LSD and fixed-internal responding in the rat.

A series of 6 doses of lysergic acid diethylamide-25 (LSD) altered the bar-pressing behavior of 6 rats maintained on a fixed-interval, 5 min (FI 5) schedule of reinforcement. High doses of LSD (0.16, 0.32 mg/kg) depressed overall rates of responding. Low response rates, which occurred during the first half of the interval between successive reinforcements, were increased by low (0.01, 0.02 mg/kg), moderate (0.04, 0.08 mg/kg), and high doses of LSD; high rates of responding which occurred during the final half of the interval were decreased only by high doses of LSD. All doses (except the lowest) decreased the Index of Curvature, a statistic describing the temporal distribution of responses. The results were discussed in terms of baseline rate of responding and the presence or absence of timing behavior.