The behavioral effects of cocaine: rate dependency or rate constancy

The behavioral effects of cocaine were studied in squirrel monkeys trained to press a response key under an 8-min fixed-interval (FI) schedule of electric shock presentation. Overall mean rate of responding increased at 0.03--0.3 mg/kg (i.m.) and decreased at 1.0--3.0 mg/kg. Increased responding during the initial and middle periods of the fixed-interval accounted for the increase in overall mean rate; response rate during the final two min of the interval did not increase at any dose. An analysis based on response rate during individual 1-min segments of the 8-min interval showed that the rate during the interval became more uniform, and the pattern of positively accelerated responding became more linear, as dose increased. At 0.3--1.0 mg/kg, response rate was relatively constant and independent of the control, pre-drug rate of responding.