Facilitation of avoidance extinction through response prevention was investigated under two levels of acquisition (three and eight successive avoidance trials) and three levels of blocking (5-sec. detainment and 15-or 60sec. trials with total avoidance response prevention). Subjects consisted of 82 albino rats. An independent test of fear reduction involved allowing Ss subsequently to approach the shock chamber for food. Results indicate that complete prevention of the response during the first five extinction trials greatly facilitates extinction, independent of trial length, whereas brief detainment does not. However, blocked animals gave evidence of greater residual fear than detained or unblocked SB which respond as do nonshocked controls. These results hold for both levels of acquisition.Identification of procedures which facilitate the extinction of learned avoidance behavior has been of continuing theoretical and applied interest. One seemingly effective method involves preventing, or "blocking," the instrumental avoidance response (

That organisms cannot remember events occurring during infancy may be the result of common forgetting processes exacerbated by the organism's increase in size during development or a unique process such as insufficient neurological development at the time of the early experience. To establish the uniqueness of infantile forgetting, size change was made irrelevant by exposing infant rats to "off-baseline" Pavlovian fear conditioning and assessing the effect of an apparatus-free conditioned stimulus upon independently established bar pressing. In Experiment 1, bar pressing by rats exposed to Pavlovian contingencies when 20-22 days old was substantially suppressed by the conditioned stimulus both 1 and 42 days after conditioning. In Experiment 2, pups conditioned when 17-19 and 20-22 days old again showed excellent retention, whereas pups conditioned when 11-13 and 14-16 days old showed total forgetting 42 days later. In Experiment 3, pups conditioned when 14-16 days old remembered well after 5 days, less well 10 days later, and not at all after 20 days. These findings suggest that size change may contribute to the forgetting of events occurring late in development, but that neurological immaturity may underly the forgetting of earlier events.

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