Researchers at the Ruhr-University Bochum in Germany and the University of Otago in Dunedin, New Zealand, have identified single neurons in the pigeon forebrain that play a role in controlling impulsive decisions -- in the pigeons' case, the preference for a small, immediate reward over a large, delayed reward.
In contrast to assumptions made by many economic theories, decisions by humans and other animals are often not rational, and their actions are not always exclusively directed toward the maximization of gain. A prominent example of such sub-optimal choice behavior is impulsive decision-making, in which case an immediate but disadvantageous reward may be preferred over a delayed but more advantageous reward.
In their new work, Tobias Kalenscher and colleagues trained pigeons to choose between a small and a large reward. At the beginning of a session, both rewards were delivered immediately after the choice was made, but the delivery of a large reward was increasingly delayed as the session progressed. At first, pigeons preferred the large reward, but once the preceding delay exceeded a critical threshold of duration, they switched to preferring the small but fast reward. Reward preference in this task therefore depended on both reward amount and time-to-reward.
Using single-cell electrophysiological recordings from freely moving pigeons, the authors provided evidence that single neurons in the avian forebrain showed increased activity in anticipation of an upcoming reward. This reward-related activation was greater when the pigeons expected a large reward than when they expected a small one. But most importantly, the degree of activation decreased as the delay between the choice and the large reward increased, until this degree of activation dropped below that of activation related to the expectance of a small reward. This drop in neural activation coincided with the pigeons' preference shift from the large, delayed reward to the small, fast reward. The neurons thus integrated reward amount and time-to-reward, and, as predicted by impulsive choice theory, the relative activation level correlated with the pigeons' reward preference.
These findings not only shed light on impulsive decision-making, but they may also aid the understanding of neuropathologies, such as drug addiction, gambling, frontal lobe syndrome, and attention-deficit disorders, that are characterized by a decreased ability to wait for a large reward.
The researchers include Tobias Kalenscher, Sabine Windmann, Bettina Diekamp, and Onur Güntürkün of the Institute of Cognitive Neuroscience at Ruhr-University Bochum; and Jonas Rose and Michael Colombo of the Goddard Laboratories at University of Otago. This study was supported by a Royal Society of New Zealand Marsden grant and grants from the Deutsche Forschungsgemeinschaft through the priority program Executive Functions and the G.-A. Lienert Stiftung.
Publishing in Current Biology, Volume 15, Number 7, April 12, 2005, pages 594-602. http://www.current-biology.com
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