Effects of D-amphetamine on temporal discrimination in pigeons.

Two experiments used a matching to sample of durations procedure to examine changes in temporal discrimination evoked by D-amphetamine (0.75, 1.50, 2.25 and 3.0 mg/kg i.m.) in pigeons. Responses to one key color were reinforced after long (8-s) stimuli: responses to the alternative key color were reinforced after short (2-s) stimuli. Stimuli of intermediate duration were reinforced in Experiment 1 and were not reinforced in Experiment 2. Sigmoid functions relating percent of choices of the key reinforced after long-duration stimuli to the duration of stimulus presented were fit by a cumulative normal function. In addition to a parameter giving the left/right position of the sigmoid curves (which indexes temporal perception), this function also provided estimates of the range and slope of the curve (measures of stimulus control). The results showed that, contrary to many published reports, amphetamine had no effect on the left/right positioning of the sigmoid curve. There was, however, an effect of amphetamine on stimulus control, as shown by the general flattening of the psychophysical functions. Little evidence of tolerance to the drug's behavioral effects was found after 30 consecutive days of behavioral testing under amphetamine.