Disruption of temporal discrimination by drugs of abuse: I. Unmasking of a color bias.

Pigeons were trained to discriminate the length of a delay period (3s vs. 10s). Under control conditions, pigeons were able to discriminate between the two delay period lengths with a high degree of accuracy (>90%). When delays of 1, 3, 5, 7, 9 and 11s were randomly presented, the percentage of responses appropriate to the 10s delay increased as a function of increasing delay length. Dose-response curves determined for a series of drugs of abuse showed that pentobarbital, diazepam and phencyclidine displayed the greatest efficacy in disrupting the discrimination. The decrease in accuracy was a function of both a decrease in the ability of the pigeon to discriminate the passage of time, and the expression of a drug-induced red color bias. When the stimulus colors were changed, these drugs still decreased accuracy of the discrimination without any evidence of a color bias. Morphine disrupted the discrimination at doses which produced marked response suppression; there was no evidence of a drug-induced color bias. Delta(9)-THC failed to produce any significant effect on the discrimination. d-amphetamine and cocaine initially had no effect; however, upon subsequent determinations and when the stimulus colors were changed during the last part of the experiment, they did disrupt discrimination performance. These results show that drugs of abuse have differential effects on temporal discrimination, with some drugs affecting temporal discrimination at doses that do not suppress responding, some affecting the discrimination at doses that decrease response rates, and others that do not appear to affect temporal discrimination. Only sedative/hypnotic drugs disrupted temporal discrimination in part by producing a red-color bias.