Single units in the pigeon brain integrate reward amount and time-to-reward inan impulsive choice task

Background: Animals prefer small over large rewards when the delays preceding large rewards exceed an individual tolerance limit. Such impulsive choice behavior occurs even in situations in which alternative strategies would yield more optimal outcomes. Behavioral research has shown that an animal's choice is guided by the alternative rewards' subjective values, which are a function of reward amount and time-to-reward. Despite increasing knowledge about the pharmacology and anatomy underlying impulsivity, it is still unknown how the brain combines reward amount and time-to-reward information to represent subjective reward value. Results: We trained pigeons to choose between small, immediate rewards and large rewards delivered after gradually increasing delays. Single-cell recordingsin the avian Nidopallium caudolaterale, the presumed functional analog of themammalian prefrontal cortex, revealed that neural delay activation decreased with increasing delay length but also covaried with the expected reward amount. This integrated neural response was modulated by reward amount and delay, aspredicted by a hyperbolical equation, of subjective reward value derived frombehavioral studies. Furthermore, the neural activation pattern reflected the current reward preference and the time point of the shift from large to small rewards. Conclusions: The reported activity was modulated by the temporal devaluation of the anticipated reward in addition to reward amount. Our findings contributeto the understanding of neuropathologies such as drug addiction, pathologicalgambling, frontal lobe syndrome, and attention-deficit disorders, which are characterized by inappropriate temporal discounting and increased impulsiveness.