Stimulus control and the response-reinforcement contingency.

Pigeons were trained under a schedule in which reinforcement was made available at varying periods of time after a prior reinforcement. The first key peck after a reinforcer was available began a timer and a second key peck, which exceeded a specified minimal time interval, produced the reinforcer. It was shown that a contingency which contains a minimal interresponse time does not necessarily weaken stimulus control by an exteroceptive stimulus.