Stimulus control of behavioral inhibition in the preweanling rat.

Previous studies demonstrate that 14-day-old rats reduce their emission of ultrasonic vocalizations and freeze when exposed to an unfamiliar adult male rat. This study sought to identify the stimulus characteristics of conspecific males that potentiate the display of behavioral inhibition. In Experiment 1, day 14 rats were isolated from the nest and exposed to either an unfamiliar prepubescent male rat or an unfamiliar adult male rat. Pups exposed to the unfamiliar adult male rat exhibited significantly elevated levels of freezing and reduced their emission of ultrasounds. In Experiment 2, pups were exposed to either a familiar or an unfamiliar adult male rat. Although several pups exposed to the familiar adult male rat exhibited freezing, pups tested with the unfamiliar adult rat showed a reliably higher duration of freezing and made fewer vocalizations. Results suggest that neither the unfamiliar factor nor cues associated with male adulthood are sufficient to account for the occurrence of behavioral inhibition when presented separately. However, the combination of unfamiliarity and adult male stimuli are highly potent stimulus features that elicit behavioral inhibition in preweanling rats.