Auditory cortical field projections to the basal ganglia of the cat

Projections to the basal ganglia from four auditory cortical fields in the cat were studied by combining microelectrode-mapping of the neurons' best frequencies with autoradiographic and histochemical tract-tracing techniques. Each auditory field is a source of projections to the homolateral basal ganglia. The distribution of labeling within the basal ganglia is related to the cortical field in which the injection site is located. The dorsal portion of the putamen and adjacent caudate nucleus are connected with cortical fields situated anteriorly and dorsally, while the ventral portion of the putamen and adjacent lateral amygdaloid nucleus are related to auditory fields situated posteriorly and ventrally. Injections of two different tracers into different best-frequency loci of one cortical field provided evidence that low best-frequency neurons project medially within the basal ganglia while high best-frequency neurons project more laterally. We concluded that there was a basic similarity among patterns of terminations in the basal ganglia from axons that originate in different auditory cortical fields. When the source of a projection was confined to a restricted portion of an auditory cortical field, labeling appeared as dense patches of silver grains separated from each other by areas of less dense labeling. Often, these patches were distributed within a sheet of tissue, elongated both dorsoventrally and anteroposteriorly. Loci having the same best-frequency representation, but situated in different auditory cortical fields, project upon overlapping but not coextensive portions of a single sheet of tissue. Thus the projections from geographically distant cortical loci possessing similar best-frequency representations are notably distinguished on a topographic basis. By comparison, two adjacent sheets of tissue were labeled when two injections were made into the low best-frequency and high best-frequency representations of the same auditory field. Double-injection, double-tracer experiments revealed that adjacent sheets of tissue received projections from different best-frequency loci. These observations suggested a degree of tonotopic organization to this projection system which was equipoise to the evidence obtained for a topographic organization.