THE AUDITORY PATHWAY IN CAT CORPUS-CALLOSUM

The cortical auditory fields of the two hemispheres are interconnected via the corpus callosum. We have investigated the topographical arran gement of auditory callosal axons in the cat. Following circumscribed biocytin injections in the primary (AI), secondary (All), anterior (AA F) and posterior (PAF) auditory fields, labelled axons have been found in the posterior two-thirds of the corpus callosum, Callosal axons la belled by small individual cortical injections did not form a tight bu ndle at the callosal midsagittal plane but spread over as much as one- third of the corpus callosum, Axons originating from different auditor y fields were roughly topographically ordered, reflecting to some exte nt the rostro-caudal position of the field of origin, Axons from AAF c rossed on average more rostrally than axons from AI; the latter crosse d more rostrally than axons from PAF and AII. Callosal axons originati ng in a discrete part of the cortex travelled first in a relatively ti ght bundle to the telo-diencephalic junction and then dispersed progre ssively. In conclusion, the cat corpus callosum does not contain a sec tor reserved for auditory axons, nor a strictly topographically ordere d auditory pathway. This observation is of relevance to neuropsycholog ical and neuropathological observations in man.