BETWEEN-SUBJECT AND WITHIN-SUBJECT EFFECTS OF US DURATION ON CONDITIONED SUPPRESSION IN RATS - CONTRAST MAKES OTHERWISE UNNOTICED DURATION DIMENSION STAND OUT

Four experiments were conducted to study the unconditioned stimulus (U S) duration effects upon conditioned suppression of licking in rats. A fter ascertaining the aversiveness of footshocks to be a direct functi on of their duration in Experiment 1, these shocks were used as USs in Experiment 2. The US duration had no systematic effect upon condition ing. The 0.7- and 4.9-s shocks, which had been proved to be different in their aversiveness in Experiment 1, were used as USs in Experiment 3. Shock duration had no effect on conditioning with a between-subject design, but the 4.9-s US produced greater conditioned suppression tha n the 0.7-s shock in a within-subject design. Experiment 4 showed that the US-duration effect was facilitated when rats had more chances to compare USs of different durations within a session. The results were discussed with reference to the results of previous experiments on the US-duration effect as well as to the Rescorla-Wagner model and Miller 's comparator hypothesis. (C) 1996 Academic Press, Inc.