Effects of d-amphetamine and morphine on delayed discrimination: Signal detection analysis and assessment of response repetition in the performance deficits

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Abstract
Signal detection analysis was used to examine the effects of d-amphetamine and of morphine on delayed visual discrimination (delay intervals: 0–4–8–16 s) in male rats. The probability of response repetition in the discrete trial two-choice discrimination procedure was used as an additional behavioral measure. d-Amphetamine (0.16–0.33 mg/kg) decreased SI (a measure of the animals' sensitivity to the discriminative stimuli) at delays between stimulus presentation and opportunity for responding of 4–16 s, and did not affect SI at the 0 s delay. Morphine (1–3 mg/kg) decreased SI at all delay conditions. d-Amphetamine, but not morphine, affected RI (a measure of the animals' bias towards responding on one lever or the other) and increased the probability of response repetition. The bias measure B″ was affected neither by d-amphetamine nor by morphine. It is concluded that d-amphetamine, but not morphine, produces a deterioration of delayed discrimination performance, probably as a result of drug-induced response perseveration. It is suggested that under the conditions of the present study, the selective deterioration of discrimination performance after d-amphetamine at delays which are longer than 0 s may not be primarily related to a drug-induced disruption of a short-term memory mechanism, but may be related to drug effects on response output.