Cognitive effort increases the intensity of rewards

An important body of literature suggests that exerting intense cognitive effort causes mental fatigue and can lead to unhealthy behaviors such as indulging in high-calorie food and taking drugs. Whereas this effect has been mostly explained in terms of weakening cognitive control, cognitive effort may also bias behavioral choices by amplifying the hedonic and emotional impact of rewards. We report parallel findings with animals and humans supporting this hypothesis. In rats, exerting cognitive effort immediately before access to cocaine self-administration significantly increased drug intake. In addition, exerting cognitive effort increased the psychostimulant effect of cocaine. The effects of cognitive effort on addiction-related behaviors were eliminated and even reversed when animals could rest in their home-cage for 2-4h before access to cocaine self-administration. Among humans, we found that expending cognitive effort increased consumption of tasty (but unhealthy) food by increasing the hedonic enjoyment of consuming the food. In addition, the effects were specific for emotionally relevant stimuli (i.e., food rewards) and did not generalize to judgment about neutral objects. Altogether these data suggest that intense cognitive effort can increase the perceived intensity of rewards and lead to their overconsumption. This effect may contribute to bad decision making induced by excessive cognitive effort and make people more vulnerable to indulge in unhealthy behaviors such as use of addictive drugs. Significance StatementPeople dieting or recovering from addiction frequently report that relapses occur during periods of stress and mental fatigue. Multiple processes may contribute to this, including beliefs about the stress-reducing effects of drugs, beliefs about ones inability to continue resisting, and lack of energy needed to sustain resistance. Here, we suggest an additional possible process: during a state of mental fatigue, rewards become all the more satisfying, thereby also increasing subsequent desire for them. We report two lines of experiments, one with rats and one with human participants, to show that intense cognitive effort leads to increased enjoyment of rewards and hence increased consumption.