CONDITIONED CARDIAC AND BEHAVIORAL-RESPONSE TOPOGRAPHY TO AN OLFACTORY CS DISSOCIATES WITH AGE

Conditioned cardiac and behavioral responses to an olfactory stimulus (CS) paired with electric shock (US) were studied in 16-, 23-, and 75- day-old rats. Heart rate and behavior were recorded during a single 15 -trial training session in which a 10-sec olfactory stimulus (amyl ace tate) was either paired or explicitly unpaired with the US. In the pre weanling rats, the conditioned cardiac response consisted of an increa se in heart rate (tachycardia) and a slight increase in investigatory behaviors. In contrast, the conditioned cardiac response in the adult rats consisted of a decrease in heart rate (bradycardia) and a strikin g increase in behavioral immobility (freezing). The 23-day-old rats sh owed a dichotomous pattern of responding; some showed preweanling-like conditioned responses and some showed adultlike responses. Developmen tal changes in neural organization and degree of learning are discusse d as possible determinants of the transition from infant to adult mode s of responding.