Connections of primary auditory cortex (A‐I) were investigated in the tamarin (Saguinus fuscicollis), a New World monkey. In each case, A‐I was defined by multiunit recordings, and best frequencies were determined for neurons at different recording sites. Microlesions were placed to mark recording sites for correlation with cortical architecture. Following mapping, separate injections of up to three different tracers (HRP‐WGA and fluorescent dyes) were placed into the representations of different frequencies within A‐I. The results support several conclusions: (1) high to low frequencies are represented in a dorsocaudal to ventrorostral sequence in A‐I, (2) intrinsic connections in A‐I are more pronounced along isofrequency contours, (3) the pattern of connections between A‐I and adjoining cortex suggests that this surrounding auditory cortex contains at least two tonotopically organized fields and possibly one or more additional auditory fields, (4) callosal connections of A‐I are largely between parts of A‐I matched for frequency representation, (5) thalamic connections of A‐I include topographic connections with the ventral division of the medial geniculate complex (MGv) and more diffuse connections with the medial (MGm) and dorsal (MGd) divisions of the medial geniculate complex and the suprageniculate nucleus (Sg), and (6) A‐I projects bilaterally to the dorsal cortex of the inferior colliculus.

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.

While several subdivisions of auditory cortex have been determined in cats and monkeys, the organization of auditory cortex in rodents is not well understood. Connections, myeloarchitecture, and response characteristics of clusters of neurons were used to subdivide auditory cortex in grey squirrels, which are rodents with large, well-differentiated brains. These data reveal that primary auditory cortex (AI) is tonotopically organized (high frequencies represented caudomedially, low rostrolaterally), heavily myelinated, and reciprocally connected with multiple locations in the surrounding cortex. Although many neuronal clusters outside of AI are broadly tuned and habituate rapidly, our findings show that some of the surrounding cortex is responsive to auditory stimulation and includes two zones: (1) a rostrolateral (to AI) region which appears to be tonotopically organized and (2) a newly discovered “parietal ventral” somatosensory field which is apparently bimodal. AI is also reciprocally connected with a caudolateral zone which is not responsive to auditory stimuli. These results suggest that auditory cortex in rodents is more complexly organized than previously thought. [Work supported by NIH Grant NS16446.]