POST-TRIAL EFFECTS OF PRESENTING VS OMITTING EXPECTED SHOCK USS IN THE CONDITIONED SUPPRESSION PROCEDURE - CONCURRENT MEASUREMENT OF BAR-PRESS SUPPRESSION AND FREEZING

Barpress suppression in a 1 min interval following CS [conditioned stimulus] trials was investigated using 16 rats in a conditioned suppression procedure with a 2-stage design. For 1 group, each CS coterminated with a brief shock US in Stage 1. Then, in Stage 2, only half the CSs ended with a shock, which in turn was followed 1 min later by a 2nd shock. For a 2nd group, the 2 stages were reversed. When CSS were followed by single shocks in Stage 1, posttrial suppression weakened across trials; but when, in Stage 2, double shocks followed half the CSs, posttrial suppression grew stronger. When half the trials were followed by double shocks in Stage 1, posttrial suppression was maintained at initial levels but weakened in Stage 2 when single shocks followed each trial. In both stages, posttrial suppression was stronger on nonreinforced than on reinforced trials. Two factors were hypothesized to control posttrial suppression. First, posttrial suppression weakens with training under the single-shock procedure because postshock temporal stimuli come to inhibit fear unless themselves paired with shock. Second, posttrial suppression is stronger on nonreinforced trials than on reinforced trials because freezing behaviors initiated during the CS are not disrupted by a US and so persist into the posttrial interval.