Corticocortical neurons projecting to the medial and lateral banks of the middle suprasylvian sulcus in the cat: an experimental study with the horseradish peroxidase method.

Corticocortical afferents to both cortical walls of the cat middle suprasylvian sulcus (MSs area) were investigated by means of retrograde axonal transport of horseradish peroxidase (HRP). the visual cortex (V-I and V-II) projects to the medial wall of the MSs, the projection from V-II being heavier. The auditory cortex (A-I, A-II,. and Ep), including cortical walls of the dorsal part of the anterior and posterior ectosylvian sulci, sends fibers to the lateral wall of the MSs. Connections from the first auditory area (A-I) are heavier than from the second (A-II). In the rostral part of the MSs, both the medial and lateral walls receive fibers from the somatosensory (S-I and S-II) cortex. A larger number of association fibers appear to arise from S-II than S-I. Although the MSs as a whole apparently receives various kinds of sensory inputs, there seems to be a parcellation of th MSs area such that the areas receiving cortical association fibers from the visual, auditory, or somatosensory cortical areas also receive thalamic projections from those parts of the thalamus receiving sensory connections of the same modality. The cells of origin of the association fibers were mostly pyramidal, the majority located in layer III (e.g., 80% in the visual cortex and 74% in the auditory cortex), some in layer V, and a few in other layers. Most (76-79%) of the labeled cell bodies were of 15-20 micrometer diameter. Smaller (8-15 micrometer) and larger (20-26 micrometer) cells constituted less than 15% in each case. The mean diameters were 17.0 +/- 2.8 (SD) in the visual cortex and 17.7 +/- 3.2 micrometer (SD) in the auditory cortex.