British Psychological Society (BPS) Annual Conference (15-17 April 2004; London, UK).

Neurobiology of delayed reinforcement

Rudolf N. Cardinal

Department of Experimental Psychology, University of Cambridge
Downing Street, Cambridge CB2 3EB, UK.

Delayed reinforcement is of interest from two perspectives. Firstly, when an animal works for a reinforcer, there is frequently a delay between its actions and their outcomes. How do animals succeed in bridging delays to reinforcement? Secondly, individuals vary in their ability to choose delayed rewards. Impulsive choice, one aspect of impulsivity, is characterized by an abnormally high preference for small, immediate rewards over larger delayed rewards, and can be a feature of attention-deficit/hyperactivity disorder (ADHD), addiction, and other neuropsychiatric disorders. Damage to the nucleus accumbens core (AcbC) causes rats to exhibit impulsive choice, choosing small, immediate food rewards in preference to large, delayed rewards. These rats are also hyperactive, but are unimpaired in tests of visuospatial attention; they may therefore represent an animal model of the hyperactive-impulsive subtype of ADHD. Lesions to the anterior cingulate or medial prefrontal cortex (ACC, mPFC), two afferents to the AcbC, do not induce impulsive choice; the ACC may instead be involved in discriminating similar stimuli on the basis of their differential association with reinforcement. In theory, impulsive choice may emerge as a result of abnormal processing of the magnitude of rewards, or as a result of a deficit in the effects of delayed reinforcement. Recent evidence suggests that AcbC-lesioned rats perceive reward magnitude normally, but exhibit a selective deficit in learning instrumental responses using delayed reinforcement, suggesting that the AcbC is a reinforcement learning system that mediates the effects of delayed rewards.