A further investigation of the developmental emergence of fear-potentiated startle in rats.

This series of experiments was designed to reexamine the ontogenetic emergence of the fear-potentiated startle response in rats. Previous results (Hunt, Richardson, & Campbell, 1994) indicated that potentiated startle to a light conditioned stimulus (CS) paired with an acoustic unconditioned stimulus (US) was not observed until 30 days of age. In the present experiments, subjects were given pairings of a light CS with a brief footshock unconditioned stimulus (US) and were tested for fear-potentiated startle 24 hr later. Subjects 23 and 30 days of age exhibited significant potentiated responding in the presence of the light, while 17-and 20-day-olds did not. Subjects 17 days of age did reliably express conditioned decreases in heart rate to the light at the 24-hr test. The failure to observe fear-potentiated startle at the youngest age was shown not to be due to a general disruption of conditioned fear responding by either (a) pretest startle stimulus presentations or (b) contextual characteristics of the startle testing apparatus. The capacity to express fear through a potentiated startle response develops later than the capacity for other defensive responses in the rat.