Proactive interference effects on short-term memory in rats: II. Effects in young and aged rats.

Whether the short-term memory impairments of aged rats in an operant delayed-matching task is attributable to increased susceptibility to proactive interference (PI) was tested. Groups of young and aged rats were trained on the task, and the previously reported delay-dependent deficit of aged rats was replicated. The aged rats showed a significantly greater decline in performance on previous-response-opposite trials compared with previous-response-same trials than did young rats, when tested over the same range of delay intervals, suggesting a higher sensitivity to PI. However, this effect was established against a higher overall baseline performance of the young animals on both types of trial. When the young and aged animals were equated for performance on previous-response-same trials (by increasing the range of delay intervals used for testing the young animals), the interaction effect was abolished. Thus, the delay-dependent deficits of aged rats are independent of their sensitivity to PI.