Abstract
Pigeons' key-pecking responses were maintained under concurrently available variable-interval schedules of reinforcement. Responses in the presence of two different key-colors were reinforced on two independent and concurrent variable-interval schedules of food reinforcement, each associated with one of the key colors (red or green). Pecks at a second key (changeover key), always white, would alternate the colors on the main key. In Exp. 1 and 2, electric shock of 50 msec duration followed immediately after changeovers. The proportion of responses in the presence of the color associated with the higher frequency of reinforcements per hour was a direct function of shock intensity contingent on changeovers. When both schedules provided equal number of reinforcements per hour, there was no systematic effect of shock intensity on response distribution. In Exp. 3, a timeout period was contingent on changeovers, and response distribution was a function of timeout length.