Fixed-ratio responding under second-order schedules of food presentation or cocaine injection

Squirrel monkeys pressed a key under second-order schedules in which every nth response resulted in a 2-sec visual stimulus (n-response fixed ratio; n = 10, 20 or 30); after a minimum fixed-interval of time elapsed (second-order t-min fixed-interval; t = 5 or 60), the completion of a fixed-ratio resulted in both the brief stimulus and either presentation of food or i.v. injection of cocaine. Under a second-order 5-min fixed-interval schedule with 15 intervals per session, rates of responding increased to a maximum and then decreased with increases in the amount of food (from 250 to 4000 mg/presentation) or cocaine (from 25 to 400 micrograms/kg/injection). Cocaine injections maintained higher maximal rates of responding than food presentation. When the brief stimuli were omitted during the 5-min intervals, average rates of responding maintained by cocaine decreased, but those maintained by food presentation did not. The presession administration of cocaine (0.3-1.0 mg/kg i.m.) increased rates of responding maintained by food to about the same level as those maintained by cocaine; these elevated rates did not decrease when the brief stimuli were omitted. Under a second-order 60-min fixed-interval with one interval per session, both food and cocaine maintained relatively high rates of responding even with large amounts of food (7.5-14 g/presentation) or cocaine (375-1500 micrograms/kg/injection). When the brief stimuli were omitted during the 60-min interval, rates of responding maintained by either food or cocaine decreased. Thus, the brief-stimulus presentations were essential for maintaining performance under the second-order 60-min fixed-interval schedule.