Magnitude of fear as a function of expected time to all aversive event

Three rats were trained to press a lever on a random-interval 1min schedule of food reinforcement. In successive phases of the experiment, electric shocks were superimposed at 1-min fixed intervals, 2-min fixed intervals, or at 1-min random intervals. In the fixed-interval conditions, there was a steep gradient of reduction in response rate as the time for the next shock approached; in the random-interval condition, the response rate following a shock was relatively constant. The present method appears to be adequate to monitor the instantaneous level of fear. The results suggest that animals were timing in units proportional to the fixed interval and that aversive events which occur randomly in time are perceived as phenomenologically random by the rat.