Cortical connections of electrophysiologically and architectonically defined subdivisions of auditory cortex in squirrels

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Abstract
Multiunit recordings with microelectrodes were used to identify and delimit subdivisions of auditory cortex in squirrels. In the same animals, cortical connections of subdivisions of auditory cortex were determined by placing injections of the tracer wheat germ agglutinin conjugated with horseradish peroxidase (WGA‐HRP) into electrophysiologically defined locations. The electrophysiological results and patterns of connections were later related to myeloarchitectonic distinctions in brain sections cut parallel to the surface of the artificially flattened cortex. As previously described (Merzenich et al.: J. Comp. Neurol 166:387–402, '76), a primary auditory field, A‐I, was characterized by (1) neurons narrowly tuned to tone frequency; (2) a tonotopic map with high frequencies, which represented caudal to low frequencies; and (3) dense myelination. A‐I was reciprocally connected with a rostral field, R, a parietal ventral somatosensory representation, PV, cortex ventral to A‐I, and other nearby regions of cortex of the same hemisphere. Callosal connections of A‐I were with A‐I, R, and two or more other regions of temporal cortex. The less densely myelinated rostral field, R, also had neurons that were frequency tuned, but the neurons were often less securely driven. R appeared to have a tonotopic organization that roughly mirrored that of A‐I. Ipsilateral connections of R included A‐I, PV, and cortex ventral and caudal to R. Callosal connections were with R, A‐I, PV, and other locations in temporal cortex. Cortex in caudal PV, ventral to A‐I, and ventral to R was responsive to auditory stimuli, but responses to pure tones were weak and inconsistent, and habituation to a repeated stimulus was rapid. The cortex responsive to auditory stimuli included some but not all of the cortex connected with A‐I and R. The results lead to the conclusion that auditory cortex of squirrels contains at least two tonotopically organized fields, possibly as many as five or more auditory fields, and at least two auditory‐somatosensory fields.