Temporal discrimination and forgetting of CS duration in conditioned suppression

Two experiments with rats investigated a temporal discrimination, and the forgetting of it, using the conditioned suppression paradigm. Experiment 1 showed that the conditioned emotional response (CER) increased as a function of time in the conditioned stimulus (CS), with a maximum near the unconditioned stimulus (US), when a long CS was immediately followed by the US. Location of the maximum response was taken as an index of the expected time of US. When the CER was recorded in time-proportional units (every fifth of the CS), the temporal discriminations found with CS durations of 50, 100, 150, and 200 s superposed. Experiment 2 showed forgetting of the temporal discrimination (flattening of the temporal discrimination gradient) after a retention interval of 20 days, but not after 3 or 7 days. The forgetting of the temporal discrimination resulted in an increase in the CER at CS onset. Thus, a high level of suppression to the whole CS was found after the retention interval of 20 days. Results were consistent over each of the CS durations employed. (C) 1996 Academic Press, Inc.