The development of frequency representation in the inferior colliculus of the kitten

While morphologically the kitten's cochlea matures first at the basal or high-frequency region, behavioural and physiological evidence suggests that it responds first to low-frequency sound. Explanations of this paradox include the suggestion that the spatial representation of frequency within the cochlea changes as a function of age. We have used the [14C]-2-deoxyglucose technique to study the development of frequency representation in the central auditory system of the kitten. We report here that while the locations within the inferior colliculus (IC) where high- and mid-frequency sounds are represented shift markedly between 10 and 35 days of age, the location where low-frequency sound is represented does not alter. The IC representation of low frequencies is adult-like by 10 days of age but that of higher frequencies continues to mature until as many as 35 days. Despite its morphological immaturity with respect to other regions, the apex of the cochlea appears to be the first region to become tuned to those frequencies to which it is tuned in the adult. We found little labelling at 5 and 7 days of age to 75-80 dB stimuli, but it is quite possible that the high-frequency region might respond to very intense low frequencies before 10 days of age. to very intense low frequencies before 10 days of age.