Tone responses in core versus belt auditory cortex in the developing chinchilla

Single‐unit responses to tone pip stimuli were isolated from numerous microelectrode penetrations of core primary auditory cortex (AI) and a dorsocaudal (DC) belt region in the ketamine‐anesthetized chinchilla ( laniger ). Results are reported at postnatal day 3 (P3), P15, P30, and from adult animals. The AI core could be distinguished from the DC belt on the basis of its strict tonotopic organization, evident in all chinchillas studied (including the youngest). Averaged by age group and compared to their core counterparts, belt neurons generally had similar absolute (spike rate) thresholds and onset latencies (at a given sound pressure level), but lower maximum spike rates, broader tuning bandwidths, and more complex (multipeaked) receptive fields. Most notably, the fraction of complex belt units in the near‐newborn (P3) group was high (∼50%), and did not systematically increase with age, while that of complex core units was ∼10% at P3 and increased steadily to about 40% in adulthood. These results provide further evidence to support the hypothesis that, at least to some extent, core and belt auditory cortex may constitute parallel processing streams which represent different aspects of complex acoustic stimuli. J. Comp. Neurol. 492:101–109, 2005. © 2005 Wiley‐Liss, Inc.