Combining stimuli signalling response-dependent food and shock.
Three rats were exposed to a multiple schedule in which separate presentations of light and tone alternated with periods during which light and tone were absent. In Phase 1, light and tone each signalled identical variable-interval schedules of food delivery. In Phase 2, light and tone signalled separate but concurrent variable-interval schedules of food and shock delivery. In both phases, the absence of light and tone was associated with the differential reinforcement of other behavior. Test presentations of light, tone, and a light-plus-tone combination indicated that in both phases, light-plus-tone controlled higher response rates than either light or tone alone. The combination continued to control enhanced responding even when the test stimuli signalled variable-interval schedules of food and fixed-ratio schedules of shock. In these latter sessions, enhanced control by the combination increased shock frequency with no corresponding change in food frequency. Apparently, the level of behavior controlled by the absence of two single stimuli may be more important than the consequences of responding in determining the effects of combined-stimulus presentations.