To review this chapter, you would do well to focus on major themes. Even details are easier to remember if tied to larger themes, or arguments, than if seen as isolated facts. Here are two themes that ran through the chapter and may help you to organize your review:
1. The mechanisms of transduction and coding All sensory systems respond to physical stimuli by producing action potentials (the process of transduction), and all sensory systems do this in such a way as to preserve useful information about the stimulus (coding). For each sense discussed in this chapter—smell, taste, pain, and hearing—you might think about each of the following questions pertaining to transduction and coding: (a) To what type of physical stimulus does this sense respond, and what is the range of stimuli to which it responds? (b) How is the sensory organ designed for receiving (and possibly concentrating or amplifying) the stimulus? (c) What are the receptors for the stimulus, and how do they respond in such a way as to generate action potentials in sensory neurons? (d) How does the transduction process code the different qualities of stimuli to which the sensory system responds? (e) How do neural mechanisms in the central nervous system alter or reorganize the input, and for what purposes? The chapter does not answer all these questions (especially not the last) completely for each sense, but the questions provide a good framework for organizing and thinking about the information that is provided.
2. The survival functions of sensory processes Our sensory systems, like all the basic mechanisms underlying our behavior, evolved through natural selection because they promoted our ancestors’ survival and reproduction. They are not unbiased recorders of physical energies but biological tools designed to pick out from the sea of energy around us the information that is potentially most useful. We are sensitive to some kinds of energies and not others, and, within the kinds to which we are sensitive, our senses extract and enhance some relationships and not others.
Here are some examples, described in the chapter, of how sensory processes can be understood in terms of their survival advantages: (a) Sensory adaptation (the decline in sensitivity to prolonged, constant stimuli) helps us to ignore stimuli that remain unchanged and to notice changes. (b) Attraction to the smell of individuals who differ in MHC may help to create genetic diversity and to avoid incest. (c) Smell and taste work together to produce flavors that are experienced as pleasant or unpleasant, in ways that are generally consistent with what is good for us or bad for us (or for our evolutionary ancestors) to eat. The association of bitter taste with poisons is an example. (d) Pain is a sensory system for warning us when our actions are damaging our tissues and for motivating us to avoid such actions. Evolved mechanisms increase pain sensitivity at times of illness, when it is best to rest. In contrast, they decrease pain sensitivity at times of threat, when strenuous action without favoring one’s wounds may be necessary. (e) The phonemic-restoration illusion helps us to hear speech in a continuous, meaningful flow and to ignore extraneous noises and interruptions.