Long-term prereward suppression in monkeys unaccompanied by cardiovascular conditioning.

In Experiment I, a 3-min tone that preceded a free pellet of food suppressed variable-interval performances maintained by the same type of pellets, but failed to elicit conditioned changes in the heart rates and blood pressures of two rhesus monkeys. Initially severe, the prereward suppression became temporally discriminated to progressively later portions of the tone, and was maintained at an attenuated level for over four months. The suppression was apparently not caused by interfering autonomic respondents, nor was it superstitiously conditioned, since 21 of the initial 25 tone-food pairings took place outside of baseline sessions. In Experiment II, a 1-min light, paired with four free pellets of food, suppressed the variable-interval responding of a second pair of similarly trained monkeys. An interresponse-time analysis showed that in one subject, mild prereward suppression of responding developed through two stages. On early trials, response rate slowed by 10% throughout the prefood interval. On later trials, the animal suppressed by pausing for a like portion of the interval, most often near the end, but otherwise responded normally during the prefood signal.