Schedule-induced escape from fixed-interval reinforcement.

Pigeons trained to peck one of two keys for food were exposed to an ascending and descending series of fixed-interval values. A response on the second key produced an escape period consisting of a visual stimulus change. During escape periods, the fixed-interval timer continued to operate and even if it timed out, a response on the food key would not operate the feeder unless preceded by an escape-key response that terminated the escape condition. As the fixed-interval schedule was increased logarithmically through six values from 30 to 960 sec, the percentage of session time spent in escape as well as the frequency, duration, and rate of escape increased to a maximum and then decreased. One subject did not develop escape behavior to any significant degree. For all pigeons, escapes usually occurred after, rather than before, reinforcement.