Cholinergic drug effects on visual discriminations: a signal detection analysis.

Signal detection analysis was used to examine the effects of scopolamine, amphetamine, and physostigmine on a brightness discrimination task. Four groups of rats were exposed to different reinforcement contingencies for correct responses in the presence of S+ stimuli and correct response failures in the presence of S- stimuli. Under non-drug conditions, orderly signal detection data were obtained with the group design. Contrary to a disinhibition hypothesis of cholinergic effects, scopolamine disrupted stimulus sensitivity and not response bias, but only when the discrimination was difficult. Drugs did not affect sensitivity of an earlier, simpler discrimination. The drug effects on response tendency for this simple discrimination task were difficult to interpret.