Central mechanisms in the perception of reward
The perception of reward is crucial in many psychological processes. Wise (1982) has suggested that the neurotransmitter dopamine is involved in producing 'hedonic' responses to rewards. Dopamine is also involved in the control of movement and hunger. Testing dopamine's role in reward perception is therefore complicated by these other actions; the effects of manipulations can often be interpreted as actions on hunger or motor control, rather than on reward. Two methods are described in this thesis which isolate reward effects. The effects of dopaminergic drugs on the rewarding impact of exploration were investigated. The use of exploration eliminates the influence of hunger, while the impact of motor deficits was reduced by using choice measures. Control experiments assessed effects on activity and emotionality. Results indicated that dopamine was involved in exploratory reinforcement independently of its roles in hunger or simple motor control. Tests of whether dopamine is involved in hedonia, or merely in engaging responses to reinforcers, used the 'behavioural contrast' paradigm. Contrast occurs when animals over-react to unexpected changes in reinforcement and allow response elicitation effects to be dissociated from effects on hedonia. Although the dopamine anatagonist a-flupenthixol did not affect contrast induced by changes in reinforcement value, the introduction or withdrawal of the drug could, itself, induce contrast effects. It is concluded that dopamine is involved in hedonia independently of any involvement in response elicitation. A speculative model of the nature of dopamine’s involvement in hedonia is proposed and its implications for our understanding of Parkinson’s disease and schizophrenia, in which dopamine dysfunctions are implicated, are discussed.