The ontogenesis of defensive reactions to shock in preweanling rats

Behavioral observations were made on the reaction to tailshock and footshock in 5- to 20-day-old hooded rats. For detection thresholds, age differences were found for footshock but not for tailshock. During intershock intervals, more generalized activity and freezing were elicited by footshock, whereas more responding directed to the shock source was elicited by tailshock. The unconditional responding to shock indicated that the older animals had a larger behavioral repertoire of defensive reactions and responded differentially to different shock intensities. The younger animals appeared to have a more limited and stereotyped repertoire of defensive reactions and to be more sensitive to the same nominal shock level. These normative data should prove useful in evaluating motivational and response repertoire problems when assessing learning processes developmentally.