After Chapter 1.4, you will be able to:
The forebrain is the most “modern” portion of the brain, and—in humans—forms the largest portion of the brain by weight and volume. The forebrain contains regions derived from the diencephalon, such as the thalamus, hypothalamus, posterior pituitary, and pineal gland; it also includes derivatives of the telencephalon, such as the cerebral cortex, basal ganglia, and limbic system.
The thalamus is a structure within the forebrain that serves as an important relay station for incoming sensory information, including all senses except for smell. After receiving incoming sensory impulses, the thalamus sorts and transmits them to the appropriate areas of the cerebral cortex. The thalamus is therefore a sensory “way station.”
The hypothalamus, subdivided into the lateral hypothalamus, ventromedial hypothalamus, and anterior hypothalamus, serves homeostatic functions, and is a key player in emotional experiences during high arousal states, aggressive behavior, and sexual behavior. The hypothalamus also helps control some endocrine functions, as well as the autonomic nervous system. The hypothalamus serves many homeostatic functions, which are self-regulatory processes that maintain a stable balance within the body. Receptors in the hypothalamus regulate metabolism, temperature, and water balance. When any of these functions are out of balance, the hypothalamus detects the problem and signals the body to correct the imbalance; for example, osmoreceptors in the hypothalamus may trigger the release of antidiuretic hormone to increase water reabsorption as part of fluid balance. The hypothalamus is also the primary regulator of the autonomic nervous system and is important in drive behaviors: hunger, thirst, and sexual behavior.
Functions of the Hypothalamus—The Four Fs:
The lateral hypothalamus (LH) is referred to as the hunger center because it has special receptors thought to detect when the body needs more food or fluids. In other words, the LH triggers eating and drinking. When this part of the hypothalamus is destroyed in lab rats, they refuse to eat and drink and would starve to death if not force-fed through tubes.
When the Lateral Hypothalamus (LH) is destroyed, one Lacks Hunger.
The ventromedial hypothalamus (VMH) is identified as the “satiety center,” and provides signals to stop eating. Brain lesions to this area usually lead to obesity.
When the VentroMedial Hypothalamus (VMH) is destroyed, one is Very Much Hungry.
The anterior hypothalamus controls sexual behavior. When the anterior hypothalamus is stimulated, lab animals will mount just about anything (including inanimate objects). In many species, damage to the anterior hypothalamus leads to permanent inhibition of sexual activity. The anterior hypothalamus also regulates sleep and body temperature.
When the Anterior hypothalamus is destroyed, one is Asexual.
In the early 1920s, researchers first discovered the hypothalamus’s role in rage and fighting through classic experiments conducted with cats. When researchers removed the cat’s cerebral cortex but left the hypothalamus in place, the cat displayed a pattern of pseudoaggressive behavior that was called “sham rage”—lashing of the tail, arching of the back, clawing, and biting—except that rage was spontaneous or triggered by the mildest touch. The researchers concluded that the cortex typically inhibits this type of response. When the researchers removed the cat’s cortex and hypothalamus together, the outcome was very different. The cat no longer showed any signs of sham rage, and much rougher stimulation was required before the cats showed any defensive behavior at all.
The diencephalon also differentiates to form the posterior pituitary gland, pineal gland, and connecting pathways to other brain regions. The posterior pituitary is comprised of axonal projections from the hypothalamus and is the site of release for the hypothalamic hormones antidiuretic hormone (ADH, also called vasopressin) and oxytocin. The functions of these hormones are described in Chapter 5 of MCAT Biology Review. The pineal gland is the key player in several biological rhythms. Most notably, the pineal gland secretes a hormone called melatonin, which regulates circadian rhythms. The pineal gland receives direct signals from the retina for coordination with sunlight.
In the middle of the brain are a group of structures known as the basal ganglia. The basal ganglia coordinate muscle movement as they receive information from the cortex and relay this information (via the extrapyramidal motor system) to the brain and the spinal cord. The extrapyramidal system gathers information about body position and carries this information to the central nervous system, but does not function directly through motor neurons. Essentially, the basal ganglia help make our movements smooth and our posture steady. Parkinson’s disease is one chronic illness associated with destruction of portions of the basal ganglia. It is characterized by jerky movements and uncontrolled resting tremors. The basal ganglia may also play a role in schizophrenia and obsessive–compulsive disorder.
The limbic system, diagrammed in Figure 1.8, comprises a group of interconnected structures looping around the central portion of the brain and is primarily associated with emotion and memory. Its primary components include the septal nuclei, amygdala, and hippocampus. In Chapter 5 of MCAT Behavioral Sciences Review, we will also explore the roles of the thalamus, hypothalamus, and cortex in the limbic system.
The septal nuclei contain one of the primary pleasure centers in the brain. Mild stimulation of the septal nuclei is reported to be intensely pleasurable; there is an association between these nuclei and addictive behavior.
James Olds and Peter Milner discovered the association between the septal nuclei and addictive behavior in the 1950s. They demonstrated that when rats could stimulate their septal regions at will by pushing a lever, they found it so pleasurable that they preferred it to eating or any other activities, even after going 24 hours without food or sleep.
The amygdala is a structure that plays an important role in defensive and aggressive behaviors, including fear and rage. Researchers base this observation on studies of animals and humans with brain lesions. When the amygdala is damaged, aggression and fear reactions are markedly reduced. Lesions to the amygdala result in docility and hypersexual states.
Heinrich Klüver and Paul Bucy performed studies that linked the amygdala with defensive and aggressive behavior in monkeys. When the amygdala of the Rhesus monkey was removed, they noted increased sexual behavior, decreased fear responses, and hyperorality, or the examination of inanimate or animate objects by mouth. These symptoms are now referred to as Klüver–Bucy syndrome.
The hippocampus plays a vital role in learning and memory processes; specifically, the hippocampus helps consolidate information to form long-term memories, and can redistribute remote memories to the cerebral cortex. The hippocampus communicates with other portions of the limbic system through a long projection called the fornix. Researchers originally discovered the connection between memory and the hippocampus through a famous patient named Henry Molaison (known as H.M. in the scientific literature until his death in 2008). Parts of H.M.’s temporal lobes—including the amygdala and hippocampus—were removed in an effort to control epileptic seizures. After surgery, H.M.’s intelligence was largely intact but he suffered a drastic and irreversible loss of memory for any new information. This kind of memory loss is called anterograde amnesia and is characterized by not being able to establish new long-term memories, whereas memory for events that occurred before brain injury is usually intact. The opposite kind of memory loss, retrograde amnesia, refers to memory loss of events that transpired before brain injury.
Learning and memory are discussed thoroughly in Chapter 3 of MCAT Behavioral Sciences Review.
Lobes of the brain: F-POT
The outer surface of the brain is called the cerebral cortex. The cortex is sometimes called the neocortex, a reminder that the cortex is the most recent brain region to evolve. Rather than having a smooth surface, the cortex has numerous bumps and folds called gyri and sulci, respectively. The convoluted structure of the brain provides increased surface area. The cerebrum is divided into two halves, called cerebral hemispheres. The surface of the cortex is divided into four lobes—the frontal lobe, parietal lobe, occipital lobe, and temporal lobe. These lobes are identified in Figure 1.9, which shows a side view of the left cerebral hemisphere.
The frontal lobe is comprised of two basic regions: the prefrontalcortex and the motor cortex. The prefrontal cortex manages executive function by supervising and directing the operations of other brain regions. This region supervises processes associated with perception, memory, emotion, impulse control, and long-term planning. In memory, for instance, the role of the prefrontal cortex is not to store any memory traces, but rather to remind the individual that he or she has something to remember at all. To regulate attention and alertness, the prefrontal cortex communicates with the reticular formation in the brainstem, telling an individual either to wake up or relax, depending on the situation.
Because it integrates information from different cortical regions, the prefrontal cortex is a good example of an association area: an area that integrates input from diverse brain regions. For example, multiple inputs may be necessary to solve a complex puzzle, to plan ahead for the future, or to reach a difficult decision. Association areas are generally contrasted with projection areas, which perform more rudimentary or simple perceptual and motor tasks. Examples of projection areas include the visual cortex, which receives visual input from the retina, and the motor cortex, which sends out motor commands to the muscles.
Damage to the prefrontal cortex impairs its overall supervisory functions. A person with a prefrontal lesion may be more impulsive and generally less in control of his or her behavior, or depressed. It is not unusual, for instance, for someone with a prefrontal lesion to make vulgar and inappropriate sexual remarks, or to be apathetic.
The primary motor cortex is located on the precentral gyrus (just in front of the central sulcus that divides the frontal and parietal lobes), and initiates voluntary motor movements by sending neural impulses down the spinal cord toward the muscles. As such, it is considered a projection area in the brain. The neurons in the motor cortex are arranged systematically according to the parts of the body to which they are connected. This organizational pattern can be visualized through the motor homunculus, as shown in Figure 1.10. Because certain sets of muscles require finer motor control than others, they take up additional space in the cortex relative to their size in the body.
A third important part of the frontal lobe is Broca’s area, which is vitally important for speech production. Broca’s area is usually found in only one hemisphere, the so-called “dominant” hemisphere; for most people—both right- and left-handed—this is the left hemisphere.
The parietal lobe is located to the rear of the frontal lobe. The somatosensory cortex is located on the postcentral gyrus (just behind the central sulcus) and is involved in somatosensory information processing. This projection area is the destination for all incoming sensory signals for touch, pressure, temperature, and pain. Despite certain differences, the somatosensory cortex and motor cortex are very closely related. In fact, they are so interrelated they sometimes are described as a single unit: the sensorimotor cortex. The somatosensory homunculus is shown in Figure 1.11. The central region of the parietal lobe is associated with spatial processing and manipulation. This region makes it possible to orient oneself and other objects in three-dimensional space, to do spatial manipulation of objects, and to apply spatial orientation skills such as those required for map-reading.
The occipital lobes, at the very rear of the brain, contain the visual cortex, which is sometimes called the striate cortex. Striate means furrowed or striped, which is how the visual cortex appears when examined under a microscope. The visual cortex is one of the best-understood brain regions, owing to the large amount of research that has been done on visual processing. Sensation and perception of visual information is discussed thoroughly in Chapter 2 of MCAT Behavioral Sciences Review. Areas in the occipital lobe have also been implicated in learning and motor control.
The temporal lobes are associated with a number of functions. The auditory cortex and Wernicke’s area are located in the temporal lobe. The auditory cortex is the primary site of most sound processing, including speech, music, and other sound information. Wernicke’s area is associated with language reception and comprehension. The temporal lobe also functions in memory processing, emotion, and language. Studies have shown that electrical stimulation of the temporal lobe can evoke memories for past events. This makes sense because the hippocampus is located deep inside the temporal lobe. It is important to note that the lobes, although having seemingly independent functions, are not truly independent of one another. Often, a sensory modality may be represented in more than one area.
In most cases, one side of the brain communicates with the opposite side of the body. In such cases, we say a cerebral hemisphere communicates contralaterally. For example, the motor neurons on the left side of the brain activate movements on the right side of the body. In other cases (for instance, hearing), cerebral hemispheres communicate with the same side of the body. In such cases, the hemispheres communicate ipsilaterally.
We distinguish between dominant and nondominant hemispheres. The dominant hemisphere is typically defined as the one that is more heavily stimulated during language reception and production. In the past, hand dominance was used as a proxy for hemispheric dominance; that is, right-handed individuals were assumed to have left-dominant brains and left-handed individuals were assumed to have right-dominant brains (because the brain communicates contralaterally with the hand). However, this correlation has not held up under scrutiny; 95 percent of right-handed individuals are indeed left brain dominant, but only 18 percent of left-handed individuals are right brain dominant.
The dominant hemisphere (usually the left) is primarily analytic in function, making it well-suited for managing details. For instance, language, logic, and math skills are all located in the dominant hemisphere. Again, language production (Broca’s area) and language comprehension (Wernicke’s area) are primarily driven by the dominant hemisphere.
The corpus callosum connects and shares information between the two cerebral hemispheres; its function was discovered in epileptic patients whose corpora callosa were severed in a last-ditch effort to limit their convulsive seizures. In these “split-brain” patients, in whom the corpus callosum has been severed, each hemisphere has its own function and specialization that is no longer accessible by the other. Thus, an object felt only by the left hand (which projects to the right hemisphere) could not be named (because language function is usually in the left hemisphere).
The nondominant hemisphere (usually the right) is associated with intuition, creativity, music cognition, and spatial processing. The nondominant hemisphere simultaneously processes the pieces of a stimulus and assembles them into a holistic image. The nondominant hemisphere serves a less prominent role in language. It is more sensitive to the emotional tone of spoken language, and permits us to recognize others’ moods based on visual and auditory cues, which adds to communication. The dominant hemisphere thus screens incoming language to analyze its content, and the nondominant hemisphere interprets it according to its emotional tone. The roles of the dominant and nondominant hemispheres are summarized in Table 1.2; remember that the left hemisphere is the dominant hemisphere in most individuals, regardless of handedness.
| Function | Dominant Hemisphere | Nondominant Hemisphere |
| Visual system | Letters, words | Faces |
| Auditory system | Language-related sounds | Music |
| Language | Speech, reading, writing, arithmetic | Emotions |
| Movement | Complex voluntary movement | – |
| Spatial processes | – | Geometry, sense of direction |