recklessness decrease with age. We also value age and life experience when appointing people into positions of leadership. In late old age, however, such as the ninth or tenth decade of life, these strengths can be eclipsed by the degeneration of brain tissue and the elderly may become considerably constricted in their independent functioning. Nonetheless, there are more than a few people who remain healthy and vigorous even into their nineties.
Alzheimer’ disease is in many ways more frightening than other diseases as it robs one of his or her memories and ability to do daily, simple tasks (iStock).
For a long time it was simply assumed that there was no production of new neurons after birth. More and more evidence shows that this is not the case. Certain areas of the brain, such as the hippocampus (an area critical for memory formation), produce new neurons throughout adulthood. We also know that dendritic branching and synaptic development can occur throughout the lifespan. Neurogenesis (the creation of new neurons) can be bolstered by physical exercise, diet, appropriate levels of rest and relaxation, mental stimulation, and by certain medications, such as antidepressants, that act on the neurotransmitter known as serotonin.
Alzheimer’s disease is an age-related brain disease, in which abnormal growths called neurofibrillary tangles and amaloid plaques destroy the brain’s ability to function properly. These growths start in the hippocampus, where short-term memory is translated into long-term memory. Thus the cardinal feature of Alzheimer’s is loss of memory. As the disease progresses, other areas of the brain are affected and other psychological abilities deteriorate, including spatial orientation, executive functions, and eventually speech. Alzheimer’s disease is a form of dementia, which refers to any condition that involves a permanent loss of cognitive abilities. While it is one of the most common forms of dementia, it is not the only one, and other dementias, such as vascular dementia, can also cause cognitive decline in old age.
The brain is an extraordinarily intricate web of one hundred billion neurons. Somehow through the trillions of connections among these neurons, the phenomenon of the human mind emerges. Do we know how the mind comes out of the brain? Yes and no. We are learning more and more about what we call the neurobiological substrates or correlates of mental life. We are learning which parts of the brain are active when we do various mental functions. But correlation is not equal to causation. Just because two things happen together does not mean that we understand how one thing causes the other. The fundamental mystery of how billions of mindless brain cells, or neurons, somehow come together to create consciousness continues to elude us.
The concept of qualia relates to the question of subjectivity—what is the yellow of yellow, the green of green? Although we know a tremendous amount about how the brain processes light waves, we still have no idea how our own experience of yellow comes to be. At present, many neuroscientists have agreed to put the question of qualia aside. By studying how different brain processes correlate with various mental processes, we can still learn an inordinate amount about the relationship of brain and mind. This next section addresses what we do know about the links between the mind and the brain.
One critical way that the brain processes information is to create maps of the world around it. From the brain stem up to the cortex, the brain processes information like a very detailed mapmaker. The brain passes information upstream in steps, moving from the lower-level, more simply organized regions to the higher-level, more complex regions. At each step, the new region maps the neural firing patterns of the region below. It does this by recreating the spatial lay-out of the neurons of the other region. In this way, the brain builds a series of representations of both internal and external reality. These maps then serve as guides for action. They help the brain to regulate the internal states of the body, respond to objects in the environment, and respond to the neural patterns underlying our own thoughts and emotions. Another term for map is representation. This term is used frequently in discussing the brain.
When an arrangement in space in one system mimics that of another system, the first system is mapping the second. For example, when you draw two intersecting lines to show a visitor how to get to your house, you are mapping the streets where you live. The brain uses time in its maps as well as space, however. The sequence of firing patterns over time is also captured in the maps of the brain, similar to the way in which a musical theme repeats throughout a symphony.
The brain stem maps information both from the outside world and from inside the body. Nerves originating in the skin, muscles, skeletal system, blood vessels, and viscera (internal organs) all connect to neurons in the brain stem. The firing patterns of these nerves are then mirrored in the firing patterns of the brain stem neurons. In this way, the brain forms a representation of the internal state of the body, and this representation is updated on a moment-by-moment basis. Likewise, some types of sensory information, such as sound and touch, are sent to the brain stem in the initial stages of perception.
The frontal cortex maps the activity of lower areas of the brain as the frontal lobe is richly connected to other brain regions. By coordinating information from so many diverse areas of the brain, the frontal lobe can map both internal and external reality in great detail.
Sensation is the immediate mapping of raw sensory data, such as light patterns, sound waves, or tactile stimulation. Perception is the next step in the process where all that raw data is synthesized into more complex maps. These maps are then linked via memory to similar maps drawn from past experience. This allows us to classify the images into known categories, for example furniture, food, or animal. Perception occurs when the initial sensory information is organized into a sufficiently complete whole (or gestalt) that it can be recognized as an object. At this point, we recognize that the pattern of light waves hitting our retina is actually a chair. Thus sensation works by analysis—by breaking down information into its smallest parts. Perception works by synthesis, by coordinating the parts back into a whole.
Anumber of neurological disorders gives us clues as to the workings of the brain. Visual agnosia, also known as blindsight, is a neurological disorder that sheds light on how the brain makes sense of visual information. People with injuries to the visual association cortex will be blind in the conventional sense of the word. They will not be able to recognize any objects visually. Likewise, they will tell you that they cannot see anything. However, if you put a large object in front of their path, they will walk around it, all the while insisting that they do not see a thing. This suggests that fundamental information about the presence and location of visual stimuli does get through. This preliminary information is processed in the primary visual cortex. However, the coordination of visual information into coherent, recognizable shapes takes place in the association cortices. Therefore, injury in those regions results in visual agnosia. In effect, a visual agnosia is sensation without perception.
Our initial sensory information comes from physical stimulation from the outside world. This might come in the form of light waves or sound waves or physical pressure against our skin. This information is picked up by our sense organs, for example our eyes, ears, nose, skin, or tongue. Our sense organs then relay this information to the primary sensory cortices via the thalamus. The thalamus acts as a gating station, blocking information it identifies as unimportant and passing on information it deems important. This is true for all our senses except the sense of smell (olfaction) which goes directly to the olfactory bulb, bypassing the thalamus and the cortex. The primary visual cortex is in the occipital lobe, the primary auditory (hearing) cortex is in the temporal lobe, and the primary sensory cortex (for touch and taste) is in the parietal lobe. The primary sensory cortex is known as the somatosensory strip.
The primary sensory cortical areas register the most fundamental aspects of sensation, such as the orientation of a line, the location of a touch, the frequency of a sound wave. Information about these fundamental sensory properties is then passed onto the association cortices, where it is synthesized into a larger whole. In this way sensation moves toward perception.
Sensory information does not only come from outside the body. We also need sensory information about the state of our body. We need to know if we are feeling dizzy or sick to our stomachs, or if our heart is beating fast. This tells us important information about our health but also tells us about our emotions. Part of the way that we know what we are feeling is through sensory information about our internal bodily states. In fact, there are neurons throughout the gastro-intestinal system—so many that it is sometimes called our second brain. The brain stem, somatosensory strip and insula (a cortical area on the inside of the cortex) are involved with processing sensory information from inside our body.
It is important to understand that the brain does not work like a camera, simply photographing outside reality. As mentioned above, the brain constructs its own version of reality by constructing a map of the world around it. In the primary sensory cortices, neurons respond to patterns of stimulation, for example, with cells that fire in response to horizontal, vertical, or diagonal lines. This information is then sent to the association cortices, where the firing patterns of the different cells are coordinated with each other to establish a broader pattern. When this information is sent to the parts of the brain associated with memory, language, and emotion, we are both able to recognize and label an object as well as appreciate what meaning it might have for us. In this way, the brain constructs an interpretation or map of reality that is deeply connected to our personal experience and history.
The association cortices are the cortical areas that synthesize the fundamental units of sensation into larger patterns and towards a recognizable whole. The visual association cortex is in the occipital lobe, just anterior to the primary visual cortex. The auditory association cortex is in the temporal lobe, close to the primary auditory cortex. The association cortex for touch is next to the somatosensory strip in the parietal cortex, in the brain regions known as S2 and S3.
After sensory information is processed in the unimodal (single sense) association areas, it is sent to the multimodal association areas, where information from different sensory modalities can be coordinated. For example, information about the sight of a chair as well as the sound and the feel of your body as you sit down in it is coordinated into the unified perception of a single object. Simultaneously, activation of the hippocampus and parts of the temporal cortex jog our memory, allowing us to place our perception in the context of our memories. Thus, this particular object is recognized as a chair.
What meets the eye is not always perceived the same from person to person. The external world that people see and hear and touch and smell is interpreted differently as it is filtered through each individual’s emotions, memories, and cognitive processes (iStock).
As mentioned above, the brain does not record reality like a camera; it constructs a representation of reality through analysis and synthesis of sensory information. Therefore, each person’s perception of any given event will be unique, which explains why people can have such differing memories of the same event. Even if the same sensory information is available to two different people, the unique history of each person’s brain will ensure that the final perception of each individual will differ, colored by variations in the individuals’ attention, memories, emotional states, etc. Moreover, the exact sensory information in any given event will never be identical for any two people because the position in space of each person’s body will necessarily differ. All these factors will continue to color the memory of the event at later times. This is well understood in legal contexts and is the reason that eye witness testimony can be highly problematic.
One-third of the brain is used to process visual information, which tells us just how important vision is to human beings. The processing of visual information starts in the eyes, the visual sense organs. On the retina, the inner back surface of the eye, there are a number of neurons that are specialized to fire in response to light. These specific cells are called rods and cones. Rods respond to night vision and process information in shades of black, white and gray. Cones fire in bright light and respond to color. The axons of rods and cones connect to other cells in the retina, where some preliminary visual processing is done. These neurons then connect to ganglion cells, whose axons bundle together, like wires in a cable, to form the optic nerve.
The optic nerve exits through the back of the retina and travels to the brain. All the ganglion cells that respond to the right side of the visual field will go to the left side of the brain. Likewise the ganglion cells that respond to the left side of the visual field will go to the right side of the brain. Any ganglion axon that needs to cross over to the opposite side of the brain is able to do so at the optic chiasm, in the middle of the brain. The two branches of the optic nerve next connect to the corresponding lobes of the thalamus. The thalamus is the gate keeper for sensory information. Based on feedback from the cortex, the thalamus lets some information through to activate neurons connected to the cortex but stops other messages in their tracks. Neurons in the thalamus send visual information up to the primary visual cortex in the occipital lobe, known as VI or Brodman area 17. From there, the fundamental features of the visual stimulus are processed. As discussed above, these neurons connect to association cortices where the basic visual features are then synthesized into larger patterns.
An elderly man was admitted to a hospital. The staff soon noticed that he became agitated when walking by pictures hanging on the walls. These were framed prints encased in glass. When passing by the picture, he would catch a reflection of himself and then turn and start yelling, “Get away from me! Leave me alone!” He would ask the nurses plaintively, “Why does that man keep following me?”
This man suffered from a disorder known as prosopagnosia, which refers to the inability to recognize faces. This bizarrely specific disorder is due to lesions in the fusiform gyrus, a multimodal association region that is located at the bottom of the temporal lobe. The fusiform gyrus integrates perceptual information about faces into recognizable wholes. When this region is damaged, the person cannot perceive faces as a recognizable whole but only as a collection of visual parts. Some readers may be familiar with this disorder from Oliver Sacks’s book The Man Who Mistook His Wife for a Hat.
The ear is composed of three sections, the outer ear, the middle ear, and the inner ear. The outer flap that we normally think of as the ear is called the pinna. That and the ear canal, the long inner tunnel where our wax collects, comprise the outer ear. The ear canal ends in the ear drum, a thin membrane that stretches across the back end of the ear canal. The middle ear consists of three tiny, delicate bones that transfer sound vibrations from the ear drum to the cochlea, a fluid-filled spiral that translates sound vibrations into neural activity. The cochlea and the vestibular apparatus, which senses balance and motion, comprise the inner ear. Sound vibrations are captured by the pinna, channeled back to the middle ear by the ear canal, and communicated to the cochlea via the bones of the middle ear.
The fluid-filled cochlea is lined with hair cells, a type of sensory neuron that responds to sounds of specific frequencies. High frequencies (which translate into high pitched sounds) are recorded by hair cells at the opening of the spiral cochlea, middle frequencies in the middle of the cochlea, and low frequencies by hair cells at the end of the spiral. The hair cells send information to the spinal cord, which then connects to several regions in the brain stem. Here the auditory information is processed for the timing and intensity. These neurons connect to the inferior colliculus in the mid-brain, where some further analysis takes place. The inferior colliculus connects to the gate-keeping thalamus. Finally, the auditory information enters the cortex at the primary auditory cortex (A1), located in the superior and posterior (upper rear) temporal lobe. Association areas are nearby, including the regions that deal with language.
One of the most common symptoms of schizophrenia and other psychotic disorders involves auditory hallucinations. This occurs when someone hears sounds that have no basis in external reality. People can hear voices talking to them—sometimes a single voice, sometimes an entire chorus of voices. The voices can maintain an ongoing commentary on the person’s behavior, can make derogatory statements, call the person names, or even instruct the person on what to do.
The most elaborate auditory hallucinations are associated with schizophrenia. Researchers have found that the auditory cortex is active when a person is hearing voices. In this way the brain actually is hearing voices although the sensory information does not come from the ear but from inside the person’s brain. It is believed that hallucinatory voices actually derive from the person’s thoughts. The thoughts get translated into a kind of inner speech, which the brain hears as if it were hearing actual speech.
While inner speech is a common phenomenon, healthy brains can distinguish between inner speech and spoken words. In psychosis, however, the distinction between inner and outer reality is lost. For reasons we do not fully understand, antipsychotic medication restores the ability to make the critical distinction between imagined and external reality.
Animals such as dogs have a much more heightened sense of smell than humans, whose olfactory bulbs are correspondingly smaller and process less information (iStock).
The sensory organs for all of our other senses are relatively small. Consider the size of the ears, nose, or eyes. The sensory organ that processes touch, however, covers our whole body. Our entire skin functions as the sensory organ for touch. Our skin is covered with many different sensory receptor cells. Some are specialized to sense changes in physical pressure, some to vibrations, some to pain, and others to temperature. These neurons send information to the cortex via a fairly long pathway, running through the spinal cord, areas in the mid brain, and the thalamus. The primary sensory area for touch is called the somatosensory strip (S1) in the anterior (front) area of the parietal lobe. Neurons responding to touch on different areas of the body are mapped along the somatosensory strip. This map is known as the homunculus (“little man” in Latin). Association cortices for touch (e.g., S2 and S3) are adjacent to S1.
The sense of smell is processed in the olfactory system, which is an evolutionarily ancient system, dating back hundreds of millions of years. The olfactory system responds to airborne chemicals, which waft into the nose. The olfactory nerve connects the olfactory receptor cells in the nose to the olfactory bulbs, two small structures on either side of the brain just below the frontal lobe. From the olfactory bulbs, axons bundled together into the olfactory nerves project to various parts of the limbic system. From there, neurons connect to other subcortical areas such as the thalamus, hypothalamus, and insula. Thus, the olfactory nerve sends information directly to the emotional centers of the brain, without the filtering role of the thalamus or the cortex. The olfactory bulb plays a much larger role in simpler and phylogenetically older animals than it does in phylogenetically younger and more complex animals like primates.
Our subjective experience of taste is actually a combination of taste and smell. If you sever the olfactory nerve, removing the sense of smell, your ability to taste your food will be much reduced. You will only be able to taste sweet, bitter, salt, and sour flavors. There is now evidence for a fifth type of taste category, however, which has been named umami. This is a Japanese word meaning delicious, savory, or meaty. Umami seems related to monosodium glutamate (MSG), a food additive used extensively in East Asian cuisine.
All of these taste categories have survival value. Sweet, umami, and salty tastes enhance our intake of carbohydrate, protein, and salt, respectively. Excessive amounts of bitter or sour taste alert us to decaying or toxic food. The relative amounts of the taste qualities of any food put in the mouth are sensed by the taste buds (gustatory papillae), tiny protrusions that cover the surface of the tongue. The taste buds are somewhat specialized to specific taste qualities, but generally respond to more than one aspect of taste (e.g., both salty and sweet). The taste buds carry information to the cranial nerves, which connect to neurons in the brain stem, which then connect to neurons in the thalamus. These project (or connect) to a particular area in the somatosensory strip specialized for the tongue.
Physical action, also called motor behavior, is the output side of the input/output system of the brain. Sensation/perception is our input and action is our output. Our physical behavior ranges from simple reflexive movements up to intentional, planned, and complex movements.
Voluntary movements are those movements that are potentially under conscious control. Examples include walking, standing up, raising our arm, getting dressed, shaking our head, etc. Involuntary movements are those movements that are not under conscious control or that generally happen automatically without conscious thought. Examples include breathing, heartbeat, posture, and motor coordination.
Different tracts of neurons are involved with voluntary and involuntary movements. Involuntary movements are processed largely through a group of brain cells known as extrapyramidal neurons. They connect input from the cerebellum and the inner ear to the brain stem. The first two areas process information about coordination and balance. The brain stem sends this information to motor neurons in the spinal cord, which connect directly to the muscles involved. Thus, information related to involuntary movement does not go through the cortex and generally travels in a relatively simple, closed circuit.
Antipsychotic medication can disrupt the function of extrapyramidal neurons. This can result in disfiguring side effects, such as tardive dyskinesia, a syndrome characterized by tremors in the extremities and facial muscles. The atypical antipsychotics, a new class of medication, have now become popular because they produce fewer extrapyramidal side effects (e.g., restlessness, tremor, and muscular stiffness).
While sensory information moves more or less from the back to the front of the brain, motor information moves in the opposite direction. Goals for physical action are processed in the pre-frontal cortex, the seat of planning and goal setting. This information is sent back to the pre-motor cortex and the supplemental motor area, which lie just in front of the primary motor cortex. Coordination of the specific movements appears to take place in these areas.
This information is then transmitted to the primary motor cortex (or M1), which lies just in front of the central sulcus, next to the somatosensory strip of the parietal lobe. The surface of the body is mapped along M1, creating a similar homunculus to the one found next door in the somatosensory strip. This region is linked to the actual execution of the movement. M1 sends information to the brain stem, which then activates motor neurons in the spinal cord. These in turn connect directly to the muscles.
It is important to understand that the brain is continuously giving and receiving feedback. Information is always moving up and down the system and modifying both input and output. For example, the motor system sends out a signal to move and the left hand reaches out to grasp a glass. Simultaneously, the brain processes important sensory feedback. The hand is too far to the left of the glass, the pinky finger brushes against the back of a chair. This feedback is automatically incorporated into the ongoing movement, and the hand is moved half an inch to the right. The impact of this new movement is then encoded into new sensory information. For the sake of simplicity, our sensory and motor systems are presented here as if they act in isolation. In reality, though, the brain is a giant web of interacting systems giving itself constant feedback.
The cerebellum, which is Latin for “little brain,” is the large, bulbous structure located below the back of the cortex. With rich connections to both the frontal lobe and the brain stem, it is integrally involved in motor control. The cerebellum mediates motor coordination, posture, and the smooth flow of movement. Damage to the cerebellum results in jerky, uncoordinated movements and problems with balance. More recent research has revealed that the cerebellum is involved with a range of cognitive functions as well.
Surprisingly, the brain processes observed movement and imagined movement in much the same way it processes actual movement. Brain imaging studies have shown that the same areas of the motor cortex are activated when people witness an action in another person, imagine performing the same action themselves, or actually perform the action themselves.
Mirror neurons are a group of neurons found in the premotor cortex that respond both to witnessed movements in other animals and to the analogous movement in the self. Similar neurons have also been found in the sensory association cortex in the parietal lobe. Some scientists have suggested that mirror neurons might be the basis of empathy. Mirror neurons were discovered when scientists implanted electrodes in monkeys’ brains to measure the electrical activity of single neurons. The cells fired both when the monkey made a particular hand movement and when the experimenter made the same hand movement. The cells did not fire when the experimenter made a different hand movement.
People who have lost their limbs to amputation often complain of feeling pain where the limb used to be. This phenomenon is known as phantom limb pain. Such an experience can be very distressing as it compounds an already devastating loss. Brain imaging studies have shown that phantom limb sensation is related to activity in the somatosensory strip and the sensory association cortices nearby. Neurons in the spinal cord still send pain signals to the parts of the brain that process sensory information from the amputated limb. This does not apply to just pain. For people who have lost their hands can imagine moving their fingers. When they do so, the corresponding area of the motor strip in the brain is activated as if the hand is still there.
People who have had a limb or other body part amputated can still experience sensations like pain because the nerves in the spinal cord corresponding to that area of the body can still send signals to the brain (iStock).
The frontal lobe is not the only source of complex goal-oriented movement. As discussed above, the frontal lobe is a recent evolutionary achievement and is most fully developed in humans. Prior to the evolution of the frontal lobe, however, animals needed some way to perform goal-oriented behaviors. Prey had to be hunted, hygiene maintained, food eaten, and social behaviors performed. In most animals, these behaviors were patterned into fairly fixed packets of behaviors that would be released rather automatically in the face of the appropriate stimulus. The mouse runs across the floor and the cat pounces. The part of the brain associated with these pre-set behavioral packets is known as the basal ganglia.
The basal ganglia refers to a group of brain structures which include the caudate nucleus, the putamen and the globus pallidus. This is an evolutionarily ancient brain region that precedes the evolution of the cortex and is found in mammals, birds, and even reptiles. The basal ganglia mediates simple motor programs. These are packets of movements that serve a purpose—riding a bike, throwing a ball, etc. Some of these automatic behaviors are learned (e.g., riding a bike) and some are unlearned, based on genetics. The unlearned motor programs are also known as fixed action patterns. In humans, the basal ganglia is heavily involved with learned motor programs. Although complex behaviors are learned via the frontal lobe, with practice the behavior becomes more automatic and the basal ganglia comes into play.
A fixed action pattern is a genetically-coded behavioral sequence that is triggered by specific stimuli. Fixed action patterns are analogous to animal “instincts” and are mediated by the basal ganglia. The behavior is fixed and relatively unmodifiable. There is very little goal correction; the behavior is a set response to a set stimulus.
For example, cats smell their urine and start digging in cat litter to bury their waste. They rarely check the results of their efforts to see if they have met their goal; they simply respond in a preprogrammed way to a set stimulus, in this case the smell of their urine or feces. Grooming behavior in multiple animals provides additional examples of fixed action patterns. Birds preen themselves, cats lick themselves, and dogs shake themselves when wet. Additional examples include gnawing behavior in pigs and horses whinnying and shaking their heads.
In humans, we see reflexes at birth (rooting, swimming, grasping, and sucking). With development, however, these basal ganglia-mediated reflexes get suppressed by the frontal lobe. For the most part, thoughtful, intentional behavior replaces automatic stimulus-response chains. In adulthood, fixed action patterns can re-emerge if there is damage to the frontal lobe. Frontal release signs, which are associated with frontal lobe damage, include several reflexes that we normally see only in early infancy. Certain psychiatric conditions may also reflect pathological activation of fixed action patterns. For example, obsessive compulsive disorder (OCD) has been linked to basal ganglia-frontal lobe circuits. It is characterized by repetitive, stereotyped, and senseless behaviors, such as compulsive hand-washing, tapping, straightening, or organizing behavior.
What is cognition? How do we define thought? At base, cognition involves the representation of events and the mental manipulation of events outside of immediate reality. In other words, the use of imagination. If you can imagine the outcome of an action, you can evaluate whether or not it is worthwhile to take such an action. Or you can imagine alternative actions that can be taken. These abilities dramatically change the animal’s relationship with action. With cognition, human beings are able to think before they act and to correct their mistakes. They are also able to plan future actions and to anticipate possible outcomes.
Cognition also allows us to compare the actual outcome of an action with the desired outcome and then modify the action accordingly. This process is called goal-correction.
Executive functions are a set of mental abilities that are mediated by the frontal lobes. These include planning, analyzing, considering alternative actions, abstraction, changing sets, etc. These critical mental functions help people adapt to a complex and ever-changing environment. When the frontal lobe is damaged, people lose executive functions. They become more impulsive, disorganized, unable to plan, and unable to monitor and regulate their own behavior. In effect they become more childlike, regressing to a time when the frontal lobe was less developed.
Impulse control is also critical. Specifically, this involves the linking of representations of events with memories or expectations of punishment. The person can consider the negative consequences of an intended action and inhibit or alter the action accordingly. People with poor impulse control often do poorly on neuropsychological tests that measure executive functions like abstraction, set switching, and planning. Impulse control is mediated by the orbital frontal cortex, which lies on the underside of the frontal lobe.
Phineas Gage (1823-1860) was a railroad worker living in the mid-nineteenth century who suffered a terrible accident while at work. An iron rod crashed through his head, tearing a large hole through his brain. Surprisingly, he survived this incident and in fact was relatively unharmed physically. His cognition and motor control were reasonably intact. However, there were marked personality changes that were very disturbing to those who had known him prior to the accident. While he had once been a sober and well-behaved citizen, after the accident he became rude, impulsive, and socially inappropriate. We now understand that he suffered from orbital frontal damage, with severe damage to the areas of the brain centrally involved with impulse control and social judgment.
We can think of emotions as behavioral packets for social animals. They are a very quick and efficient way to respond to different circumstances in the environment. Such circumstances might include aversive conditions, such as danger or aggression, or rewarding conditions such as food, sex, safety, or social bonds. All emotional reactions involve coordination of several features. These include autonomic nervous system arousal, facial expression, muscular tension, and subjective experience. This packet of responses is almost like a computer macro that serves as preparation for a specific situation. For example, when people are angry their blood pumps faster, their face becomes flush, their brow furrows, their mouth purses, the large muscles in their arms and legs tense, and they have the distinct subjective experience of anger. In this way, their body is prepared not only for action, but for aggressive action in particular.
All emotions serve at least three purposes: they prepare the individual for appropriate action; they alert the individual of the salience (or significance) of the situation; they communicate to others how the individual is reacting. For example, in a situation of danger, the emotional response of fear alerts the individual to the danger of the situation, prepares the individual for physical flight, and communicates to other people via facial expression, vocalization, and physical posture that the individual senses danger.
The core emotions are generally recognized to be anger, fear, disgust, surprise, joy, and sadness. These are biologically encoded, unlearned human responses that are immediately recognizable regardless of linguistic or cultural differences. Many of these emotions, such as fear and anger, are also found in other mammals. However, the palette of human emotions is broader than these six core emotions. Self-conscious emotions—such as shame, embarrassment, pride, or guilt—are also part of the human emotional repertoire. These more complex emotions depend upon a certain degree of cognitive development and relate to one’s place in the social group.
Feeling something and knowing what we are feeling are two different things. In fact, the neuroscientist Antonio Damasio makes a distinction between emotion, which is the body’s physiological response, and feeling, which is the conscious experience of emotion. Infants are born with emotional responses—they are literally born crying. In contrast, the ability to recognize and verbally label emotion (“Oh, I’m feeling sad.”) is something that develops with age and is to some extent dependent on appropriate social feedback. Relatedly, the basis of psychodynamic psychotherapy is the idea that emotional maladjustment comes from lack of knowledge of one’s own emotions. The inability to recognize one’s own emotions is called alexythymia.
Our knowledge of the neurobiology of emotion is far less developed than our knowledge of the role of the brain in cognition. But we do know that the group of brain structures called the limbic system is centrally involved with emotion. The limbic system refers to a group of subcortical brain structures that wrap around the thalamus. Although there is disagreement about the exact boundaries of the limbic system, the term is usually understood to include core brain structures involved in the processing of emotion.
The amygdala is a small, almond shaped structure that lies just below the basal ganglia (“amygdala” is Greek for “almond”). The amygdala is a critical player in emotional reactions. It is an early responder to emotionally salient stimuli, particularly fearful stimuli. The amygdala has rich connections with other limbic areas as well as lower brain regions, such as the midbrain and the brain stem. In particular, it has many connections to neurons in the midbrain, which manufacture neurotransmitters highly relevant to emotional life.
For example, the raphe nucleus generates serotonin, the ventral tegmental area dopamine, and the locus ceruleus norepinephrine. Most psychiatric drugs target one or more of these neurotransmitter systems. The amygdala also connects to the frontal and temporal lobes. In this way, the amygdala acts as a way station between the thinking and perceiving areas of the brain and the physiological control centers.
The HPA axis is one major route by which the hypothalamus can step on the gas pedal and rev the body up. The HPA axis includes the hypothalamus, pituitary gland, and adrenal glands. This triad is centrally involved in the stress response of the body. The pituitary gland is a small structure below the hypothalamus and the adrenal glands lie just above the kidneys. The hypothalamus secretes a hormone known as cortisol releasing hormone (CRH), which travels down to the pituitary gland, where it stimulates the release of adrenocorticotropic hormone (ACTH). This is turn travels down to the adrenal glands, where it stimulates the release of cortisol and other corticosteroids. These hormones activate the sympathetic nervous system. Cortisol influences many psychological responses, including stress, mood, and some forms of mental illness.
One of the brain regions that the amygdala connects to is the hypothalamus. This important structure is involved with motivational drives such as hunger, sex, and thirst and also serves as a coordinator for the physiological centers of the brain. The hypothalamus is the master control center for the autonomic nervous system. This whole body system gears the body up for action by mobilizing the cardiovascular, respiratory, muscular, and gastro-intestinal systems.
The work of the autonomic nervous system is evident when we feel emotionally aroused. Our heart beats faster, we start to sweat, our stomach churns, and our breath grows rapid and shallow. (More specifically, this is the work of the sympathetic nervous system, which speeds us up. The parasympathetic nervous system slows us down.) The hypothalamus activates the autonomic nervous system in two major ways. The first is the more traditional route of synaptic connections between neurons. The second is through the release of hormones, free-floating chemical messengers that largely travel through the blood stream.
The hippocampus is involved with memory, which is critical to our evaluation of any new stimulus. Have I encountered this person/situation/object before? Is this friend or foe? Although the hippocampus is not directly involved in emotion, it is located near the other limbic structures and has neuronal connections with several of them, including the amygdala, the hypothalamus, and the cingulate gyrus.
The insula is involved with the representation of internal bodily states. It helps process sensory information from inside our bodies, for example butterflies in our stomach or cramps in our intestines. More specifically it has been associated with the processing of aversive food tastes and the related experience of disgust. The insula is located on the inside of the cortex, surrounded by the temporal, frontal, and parietal lobes. Brain imaging studies have shown that this region is activated in many different emotional states. This is because sensory information about the physical state of the body plays a critical part in the subjective awareness of emotion. Try to remember the times when you have felt intensely happy, angry, or frightened. Do you recall the sensation of changes in muscle tension, energy level, heartbeat, etc? Can you imagine experiencing intense emotions without feeling these bodily changes? In sum, the amygdala and hypothalamus primarily provide emotional output—they activate the emotional responses. In contrast, the insula gives emotional input. It helps translate biological emotion into conscious feeling by giving us information about what our body is doing.
Many mammal species are very social. These elephants, for instance, are engaging in affectionate social behavior. Our emotions evolved in large part to help us negotiate the social world (iStock.com).
Emotions are part of the evolutionary tool kit of mammals. Mammals are, for the most part, very social animals and emotions help in the core aspects of social functioning. For example, emotions cement social bonds, in the interest of both mating and parenting. They support intra-species competition, as when animals compete for status, resources and mates. They also offer invaluable help in responding to and communicating danger and, finally, they give us information as to how other group members are responding to ourselves and to the environment.
Core emotions are clearly evident in our household pets, our cats and dogs, which is why we grow so attached to them. Affection, aggression, fear, contentment, and excitement are all impossible to miss. When a dog wags its tail and assumes a play position, it is hard not to respond with positive emotion. Likewise, when our pet cat rubs against us and purrs, this typically elicits a similarly content and affectionate response.
Because of our greater brain complexity, our emotions tend to be more nuanced and complex, more influenced by cognition and by our considerations of the past and the future. Animal emotions are simpler, more directly tied to actions and to the here and now. Dogs may growl at a stranger, rub against a beloved and trusted owner, or wag their tails at a sign that they are about to be taken for a walk. In contrast, human emotion is much less tied to the present. We regret the past and worry about the future. And our emotional responses are not restricted to our immediate environment. Human beings might become depressed after reading a newspaper article on global warming, worry about what their friends think of their lovers, or feel jealous of a high school friend’s success at work.
The limbic system, the seat of emotions, has not changed as much as the frontal lobe across evolution. Our cortex is quite different from that of our mammalian cousins; our limbic system is not. This is why we can make emotional attachments to animals of different species. Even though our intellectual brains are vastly different, our emotional brains are relatively similar.
The frontal lobe serves to control the limbic system. Correspondingly, thought serves to control emotion. The frontal lobe is richly connected to brain regions throughout the limbic system and many frontal lobe neurons that connect to limbic regions have inhibitory effects. Emotions are blunt instruments—they are very fast, but not very precise. The frontal lobe helps us refine our emotional responses; to ensure that our responses are proportional to the situation. This is done with the use of thought, by interjecting thought between emotion and response.
Sometimes cognitive analysis of the situation may increase the emotional response. Think of the times when your initial response to a situation was muted but then escalated the more you thought about what happened. On the other hand, cognitive analysis frequently serves to dampen the emotional response by helping the individual consider the consequences of acting on emotions. (If I punch him, he might punch back. If I quit my job, I won’t be able to pay my rent.) Cognition also helps people consider alternative explanations for a situation. (Hmmm, maybe this wasn’t an insult. Maybe he just didn’t see me.)
Much of psychoanalytic theory has required modification since Sigmund Freud’s original work. Nonetheless, Freud’s concepts of the ego and the id have held up remarkably well over these past 100 years. The ego is the seat of the reality principle. It helps us adapt our wishes and urges to cold reality. The id, on the other hand, is the seat of our most primitive and animalistic desires. It is the source of our passions. As is suggested by Freud’s famous phrase “Where id was, there shall ego be,” the ego serves to control the id.
Surprisingly, modern neuroscience has completely supported this notion. The id can be equated with the limbic system, the seat of our emotions. The ego can be equated with the frontal lobe, or, more specifically, the pre-frontal cortex, which mediates cognition and behavioral control. Just as the ego serves to control the id, the frontal lobe serves to control the limbic system.
The orbital frontal region lies on the underside of the frontal lobe, just above the eyes. This brain region is particularly important for impulse control, for the inhibition of dangerous or reckless actions. People with damage to their orbital frontal cortex show disinhibited, impulsive, and socially unacceptable behavior. The case of Phineas Gage is a famous example of orbital frontal damage. Orbital frontal inhibition probably works through linking representations of future events with representations of past or future punishment.
New research suggests that a part of the frontal lobe, the superior medial frontal cortex, is involved with social cognition via connecting emotional memories with cognition. This part of the brain, which lies in the middle of the frontal lobe, has been linked to the perception of self, of others, and of mental states. Although this research is still fairly new, it may be quite radical, in that it provides the first evidence of the neurobiological substrates of certain aspects of personality.
As the frontal lobe is the last to develop across childhood, and in fact is not fully developed until adulthood, emotional control also does not fully mature until adulthood. This is intuitively obvious if we think of the relative emotional immaturity of children and even adolescents.
When the frontal lobe deteriorates, we can see re-emergence of more primitive behaviors that had previously been inhibited by the frontal lobe. The Babinski reflex and frontal release signs are examples of this. Likewise, there is less control over primitive limbic responses. Consequently, the person loses social judgment, impulse control, and the ability to plan and to analyze situations effectively. This is why people with Alzheimer’s disease or other dementias need continuous supervision. When the frontal lobe goes, the person in effect regresses to childhood.
The frontal release signs refer to a group of reflexive behaviors, controlled by the basal ganglia, which are normally evident only in earliest infancy. Examples of these behaviors include rooting (turning the face toward an object if it touches the cheek near the mouth) and puckering of the lips in response to touch of the skin above the upper lip. These instinctual behaviors promote nursing behavior in an infant.
The palmar grasp reflex helps an infant hold onto its mother. In this reaction, the infant grasps at anything that strokes its palm. The Babinski reflex, in which the foot arches away from tactile stimulation on the sole of the foot, is another of these early reflexes. With development of the frontal lobe, these crude automatic behaviors are suppressed. When there is damage to the frontal lobe in adulthood, however, these early reflexes may re-emerge. The presence of frontal release signs in adulthood, therefore, is a sign of significant brain damage.
Neurotransmitters are perhaps the main chemical messengers in the brain. They are the means by which neurons communicate with each other. It is through neurotransmitters that one neuron tells another neuron to fire. If we think of the neuronal networks of the brain as a vast economy, neurotransmitters can be seen as the currency of that economy. The exchange of neurotransmitters stimulates neurons to act.
Neurotransmitters are stored in sac-like vesicles in the axon terminals of the neuron. When the neuron fires, its axon terminals release neurotransmitters into the synaptic cleft, the space between the pre-synaptic and post-synaptic neuron. When the neurotransmitters bind to the receptor sites on the post-synaptic neuron, they impact the likelihood that the neuron will fire. Excitatory neurotransmitters increase the likelihood of firing while inhibitory neurotransmitters decrease it.
Dopamine, norepinephrine, and serotonin are probably the best known of the neurotransmitters and the ones most frequently targeted by psychiatric medication. All three are classified as monoamines based on their chemical structure. Glutamate is a general excitatory neurotransmitter—it increases the likelihood that neurons will fire. GABA is a general inhibitory neurotransmitter—it decreases the likelihood of neurons firing. Histamine is known for its involvement in the allergic response. Acetylcholine is involved with memory and is targeted by anti-Alzheimer’s drugs.
The table below lists the major classes of psychiatric medications, sample drugs in each class, and the primary neurotransmitter systems targeted by each medication class. Cognitive enhancers are a fairly new class of drugs, developed to treat Alzheimer’s dementia.
A model of a dopamine molecule. Dopamine is a neurotransmitter frequently targeted by psychiatric medications (iStock).
Neurotransmitters and Psychiatric Medications
| Class of Medication | Specific Drugs | Neurotransmitter |
| Typical antipsychotics | Haloperidol (Haldol) Chlorpromazine (Thorazine) | Dopamine |
| Atypical antipsychotics | Risperidone (Risperdal) Olanzapine (Zyprexa) | Dopamine, Serotonin, Histamine, Norepinephrine |
| SSRI Antidepressants | Fluoxetine (Prozac) Sertraline (Zoloft) | Serotonin |
| Tricyclic Antidepressants | Amitriptylene (Elavil) Clomipramine (Anafranil) | Serotonin, Norepinephrine |
| Benzodiazepine antianxiety medications | Diazepam (Valium) Clonazepam (Klonopin) | GABA |
| Stimulants | Methylphenidate (Ritalin)
Dextroamphetamine (Dexedrine) |
Dopamine, Norepinephrine |
| Cognitive Enhancers: Cholinesterase Inhibitors | Donepezil (Aricept) Tacrine (Cognex) | Acetylcholine |
| Cognitive Enhancers: NMDA Receptor Antagonists | Memantine (Namenda) | Glutamate |
What functions does dopamine serve?
The dopamine pathways serve many functions. There are, in fact, several dopaminergic tracts. The nigro-striatal tracts originate in the substantia nigra in the midbrain and project to the basal ganglia. These tracts are involved with motor control (control of physical movements) and are the neurons damaged in Parkinson’s disease. Antipsychotic drugs can also cause problems with this system, resulting in abnormal movements. The second major dopaminergic tract is known as the mesolimbic pathway. It originates in the ventral tegmental area (also in the midbrain) and projects to the nucleus accumbens and some areas of the limbic system. This tract is associated with the reward system. The third major dopaminergic tract is the mesocortical tract. Likethe mesolimbic tract, this originates in the ventral tegmental area. It projects to the cortex, with particularly rich connections in the frontal lobe. This pathway is associated with psychotic symptoms and is targeted by many antipsychotic medications.
Many of the major neurotransmitters are distributed by single tracts of neurons. The cell bodies of these neurons are located deep in the midbrain or brain stem but their axons travel great distances across the limbic system and the cortex, branching off to form synapses with many other neurons along the way. In this way, a neurotransmitter tract acts somewhat like a subway, with the neurotransmitters acting as passengers. In the same way that the No. 1 line in New York City runs through City College, Columbia University, the theater district, and the financial district, each neurotransmitter tract runs through specific areas of the brain that serve specific functions. If you blockaded the No.1 line running down Broadway, you would have reduced activity in the four districts mentioned above. Likewise, if you blockade dopamine activity in the nigrastriatal tract, you will have reduced activity in the basal ganglia and subsequent problems with motor coordination will arise. This is exactly what happens with Parkinson’s disease.
The reward system refers to a tract of dopamine-containing neurons that are centrally involved in the experience of desire. The object of desire is not important. This is an all-purpose motivation machine that is active in drug craving (cocaine, methamphetamine, alcohol, and cigarettes) and in gambling, eating, and sex. It may be active as well in many other activities that elicit strong motivation and desire. The reward system is composed of the mesolimbic dopaminergic tracts, which reach from the ventral tegmental area in the midbrain to the nucleus accumbens in the forebrain.
Serotonin is an evolutionarily ancient neurotransmitter system and is found in animals as primitive as sea slugs. In humans, not surprisingly, it is involved with a very wide array of functions, ranging from the simplest to the most advanced. For example, serotonin is involved with hunger, sleep, migraine headaches, and sexual function. It is also involved with mood, anxiety, and anticipation of harm. People with low levels of serotonin demonstrate impulse control problems, while people with high levels of serotonin manifest excessive levels of caution and inhibition. The serotonin tracts originate in the raphe nuclei in the brainstem and project widely throughout the cerebral cortex. Certain serotonergic tracts also project downwards to the spinal cord.
Serotonin is targeted by the selective serotonin reuptake inhibitors (SSRIs), the widely used class of antidepressants. Examples of SSRIs include fluoxetine (Prozac), sertraline (Zoloft), and paroxetine (Paxil). SSRIs are also effective in the treatment of anxiety disorders and obsessive compulsive disorder.
Norepinephrine appears to be related to arousal and attention. When norepinephrine, also known as noradrenaline, is released into the brain of an animal, the animal becomes more alert and vigilant to its environment. Likewise, norepinephrine may be involved in attention deficit disorder (ADD). Norepinephrine also activates the autonomic nervous system during the fight/flight response, affecting activity in the cardiovascular, muscular, and digestive systems. In fact, beta blockers, a class of medication used to treat high blood pressure, act on the noradrenergic system, the tract of neurons that release norepinephrine. In addition, the noradrenergic system is targeted by a class of antidepressant medication known as tricyclics. This suggests that norepinephrine may also be involved in mood.
These two neurotransmitters have broadly distributed effects; they are found throughout the brain. Glutamate is the main excitatory neurotransmitter in the brain; it activates the nervous system and appears to be involved in learning and memory. New research suggests it is also involved with schizophrenia. In contrast, GABA is an inhibitory neurotransmitter; it calms the nervous system. GABA neurotransmitters are targeted by the benzodiazepines, antianxiety medications that also act as tranquilizers. Examples of benzodiazepines include clonazepam (Klonopin), lorazepam (Ativan), diazepam (Valium), and alprazolam (Xanax). Because of their enjoyably relaxing effect (as well as their addictive potential), these GABA-ergic medications are sometimes used as drugs of abuse.
Many of the brain chemicals known to influence psychological processes are not actually neurotransmitters. These alternative forms of brain chemicals are also known as neuromodulators because they modulate the action of neurotransmitters. Such chemicals may include neuropeptides or neurohormones. Examples include opioids, oxytocin, and vasopressin, which are involved in pain processing and social behaviors, respectively.
Opioids are a form of neuromodulator that serves to dampen our pain response. They are our homemade analgesics, our natural painkillers. One way that opioids work is to inhibit the effect of the neurotransmitter glutamate. As glutamate is an excitatory neurotransmitter, inhibiting glutamate serves to reduce brain activity, in effect to calm down the brain.
A series of studies comparing the montane vole and the prairie vole has given us important insights into the workings of oxytocin and vasopressin. Voles are small rodents found in several different habitats. The montane voles live in fairly isolated mountain burrows, while the prairie voles live in densely populated colonies.
Consequently, the two types of voles demonstrate very different social behaviors. The prairie vole, but not the montane vole, displays monogamous mating patterns and a generally high level of social behavior. Prairie voles also have higher levels of oxytocin and vasopressin. Vasopressin has been directly linked to social behaviors displayed by the male prairie vole but not his montane cousin. Specifically, male prairie voles display both partner preference and mate guarding. This means that they prefer to sit by their mates over other animals and they show aggressive behavior towards any male who comes near their mate.
When vasopressin is blocked in the male prairie vole’s brain, the animal no longer demonstrates partner preference or mate guarding, although sexual and aggressive behavior are otherwise intact. Likewise, when oxytocin is blocked in the female prairie vole’s brain, both partner preference and maternal behavior decline.
Studies of the humble vole have demonstrated how oxytocin and vasopressin affect behavior.
Opiates are essentially the plant form of opioids. Opiates are extracted from the sap of the opium poppy. Synthetic versions of this chemical are also called opiates. When ingested, opiates bind to the opioid receptors in the human brain. Thus the brain responds to opiates the same way that it responds to our own endogenous (internally created) opioids. Several very potent painkillers, such as morphine, heroin, and opium, are made from opiates. Because of the relaxing and euphoric effects of opiates, opiate-based medications are popular drugs of abuse.
Oxytocin and vasopressin are two neuropeptides that perform a wide range of important functions. For example, vasopressin is involved with kidney function. However, they are best known within the field of psychology for their involvement in social behavior. Oxytocin has been linked to childbearing and lactation and both oxytocin and vasopressin have been linked to parenting behavior as well as to sexual orgasm and the emotional connection formed during sexual activity. A famous series of studies on the social behavior of the vole (see Sidebar) suggests that these chemicals are less related to sexual behavior per se than to the formation of emotional bonds.
The majority of psychiatric drugs act by altering one or more neurotransmitter systems. Typically the medications do not contain the actual neurotransmitters but instead contain various chemicals that regulate the action of neurotransmitters. For example, the SSRI antidepressants block the re-absorption of serotonin. This keeps the serotonin molecules in the synapse longer, giving them more time to bind to receptor sites and, therefore, stimulate the firing of the post-synaptic neuron. To visualize this process, imagine someone standing at your door, continuously pressing his or her finger on your doorbell.
Drugs of abuse and legitimate medications act similarly on our brain. In fact, a number of psychiatric medications are sometimes misused as drugs of abuse. Drugs of abuse tend to cause quicker and more intense pleasurable effects than other drugs. It is this “high” that makes these drugs attractive as recreational drugs.
Because of the direct effect of illicit substances on neurotransmitter action, there is often a dramatic alteration of neurotransmitter receptor activity. For example, in response to foreign chemicals that mimic the activity of neurotransmitters, the neurons may decrease their own neurotransmitter production or activity. Receptor sites may die off. This change of the actual structure of the neurons contributes to the addictive process. When the brain makes less neurotransmitter or is less able to \2 process it, craving sets in. Drug tolerance, the need for more and more of the same drug to achieve the same psychological effect, is likewise related to the changes in the structure of the neuron.
The list below shows which drugs affect which brain chemical systems:
More and more research is revealing the powerful ways that learning and experience shape the brain. While this is most true in childhood, the brain continues to be modified by experience throughout adulthood. Although genetics are crucially important in prenatal brain development, much of postnatal brain development is dependent on learning. In fact, every time the brain fires, it is slightly altered. One way that memory takes place is through a process known as long-term potentiation. The neurons fire in a particular pattern and the connections between the neurons involved are strengthened. Experience can change the brain in other ways as well. The receptor sites at a synapse can increase or decrease. New dendrites can branch out as can new axon terminals to form new synapses with nearby neurons. In these varied ways, the brain is very plastic.
One of the oldest debates within psychology (dating back to Plato and Aristotle, at least) involves the importance of nature vs. nurture with regard to human psychology. How much of who we are is innate and how much is learned, a product of our environment? In modern terms, this is a debate between genetics and learning. How much is genetic and how much is learned? The growing body of evidence showing the plasticity of the brain, however, throws a wrench into this debate. If the brain is a function of genetics—and few people would contest the importance of genetics in the development of the brain—then how do we understand the plasticity of the brain?
The brain is both genetically determined and shaped by lived experience. One way to resolve this nature/nurture dilemma is to assume that genetics sets the outer boundaries of brain development. The basic structure of the brain is genetically determined. Even if you bring a child up among horses, the child will not develop the brain of a horse. However, the specific connections among neurons, the density of these connections, and the neuronal connections that die off are in large part determined by learning and environment.
Eric Kandel experimented with sea slugs to show how conditioning behaviors actually change neuron synapses (iStock).
An enormous amount of brain development takes place during childhood, particularly early childhood. Because of the central role of experience on brain development, the nature of environmental input in childhood is critical. Nutrition, education, verbal exposure, language, emotional, and interpersonal experience—all critically impact the formation of connections between neurons, the building of neural networks. This kind of environmental input affects which synaptic connections survive and which die off. In this way, early environment becomes hardwired into the child’s brain. With time, these early environmental influences become increasingly difficult and sometimes even impossible to change.
The brain essentially quadruples in weight from birth to adulthood. Although there is a peak period of synaptogenesis (creation of new synapses) in the first two years of life, synaptogenesis continues rapidly throughout a child’s first decade. All this growth takes fuel. Just as the spurt in the body’s growth during adolescence takes fuel (and A most parents can attest to the enormous amount of food their adolescent sons take in), brain growth also takes fuel. Therefore, if there is inadequate nutrition, brain growth in childhood will be hampered. Moreover, when children are hungry, their concentration is impacted and their ability to learn in school or in any other environment will suffer accordingly. Although it seems to make intuitive sense that the quality of food will also affect brain development during childhood, the research is less robust on the effect of specific diets on learning.
The photograph (on the opposite page) is of a species of sea slug similar to the one that won Eric Kandel the Nobel Prize in 2000. Kandel’s great contribution was to demonstrate in an animal with a very simple nervous system how learning changes the brain. By altering the way he touched the tail of the sea slug, Kandel trained the animal to either amplify or minimize a protective reflex. When he examined the sea slug’s nervous system, he found that the neurons’ synapses had been changed as a result of the conditioning.
The human brain is uniquely designed to learn and process language. This differentiates us from every other animal on earth. Children are born with the capacity to recognize sounds made in any language. With exposure to their native language, however, the neural circuits associated with sounds from their own language will be strengthened and those associated with the sounds of other languages will atrophy. In this way, children become hardwired to speak and understand their native language. Children can learn other languages, of course, but second languages are processed somewhat differently than are native languages. Moreover, as children grow older, it becomes more difficult to learn new languages.
There is now considerable evidence that severe psychological trauma, and particularly childhood trauma, has long-lasting impact on the brain. Since the time of Freud, psychotherapists have been aware of the severe and persistent psychological damage caused by traumatic experiences, but neuroscience is now catching up with the clinicians. Trauma triggers the body’s stress response, mediated through the HPA axis. Over-activation of the HPA axis, as can occur with chronic trauma such as child abuse, dulls its flexibility, sort of like a rubber band that has been stretched out of shape.
This results in people with either overactive or underactive stress responses. Frequently, they have both. When the stress response is underactive, people can be somewhat dissociated, as if they are not processing what is going on around them. When the stress response is overactive, people will be hyper-reactive to any possible threat. Another set of studies has suggested that there is a reduced volume in the hippocampus in people with trauma histories. This may relate to the distortions of memory that often accompanies trauma.
Psychologists have long understood the importance of early interpersonal relationships in child development. Several different branches of psychology—including psychoanalysis, attachment theory, and even cognitive therapy—explain personality development in terms of the profound impact of early interpersonal experience. While our understanding of the neurobiology behind these observations is relatively young, we are slowly gaining a richer understanding of how these potent early experiences shape the brain. For one, there is suggestion that representations of early childhood relationships are processed in the superior medial pre-frontal cortex.
Once these representations have been encoded, it is hard to change them. In this way, our view of relationships become somewhat hardwired. It has also been suggested that the emotional tone of early childhood experiences is preserved in the underlying neural circuitry. More specifically, neural circuits underlying positive emotions are strengthened or weakened depending on the degree of positive emotion experienced in childhood. Further, the circuits related to the stress response, particularly the HPA axis, are strongly influenced by the degree of stress experienced during childhood. Over time, this affects the flexibility and resilience of the body’s stress response. We all know people who are entirely overwhelmed by stress. Their stress reactions are easily triggered and they can only calm down with difficulty. In some cases, this might be the result of abnormal levels of stress during childhood.
