Repeated acquisition and delayed performance as a baseline to assess drug effects on retention in monkeys

As an extension of previous research on repeated acquisition, a new baseline was developed to assess the effects of phencyclidine on retention in patas monkeys. Each session was divided into three phases: acquisition, delay, and performance. During acquisition, the subject acquired a four-response chain (which was different each session) by responding sequentially on three keys in the presence of four geometric forms. When the acquisition criterion (20 consecutive correct responses) was met, the keylights turned off and the delay (retention interval) began. After the delay, the keylights and a white light above the keys were turned on for 10 min (performance). The white light indicated that the response chain was the same as the chain acquired before the delay. Retention of the acquired response chain, as measured by percent "savings" in errors to criterion, decreased as the delay was increased from 5 to 180 min, and this "forgetting curve" tended to shift to the left with increasing doses of phencyclidine (administered IM 5 min before the performance phase). "Overlearning" the response chain before the 180-min delay increased retention and attenuated the disruptive effects of the lower dose of phencyclidine.