Forgetting of the CS Duration in Rats: The Role of Retention Interval and Training Level

Abstract

Three experiments were conducted on long-term memory for conditioned stimulus (CS) duration using the conditioned suppression paradigm with rats. After training in each experiment, suppression systematically increased during successive fifths of a 150-s CS that ended in shock, indicating a temporal discrimination. Experiment 1 tested the effect of a retention interval (3, 7, 15, or 20 days). The other two experiments tested the effect of two different levels of training (45 or 90 trials) on the retrieval of temporal information after retention intervals of 3 or 20 days (Experiment 2), and 7 or 15 days (Experiment 3). The temporal discrimination was well retained 3 days after training but was progressively forgotten as the retention interval increased. Forgetting of time was inferred from the flattening of the temporal discrimination gradient, which was complete after the 20-day retention interval. This flattening was accompanied by an increase in conditioned suppression that was greater at the beginning of the CS. These results suggest that rats forgot the CS duration but remembered the CS–US excitatory relationship. Training level had a slight effect on the rate of forgetting and reacquisition but did not affect the way it happened.