DISRUPTION OF TEMPORAL DISCRIMINATION BY DRUGS OF ABUSE .1. UNMASKINGOF A COLOR BIAS

Pigeons were trained to discriminate the length of a delay period (3 s vs. 10 s). Under control conditions, pigeons were able to discriminat e between the two delay period lengths with a high degree of accuracy ( >90%). When delays of 1, 3, 5, 7, 9 and 11 s were randomly presented , the percentage of responses appropriate to the 10 s delay increased as a function of increasing delay length. Dose-response curves determi ned for a series of drugs of abuse showed that pentobarbital, diazepam and phencyclidine displayed the greatest efficacy in disrupting the d iscrimination. The decrease in accuracy was a function of both a decre ase 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 disc rimination without any evidence of a color bias. Morphine disrupted th e discrimination at doses which produced marked response suppression; there was no evidence of a drug-induced color bias. Delta(9)-THC faile d to produce any significant effect on the discrimination. d-amphetami ne and cocaine initially had no effect; however, upon subsequent deter minations and when the stimulus colors were changed during the last pa rt of the experiment, they did disrupt discrimination performance. The se results show that drugs of abuse have differential effects on tempo ral discrimination, with some drugs affecting temporal discrimination at doses that do not suppress responding, some affecting the discrimin ation at doses that decrease response rates, and others that do not ap pear to affect temporal discrimination, Only sedative/hypnotic drugs d isrupted temporal discrimination in part by producing a red-color bias .