Ontogenetic change in the analysis of sound frequency in the infant rat

Previous research has shown that there is an ontogenetic shift in the spatial code of sound frequency in the cochlea and central auditory nuclei. During ontogenesis, a given area of the basilar membrane or central auditory nuclei is maximally stimulated by sounds of progressively higher frequencies. Here, a similar ontogenetic shift in the rat's "perception" of sound frequency is reported. Animals were trained with a tone (CS) paired with footshock (US) and generalization of their conditioned response (CR) to different frequencies was assessed either 0.5 or 72 hr later. When 15-day-old animals were trained with an 8-kHz tone and tested 0.5 hr later, greater suppression of activity (CR) was evoked by the 8-kHz tone than by either a 12-kHz or a 6-kHz tone. When tested 72 hr after training, however, greater suppression of activity was evoked by a 12-kHz tone than by the 8-kHz training stimulus or by a 24-kHz tone. This shift in the peak of the generalization gradient could not be accounted for by changes in unconditioned responses to the test stimuli. Additionally, the observed shift in the generalization gradient was age-dependent. It was not observed when pups were trained at 18 days old and tested 72 hr later. The implications of these data for studies of the ontogenesis of learning and memory are discussed.