Ipsilateral cortical connections from the second and fourth somatic sensory areas in the cat.

The ipsilateral corticocortical connections of the second and fourth somatic sensory areas (SII and SIV) were traced with the aid of anterograde or retrograde axonal transport techniques involving horseradish peroxidase conjugated to wheat germ agglutinin (HRP-WGA) or tritiated amino acids. The injections were placed into physiological defined components of the body representation in SII or SIV. The results from cases with localized injections into SII showed precise topographically organized, reciprocal connections with SI and motor cortex area 4. The distribution of connections in SI included areas 3a, 3b, and 1-2. A uniform pattern of cell and fiber labeling was seen across area 3b and 1 within the zones that were homotypical to the injection site in SII as though only a single representation of the cutaneous surface of the body existed in SI. Intrinsic connections within SII were also topographically arranged. Additional areas found to be interconnected with SII included, in decreasing order of density: area 5, insula, perirhinal cortex (area 36), and ventrolateral orbital cortex. SII connections with area 6 were seen only in the region of the lateral bank of the presylvian sulcus. There may be interconnections between SII and SIV but these were from possible local intrinsic connections in the AEG. The results from injections involving SIV showed reciprocal connections with area 5, the suprasylvian fringe, insula, dorsolateral orbital area, and area 6. The densest connections for SIV were with area 5. No topography was noted in the connections for SIV.