As a graduate student, I ran across a motivational mystery involving some odd rat behavior: a series of articles by William Pare at the Maryland Veterans Hospital on the topic of activity-stress (Pare, 1975, 1976, 1977). I’ve been conducting research on this topic and considering the underlying motivational factors ever since.

The activity-stress research condition involved housing rats in cages, restricting their feeding to one hour per day, and giving them unlimited access to activity wheels (Brain Scene Investigation Box  Figure 8.1). The rats could eat as much as they wanted for that one hour. For the remainder of the time, they could either rest in their cage or run in the wheel. That wasn’t so bad…was it? Pare’s initial studies indicated that the health consequences for these animals were severe. Rats are notorious for their tendency to run one to 15 miles per day when housed in cages with activity wheels (Mather, 1981), but in this case, food was restricted. Didn’t it make sense to conserve energy when resources were limited? Apparently not.

When I conducted my own version of this experiment as a graduate student, I observed rats run up to seven miles per day. Although rats are typically active at night, the rats in this experiment became increasingly active during the light hours. Additionally, when food was eventually placed in their cages, instead of jumping on top of the food bowl as expected, the rats typically hopped in their activity wheels and ran before finally orienting toward the food bowls. If an experimenter didn’t intervene, these animals would literally run themselves to death, exhibiting stomach ulcers (stress ulcers), hypertrophied or swollen adrenal glands, and shriveled thymus glands that are involved in immune functions. Similarities to anorexia nervosa became obvious—restricted eating, increased activity, and significant weight loss.

The altered feeding schedule had stressed the rats exposed to this paradigm. Many research studies have explored these stress effects in an attempt to determine which variables (e.g., temperature, type of diet, duration of food exposure) made the animals more susceptible to the ulcers and other maladaptive health effects (reviewed in Lambert, 1993). More relevant to this chapter, however, is the question of what specific biological mechanisms fuel such bizarre, seemingly self-destructive behavior. What factors would motivate an animal to do this? And, if identified, would such factors provide clues to maladaptive compulsive behavior observed in humans?

Behind the Scenes

A plausible explanation is that running somehow becomes more reinforcing than eating. In one study, when dopamine was blocked by an antagonist (at a dose that didn’t interfere with movement), the rats’ excessive amounts of running subsided (Lambert & Porter, 1992). As you learned in Chapter 4, dopamine fuels a reward circuit in the brain that involves the nucleus accumbens. In another study, rats kept in cooler ambient temperatures exhibited greater amounts of running than did rats kept in warmer temperature climates (Lambert & Hanrahan, 1990). Apparently, the more uncomfortable the laboratory conditions became, the more likely it was that these activity-stress rats would run.

Was this a case of extreme maladaptation? Probably not. In a more natural context, this would probably be a life-saving response. If food in the wild became scarce, movement to another area would likely lead the animal to locate more plentiful food. A caged rat has the same biological urge to run, but in this case, the behavior leads to its demise rather than to its rescue from starvation. Additionally, the enhanced response is consistent with a classic learning effect known as the reinforcement-omission effect, in which responses strengthen when an animal isn’t rewarded, perhaps due to frustration (Amsel & Roussel, 1952; Stout, Boughner, & Papini, 2003). For example, if a rat expects a reward after running down an alley and doesn’t receive the reward, the next time, it runs faster!

As this chapter indicates, the brain’s reward system is the product of a complex circuit of brain areas and neurochemicals. Altered environmental conditions and availability of resources can recalibrate these responses. As seen in the activity-stress paradigm, these recalibrations—run even more to receive food– may be highly detrimental.

While it’s a stretch to move from rats to humans, the activity-stress paradigm provides clues about the impact of increased activity and decreased food consumption on the brain and body. Research with patients who have anorexia nervosa suggests that, like the rats, these individuals are also searching for something to make them feel less anxious and more secure. Further, dopamine has been implicated in the excessive activity and dieting that are characteristic of this disorder (Kontis & Theochari, 2012). In this chapter, we’ll examine influential factors in both eating disorders and many behaviors we engage in every day.