After Chapter 2.1, you will be able to:
In common parlance, we often use the terms “sensation” and “perception” interchangeably, as synonyms. However, in the field of psychology, these two terms have very specific definitions and are commonly contrasted. Sensation more appropriately aligns with transduction, which is the conversion of physical, electromagnetic, auditory, and other information from our internal and external environment to electrical signals in the nervous system. Sensation is performed by receptors in the peripheral nervous system, which forward the stimuli to the central nervous system in the form of action potentials and neurotransmitters. Sensation can therefore be thought of as a raw signal, which is unfiltered and unprocessed until it enters the central nervous system.
Perception, on the other hand, refers to the processing of this information to make sense of its significance. These complex manipulations include both external sensory experience and the internal activities of the brain and spinal cord. Perception thus helps us make sense of the world. The difference between sensation and perception is key to the challenge of creating artificial intelligence: we can easily create sensors for robots to pick up information from their environment, but teaching them how to comprehend and respond to that information is far more challenging.
Sensory processing is a common topic on the MCAT; you should understand not only the definitions of these terms, but be able to apply the concepts herein to your own day-to-day sensory experiences.
Sensory receptors are neurons that respond to stimuli and trigger electrical signals. Some of these stimuli originate outside of the body and are referred to as distal stimuli prior to reaching the body, as they are part of the "outside" world. For example, a campfire is a distal stimulus. The photons that reach the observer's rods and cones, as well as the heat the observer feels, are proximal stimuli. Proximal stimuli directly interact with and affect the sensory receptors, and inform the observer about the presence of distal stimuli. Sensory receptors may encode multiple aspects of a stimulus. For example, photoreceptors respond to light and can encode not only the brightness of the light, but also its color and shape. The relationship between the physical nature of stimuli and the sensations and perceptions they evoke is studied in the field of psychophysics.
In order to inform the central nervous system, the signals from these stimuli must pass through specific sensory pathways. In each case, different types of receptors—generally nerve endings or specific sensory cells—receive the stimulus and transmit the data to the central nervous system through sensory ganglia. Ganglia are collections of neuron cell bodies found outside the central nervous system. Once transduction occurs, the electrochemical energy is sent along neural pathways to various projection areas in the brain, which further analyze the sensory input.
Sensory receptors differ from one sense to another. There are over a dozen recognized sensory receptors, but the MCAT is unlikely to test even half of those. The most heavily tested receptors include:
Perception, like sensation, is closely tied to the biology and physiology of interpreting the world around us. However, unlike sensation, perception is inextricably linked to experience and both internal and external biases. Sensations are relayed to the brain, which perceives the significance of the stimulus; for example, determining whether something is hot or cold. The same sensation can produce radically different perceptions in different people, and because these variations must be explained by central nervous system activity, perception is considered a part of psychology.
A good example of the psychological element of perception is a threshold—the minimum amount of a stimulus that renders a difference in perception. For example, the temperature may noticeably change from warm to cool when the sun sets, but subtle fluctuations in temperature throughout the day are generally unnoticeable because they are below the difference threshold. If sound increases 10 dB (ten times the sound intensity), this is usually very obvious; if it increased only 0.1 dB, it may be too small to detect. There are three main types of thresholds: the absolute threshold, the threshold of conscious perception, and the difference threshold.
On the MCAT, thresholds will frequently be used in conjunction with subjects in studies. Be on the lookout for experimental design questions when thresholds appear in a passage.
The absolute threshold is the minimum of stimulus energy that is needed to activate a sensory system. It is therefore a threshold in sensation, not in perception. Sounds of extremely low intensity may still cause slight vibrations in the sensory receptors of the inner ear, but these may not be significant enough to be converted to an action potential through transduction. While most human sensory systems are extremely sensitive, all systems also have this minimum sensory level below which the stimulus will not be transduced to the central nervous system. For example, the absolute threshold for sweet taste is a teaspoon of sucrose dissolved in two gallons of water. On a clear, dark night with no other lights shining, the eye can just detect the light of one candle burning thirty miles away. When we are talking about an absolute threshold, we’re talking about how bright, loud, or intense a stimulus must be before it is sensed.
The absolute threshold is the minimum intensity at which a stimulus will be transduced (converted into action potentials).
You already know one of the absolute thresholds from the discussion of sound in Chapter
7 of MCAT Physics and Math Review. Remember that
in the equation for sound level is the absolute threshold of normal human hearing.
It is possible for sensory systems to send signals to the central nervous system without a person perceiving them. This may be because the stimulus is too subtle to demand our attention, or may last for too brief of a duration for the brain to fully process the information. Thresholds can also be called limina. Thus, subliminal perception often refers to the perception of a stimulus below a given threshold. Usually, this term refers to the threshold of conscious perception. Note the difference between the absolute threshold and the threshold for conscious perception: a stimulus below the absolute threshold will not be transduced, and thus never reaches the central nervous system. A stimulus below the threshold of conscious perception arrives at the central nervous system, but does not reach the higher-order brain regions that control attention and consciousness. Contrary to common thinking, there is actually little practical value to using subliminal perception to sell products.
One common way to analyze this limit in human perceptive ability is to use discrimination testing, also referred to as psychophysical discrimination testing. A participant is presented with a stimulus that is varied slightly, and then is asked to identify whether there is a difference in the second stimulus. The difference between the current stimulus and the original is increased until the participant reports
noticing a change. For example, a participant may be shown a shade of blue
and asked to indicate when the shade has actually changed as the experimenter displays shades that are increasingly different from the reference blue.
The difference threshold or just-noticeable difference (jnd) refers to the minimum difference in magnitude between two stimuli before one can perceive this difference. For example, most individuals without formal ear training find it impossible to discriminate between two sound waves at 440 Hz and 441 Hz. While they are different frequencies, the perception of the tones is that they are the same. In this range of sound frequencies, the just-noticeable difference is about 3 Hz; thus, most individuals just begin to hear a difference between sound waves at 440 Hz and 443 Hz.
While the jnd was given as 3 Hz above, it is far more important to focus on the ratio between the change in stimulus and its original value, rather than the actual difference between the frequencies. Thus, the jnd for sound frequency is more accurately quantified as 0.68 percent (3 Hz ÷ 440 Hz). This relationship has been formalized in Weber’s law, which states that there is a constant ratio between the change in stimulus magnitude needed to produce a jnd and the magnitude of the original stimulus. Thus, for higher-magnitude stimuli, the actual difference must be larger to produce a jnd. If we’ve calculated the jnd as 0.68 percent for sound frequency, then an individual would be expected to be able to discriminate between sounds at 1000 Hz and 1006.8 Hz (6.8 Hz = 0.68% of 1000 Hz), but not between 1000 Hz and 1003 Hz (3 Hz = 0.3% of 1000 Hz). Weber’s law appears to be accurate for all sensory modalities, except at the extremely high and low ends of each range.
When the MCAT brings up Weber’s law, questions will usually give a numerical relationship and then ask for it to be applied; typically, it simply amounts to applying a ratio.
Perception of stimuli can also be affected by nonsensory factors, such as experiences (memory), motives, and expectations. This concept is termed signal detection theory, which focuses on the changes in our perception of the same stimuli depending on both internal (psychological) and external (environmental) context. For example, how loud would someone need to yell your name in a crowd to get your attention? Part of the answer comes from psychology: If you heard something that sounds vaguely like your name, would you likely acknowledge it or not? The answer is not merely a yes or no, but would depend on the size of the crowd; your expectation of being called; social factors, like the makeup of the crowd and your comfort with the other individuals; and personality: highly sociable, extroverted individuals tend to hear their names more easily than quieter, introverted individuals.
Signal detection theory also allows us to explore response bias, which refers to the tendency of subjects to systematically respond to a stimulus in a particular way due to nonsensory factors. A basic signal detection experiment consists of many trials; during each trial, a stimulus (signal) may or may not be presented. Trials in which the signal is presented are called catch trials, whereas those in which the signal is not presented are called noise trials. After each trial, the subject is asked to indicate whether or not a signal was given. There are therefore four possible outcomes for each trial, as illustrated in Figure 2.1: hits, in which the subject correctly perceives the signal; misses, in which the subject fails to perceive a given signal; false alarms, in which the subject seems to perceive a signal when none was given; and correct negatives, in which the subject correctly identifies that no signal was given. A significant proportion of misses or false alarms gives an indication of response bias in the subject.
On the surface, signal detection experiments would appear to be easy tasks—shouldn’t an individual easily be able to tell if he or she perceived something or not? However, consider the thought processes that occur when you’re quietly studying in the library with your phone on silent and you suddenly think you may have heard a buzz. Is my phone ringing? you wonder. You freeze in place and wait for another buzz; even if it doesn’t come, you may still be so convinced you heard a signal that you still check your phone. Perception is not a passive matter!
Our detection of a stimulus can change over time through adaptation. Adaptation can have both a physiological (sensory) component and a psychological (perceptual) component. For example, the pupils of the eyes will dilate in the dark and constrict in the light to make our vision more similar in different environments as part of physiological adaptation. In loud environments, we contract small muscles in the middle ear to reduce the amount of vibration of the ossicles, reducing sound intensity. We also adapt to somatosensory stimuli; cold water no longer seems so cold once our bodies “get used to it.” Once we’re dressed, we stop feeling the clothes on our bodies until we have a reason to think about them. Adaptation is one way the mind and body try to focus attention on only the most relevant stimuli, which are usually changes in the environment around us.