Ontogenesis of learning: II. Variation in the rat's reflexive and learned responses to acoustic stimulation.

The ontogenesis of the rat's reflexive and simple learned reactions to acoustic stimulation was investigated in five experiments. The pattern of results suggests that (a) reflexive reactions to tonal stimulation emerge earlier in ontogenesis than the capacity to learn about these stimuli, and (b) the age at which simple Pavlovian conditioning is first observed depends on the nature of the acoustic conditioned stimulus (CS). Specifically, these data suggest that pups are not capable of simple Pavlovian conditioning to a 2000-Hz tone (CS) paired with an oral infusion of sucrose (US) until they are 14-15 days old. The same acoustic stimulus will, however, evoke reflexive reactions (startle inspiration) in pups as young as 10-12 days old. Additionally, when a different auditory stimulus (an intermittent train of clicks) serves as the CS, there is evidence of conditioning in pups only 12-13 days old. It is suggested that these age-related dissociations in the emergence of reflexive and simple learned responses to sound reflect a caudal-to-rostral maturational sequence of the components of the ascending auditory system.