The personality of railroad foreman Phineas Gage took a turn for the worse when an accidental explosion shot a tamping iron through the frontal lobe of his brain. Once a model of responsibility, Gage became so rude and impulsive that he never worked as a foreman again. Now researchers report that damage to a brain region called the orbital prefrontal cortex (OPFC) brings out impulsive behavior in rats as well, causing impulsive behavior. The study suggests the OPFC contributes to decision-making by evaluating the trade-off between small, immediate rewards and large, delayed rewards.
Previous studies in humans and a variety of animals have shown that the OPFC is part of a network of brain regions that respond to food, sex, and other rewards. In brain imaging studies, the OPFC is highly active when people deliberate between actions with uncertain risks and rewards, and clinical reports describe patients, such as Gage, whose inability to consider long-term consequences can lead to tragic outcomes. But no one knew exactly what role the OPFC played in decision-making.
To address this question, Sirous Mobini and colleagues at the University of Nottingham, U.K., used a toxin to destroy the OPFC in rats. They then offered the rats a choice between two levers: one that delivered a food pellet immediately and another that delivered two pellets after a delay ranging from 1 to 30 seconds. For most delays, the lesioned rats were about twice as likely as their normal counterparts to choose the immediate reward over the delayed reward. In a second experiment, the researchers tested whether the lesioned rats preferred certain, small rewards (a single pellet delivered every time the rat pressed the lever) or uncertain, large rewards (two pellets delivered with a probability ranging from 8% to 88%). In this test, the lesioned rats were twice as likely as unlesioned rats to choose the certain, small rewards. The team concludes that the lesioned rats' eagerness to take small, certain, immediate rewards over large, uncertain, delayed rewards indicates a tendency to make impulsive choices.
The study "makes an important contribution to our knowledge of the pathogenesis of impulsive choice," says Rudolf Cardinal of the University of Cambridge. But he adds that it remains to be seen how the OPFC interacts with other areas of the brain, such as the amygdala and nucleus accumbens, that also play important roles in motivation and emotion.