Operant acceleration during a pre-reward stimulus.

Stimuli of 20, 40, and 80 sec duration terminated with five non-response-contingent food pellets were superimposed upon lever pressing reinforced with single pellets on a DRL 30-sec schedule. Two rhesus monkeys served as subjects. No change in response frequency was observed during the 20- and 40-sec stimuli. During the 80-sec pre-food stimulus, overall response frequency increased to approximately 150% and 220% of pre-stimulus levels, and the temporal distributions of interresponse times shifted toward the shorter intervals. When the 80-sec stimulus was no longer terminated with food, the response frequency decreased and the temporal distributions of interresponse times gradually approached pre-stimulus levels. An increased frequency of short interresponse times and an increase in response rate was again observed when the pellet termination procedure was reinstituted with the 80-sec stimulus. No change in response frequency or interresponse times was observed in the absence of the conditioning stimulus, and performance efficiency, as reflected in the ratio of responses to reinforcements during non-stimulus periods, remained stable throughout the experiment.