The effect of intertrial and intersession interval on the relearning of an incompletely learned avoidance response was studied in a factorial experiment having three intertrial intervals—½, 1, and 2 min.—and five intersession intervals—.08, 1, 4, 24, and 168 hr. Original learning was avoidance training to a criterion, and relearning was 40 trials of the same training. Five groups of 10 rats, matched for rate of original learning, were trained at each intertrial interval. The number of avoidance responses during relearning was a U-shaped function of intersession interval for all intertrial intervals. During the first 10 relearning trials, the maximum interfering effect occurred after an intersession interval of 24 hr. in the ½-min. groups and after 4 hr. in the 1- and 2-min. groups. During the rest of relearning, maximum interference was reached after 1 hr., and the intersession interval functions were independent of intertrial interval, except for over-all level. These results were interpreted in terms of a “parasympathetic over-reaction” following fear conditioning.

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