Organization of Response Areas in Ferret Primary Auditory Cortex
MARYLAND UNIV COLLEGE PARK SYSTEMS RESEARCH CENTER
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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- auesthetized ferret. Using a two-tone stimulus, the excitatory and inhibitory portious of the response areas were determined and then parametrized 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 initiatory 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 hands that paralleled the tonotopic axis. One functional 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.
- Anatomy and Physiology
- Medicine and Medical Research