As you review the mechanisms of reward, hunger, sex, sleep, and emotions described in this chapter, you might find the following two points useful in organizing some of your thoughts.
1. The attempt to relate behavioral states to physiological states Hunger, sexual drive, sleepiness, and emotions are examples oihehavioral states— sustained but reversible changes in the way a person or animal behaves in a given environment. In this chapter, you read many examples of research aimed at finding relationships between behavioral states and physiological states. The general goals of such work are to identify the changes in the brain that provide the foundations for behavioral states and to identify the various neural and chemical inputs that control those changes. These goals are both fascinating and difficult to achieve because the machine being studied is so extraordinarily complex (not to mention alive and mortal). As you think about each type of behavioral state discussed in this chapter, ask yourself: What changes in the brain correspond to this state, and how are these changes regulated through means that tend to promote the individual’s survival?
2. Two categories of methods in the physiological study of states Like the rest of the book, this chapter is about methods and evidence, not just findings. As you review the specific methods described in the chapter, you will find that they fall into two broad categories. One category involves intervention in ongoing physiological processes to see what happens to the behaviorally measured state. What happens to a drive or emotion if a particular part of the brain is destroyed or stimulated, or if the activity of a particular hormone or neurotransmitter is increased or decreased in some way? As you review the chapter, notice how often such methods were mentioned.
Intervention is a powerful way to identify causal relationships between physiology and behavior. But most intervention procedures are harmful or at least risky to the subject, so they are used primarily in studies of animals other than humans. The intervention approach is approximated, however, in studies of people whose natural physiology has been disrupted through accident or disease. Several studies of that sort were described in the chapter.
The other category involves the measurement of physiological processes and the correlation of the measures with changes in behavioral state. What natural changes in brain activity (measured by EEG or by brain imaging methods) or in hormonal secretions, heart rate, or other physiological variables accompany specific changes in behavioral state? Most measurement procedures are safe and can be used with humans. Notice, as you review the chapter, how often this correlational method was used in the human studies described. This method helps identify reliable relationships but does not, by itself, inform us about cause and effect. The observation that brain waves slow down during sleep, or that skin temperature rises during anger, or that the amygdala is active during fear, tells us about correlations but not necessarily about cause and effect. To test cause–effect hypotheses, researchers seek to manipulate the physiological system and measure the effect on behavior. That is why intervention experiments with nonhuman animals often go hand-in-hand with correlational studies in humans to get at the full story.