1. ¹Department of Psychiatry, School of Clinical Medicine, Addenbrooke's Hospital, University of Cambridge, Cambridge, UK
2. ²MRC Centre for Behavioural and Clinical Neuroscience, University of Cambridge, Cambridge, UK
3. ³Department of Experimental Psychology, University of Cambridge, Cambridge, UK
4. ⁴Cambridge Drug & Alcohol Service, Brookfields Hospital, Cambridge, UK

Correspondence: Professor BJ Sahakian, Department of Psychiatry, School of Clinical Medicine, University of Cambridge, Addenbrooke's Hospital, Box 189, Cambridge, Cambridgeshire CB2 2QQ, UK. Tel: +44 1223 331209; Fax: +44 1223 336968; E-mail: ke220@cam.ac.uk

Received 20 January 2005; Revised 19 April 2005; Accepted 22 May 2005; Published online 6 July 2005.

Abstract

Reinforcing properties of psychoactive substances are considered to be critically involved in the development and maintenance of substance dependence. While accumulating evidence suggests that the sensitivity to reinforcement values may generally be altered in chronic substance users, relatively little is known about the influence reinforcing feedback exerts on ongoing decision-making in these individuals. Decision-making was investigated using the Cambridge Risk Task, in which there is a conflict between an unlikely large reward option and a likely small reward option. Responses on a given trial were analyzed with respect to the outcome on the previous trial, providing a measure of the impact of prior feedback in modulating behavior. Five different groups were compared: (i) chronic amphetamine users, (ii) chronic opiate users in methadone maintenance treatment (MMT), (iii) chronic users of illicit heroin, (iv) ex-drug users who had been long-term amphetamine / opiate users but were abstinent from all drugs of abuse for at least 1 year and (v) matched controls without a history of illicit substance use. Contrary to our predictions, choice preference was modified in response to feedback only in opiate users enrolled in MMT. Following a loss, the MMT opiate group chose the likely small reward option significantly less frequently than controls and heroin users. Our results suggest that different opiates are associated with distinctive behavioral responses to feedback. These findings are discussed with respect to the different mechanisms of action of heroin and methadone.