Retention of active and passive avoidance responses tested in extinction

Retention by rats of an active two-way shuttlebox avoidance response was tested in extinction following training to a criterion of 10 consecutive avoidance responses and was found to be a U-shaped function of the retention interval. A similar function was found in an extinction test of retention of suppression of food-rewarded lever pressing by response-contingent electric shock (passive avoidance), but not following noncontingent shock. The minima of both U-shaped functions were reached approximately 2 hr after training, and in both cases exogenous ACTH administered at the beginning of the retention interval blocked the deterioration of performance 2 hr later. The results show that the U-shaped retention functions is not limited to poorly learned avoidance behavior; is not mediated by contemporaneous changes in responsiveness to electric shock; is characteristic of both active and passive avoidance responses and therefore is probably not mediated by changes in spontaneous activity. The fact that ACTH facilitated performance in extinction tests at intermediate retention intervals following both active and passive avoidance training suggests that the action of ACTH is not mediated by changes in reactivity to shock or by changes in spontaneous activity. These findings are seen as consistent with both memorial and motivational interpretations of retention.