Delayed development of fear-potentiated startle in rats.

The developmental emergence of fear-potentiated startle was examined in rats ranging in age from 16 to 75 days. In Experiment 1, a pure tone served as the conditioned stimulus (CS) and an acoustic startle pulse served as the unconditioned stimulus (US) for fear conditioning. Fear-potentiated startle by the tone CS was observed in rats 23 days of age and older but not in rats 16 days of age. In Experiment 2, a light served as the CS. Rats 30 days of age and older showed fear-potentiated startle, whereas 23-day-old rats did not. The final experiment demonstrated that another behavioral index of fear, stimulus-elicited freezing, was observed earlier in development than fear-potentiated startle, confirming the effectiveness of the training procedure for conditioning fear. The results suggest that fear-potentiated startle is a relatively late-emerging response system, paralleling the development of conditioned autonomic changes (e.g., heart rate) rather than that of freezing.