The experiments reported here were designed to determine the role of associative conditioning in reflex modification of the acoustic startle response, using gaps in background noise. The first experiment was done to characterize the efTectsof repeated testing for 9 days with 20-msec gaps in white noise as the preliminary stimulus (81) and a 120-dB, 40-msec 13-kHz tone as the eliciting stimulus (82). The second experiment was a test of associative conditioning. Three groups of rats were tested daily for 6 days under one of the following conditions: SI and 82 paired in a contingent manner, 82 only, or 81 only. All groups then received the contingent pairing of81 and 82 for an additional 9 days of testing. In the third experiment, aseparate group of rats was tested with either contingent or noncontingent presentation ofthe 81 and 82 for 6 days, and then, on the 7th day, both groups received the stimuli in a contingent fashion. Results indicate that the amount of inhibition increases with repeated, daily testing, and that it achieves asymptotic levels of inhibition after 5-6 daily sessions. The presentation of 81 and 82 together is a necessary, but not sufficient, condition for normal development of inhibition. Alternatively, the contingency that exists between the two stimuli is likely to be the determining factor necessary for development of the inhibition. These data imply that an associative learning process may be a rnajor factor in the gap-inhibition phenomenon.Reflex modification of the acoustic startle response (ASR) is the change in a stimulus-elicited response due to a perceptible antecedent change in the sensory environment (for a review, see Hoffman & Ison, 1980). A wide variety of sensory stimuli have been demonstrated to inhibit the ASR in the rat, including white noise and tone bursts (

In a series of four experiments, social and experiential factors that influence the development of motor activity in rat pups were examined. Motor activity was monitored from postnatal Days 13 to 21 as photocell interruptions in figure-eight mazes and comparisons were made between pups maintained in a nest box containing a dam and siblings and allowed access to the maze for 23 hr/day, pups tested daily for 1 hr/day vs pups tested only on postnatal Days 15, 18, or 21, pups tested daily for either 5 min, 30 min, or 1 hr/day, and pups tested daily for 30 min/day either singly in a maze, paired with a littermate, or paired with an anesthetized pup of the same age. A monotonic increase in activity was seen for nest-box testing, minimal developmental change was seen for pups tested on only a single day or for pups tested with an anesthetized pup, whereas all other groups showed an inverted U-shaped profile of activity which was influenced by the duration of testing and/or the presence of a littermate. These data emphasize the relevance of environmental factors as determinants of preweaning behavior.

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