Catania, A. C. (1963). Concurrent performances: A baseline for the study of reinforcement magnitude. Journal of the Experimental Analysis of Behavior, 6, 299-300.

When a pigeon's pecking on a single key was reinforced by a variable-interval (VI) schedule of reinforcement, the rate of pecking was insensitive to changes in the duration of reinforcement from 3 to 6 sec. When, however, the pigeon's pecking on each of two keys was concurrently reinforced by two independent VI schedules, one for each key the rate of pecking was directly proportional to the duration of reinforcement.