Organization of Response Areas in Ferret Primary Auditory Cortex

Abstract

We studied the topographic organization of the response areas obtained from single - and multi-unit recordings along the isofrequency planes of the primary auditory cortex (AI) in the barbiturate-anesthertized ferret. Using a two-tone stimulus, the excitatory and inhibitory portions of the response areas were determined and then parameterized in terms of an asymmetry index. The index measures the balance of excitatory and inhibitory influences around the best frequency (BF). The sensitivity of responses to the direction of a frequency-modulated (FM) tone was tested and found to correlate strongly with the asymmetry index of the response areas. Specifically, cells with strong inhibition from frequencies above (below) the BF preferred upward (downward) sweeps. Responses to spectrally shaped noise were also consistent with the asymmetry of the response areas: cells that were strongly inhibited by frequencies higher than the BF responded best to stimuli that contained least spectral energy above the BF. In a local region, most cells exhibited similar response area types and other response features. the distribution of the asymmetry index values along the isofrequency planes revealed systematic changes in the symmetry of the response areas. At the center, response areas with narrow and symmetric inhibitory sidebands predominated. These gave way to asymmetric inhibition, with high frequency inhibition (relative to the BF ) becoming more effective caudally, and low frequency inhibition more effective rostrally. These response types tended to cluster along repeated bands that paralleled the tonotopic asix. One functionaly implication of the response area organization is that cortical responses encode the locally averaged gradient of the acoustic spectrum by their differential distribution along the isofrequency planes. This enhances the representation of such features as the symmetry of spectral peaks, edges, and the spectral envelope. This scheme can be viewed as the one- dimensional analogue of spatial phase sensitivity in simple cells of the primary visual cortex, which there gives rise to spatial frequency channels and orientation columns.