Dissociating the effects of altered dopaminergic function on performance and learning.

Since the early days of physiological psychology it has been recognized that the behavioral effects of many manipulations often could not easily be attributed to impairments in learning because of possible changes in performance. Current psychopharmacological research concerned with evaluating the possible role of dopamine in reward-related learning is faced with the same problem. It is well known that manipulations that increase dopaminergic neurotransmission increase locomotor activity and manipulations that decrease dopaminergic neurotransmission decrease locomotor activity. These performance effects potentially influence any possible effects of manipulations of dopaminergic function on learning. To untangle these possible effects researchers have developed a number of approaches. These include observing patterns of responding on operant schedules within and across sessions, comparing avoidance responding in trained and untrained animals, conditioned reward and place conditioning procedures that separate drug and test sessions, stimulant self-administration procedures and the use of more than one conditioned stimulus to control responding. In each case data continue to support the conclusion that dopamine is involved in reward-related learning.