Shock-duration reduction on the basis of interresponse times: the effect of the interresponse time limit.

Rats were trained on a free-operant procedure in which the duration of randomly occurring shocks depended on the interresponse times of lever presses. Shocks of 1.6-mA intensity were delivered at random intervals with an average density of 10 shocks per min. Each shock that was delivered lasted 0.3 s as long as the interresponse times were within a preset limit. Whenever the interresponse time exceeded the limit, the shocks that were delivered lasted 1 s until the occurrence of a response that met the limit. The limit was reduced in 3-s steps from 15 s to either 6 s or 3 s, at which point 3 of the animals were exposed to an ascending series. The avoidance of long-duration shocks was highly efficient at the 15-s and 12-s limits, and it decreased at the 9-s limit. With the exception of one animal, performance was substantially worse at the 6-s limit and it deteriorated for all the animals that were exposed to the 3-s limit. The data suggest that shock-duration reduction is quite effective as negative reinforcement for avoidance but is perhaps less effective than shock-intensity reduction.