Summation of responding maintained by fixed-interval schedules.

A light and tone were separately correlated with responding maintained by fixed-interval schedules, in which the level of responding varied continuously throughout the duration of the interval. Responding during the presence of the single stimuli and their compound was compared during the successive segments of the interval. The following results were obtained: (1) more responses were emitted during compounding than were emitted during either stimulus alone in all segments of the interval; (2) increases in the number of responses across the interval during compounding paralleled increases during single-stimulus presentations. The sum of the responses emitted during the single stimuli was similar to the number of responses emitted during compounding, suggesting that the response tendencies correlated with the single stimuli combined in a summative or additive fashion.