In three experiments, the nictitating membrane response of rabbits was conditioned for 10 daily sessions at interstimulus intervals (ISIs) ranging from 48 to 125 msec, followed by a shift to 250 msec for 5 days. At tested ISIs shorter than 67 msec, there was no evidence of conditioning, and postshift performance revealed neither facilitation nor interference as a result of the first 10 conditioning sessions. Postshift performance of groups conditioned at preshift ISIs of 67 msec or longer revealed a gradient of increasing savings with increasing lSI. One of the groups in Experiment 1, initially conditioned at 250 msec lSI and then shifted to 48 msec, exhibited extinction of the previously well-conditioned response. Analysis of CR-onset latencies substantiated the absence of associative effects at very short ISIs. It was concluded that there is a temporal limit below which classical conditioning of the nictitating membrane response of rabbits employing forward CS-US pairing does not occur.The detailed interstimulus interval (lSI) function for the rabbit nictitating response (NMR), established by Smith, Coleman and Gormezano (1969), indicates that the minimum lSI required for conditioning is between 50 and 100 msec. Smith et al. found no evidence of conditioning at ISIs of -50, 0 or 50 msec, but increasingly rapid acquisition at ISIs of 100 msec and higher. Recent studies, however, have seemed to show substantial conditioning at 50-msec lSI (Patterson, 1970) as well as simultaneous and backward conditioning in rats and rabbits (e.g
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