Postnatal development of evoked responses in the auditory cortex of the cat

Postnatal development of the auditory evoked cortical response was investigated in the cat by laminar field potential analysis. Responses were induced by electrical stimulation of the cochlear nuclear complex and by sonic stimulation under sodium pentobarbital anesthesia. As is well known, the auditory evoked cortical response in adult cats is a diphasic positive-negative wave in the superficial cortical layers and a negative-positive wave in the deeper cortical layers. By contrast, the cortical response in neonatal kittens was a monophasic negative wave in the superficial cortical layers and a positive wave in the deeper cortical layers (sN-dP wave). After 1 week, the sN-dP wave was preceded by a small wave which was positive in the superficial cortical layers and negative in the deeper cortical layers (sP-dN wave). As animals grew older, the sP-dN wave became dominant over the sN-dP wave to take an adult-like configuration of responses at 3 to 4 weeks of age. The sN-dP wave was separable from the preceding sP-dN wave by double shock stimulation at a certain time interval. Therefore, these two wave components are presumably mediated by two different types of thalamocortical projections. The level of potential reversal in each wave component shifted from the deeper cortical layers to the more superficial cortical layers during maturation. The latency of the response decreased sharply from 44 msec at birth to 11 msec by 3 weeks, and thereafter gradually to 5 msec in adulthood.