Contrasting baseline-dependent effects of amphetamine, chlorpromazine and scopolamine on response switching in the pigeon.

Both amphetamine and scopolamine increase low rates and reduce high rates of responding. It has been suggested that the dependence of the effects of these drugs on control rate may be due to their non-specific disruptive cue properties rather than to specific pharmacological actions. To examine whether this possible non-specific disruption also applied to response-choice, pigeons were trained under a schedule in which 30 key-peck responses were required. This fixed-ratio could be completed by responding in any order on either or both of two keys, and then the first switch between the two keys was reinforced by the presentation of food. Under control conditions, the probability of the birds switching between the keys increased as the ratio progressed, resulting in performance which could be analysed in a manner analogous to rate-dependent analyses of responding under fixed-interval (FI) schedules. The birds were then treated with amphetamine (0.4-3.2 mg/kg), chlorpromazine (1.0-30 mg/kg) and scopolamine (0.01-0.10 mg/kg). Amphetamine increased switching at all baselines, to probabilities greater than chance (i.e. P. switch greater than 0.5). Scopolamine resulted in response choice converging towards chance, whereas chlorpromazine reduced switching when the baseline probabilities were high. Thus: amphetamine increases switching in pigeons as it has previously been shown to do in rats, and this effect is not due to regression towards random choice, the opposite effect of a reduction in switching can occur after chlorpromazine treatment, and whereas the effects of amphetamine and scopolamine on response rate may be similar, the effects of the two drugs on response choice are dissociable, with only scopolamine resulting in a randomisation of performance.