In 11 cats, small quantities of horseradish peroxidase conjugated to wheat germ agglutinin were placed into cortical zones of somatosensory area I representing the distal digits (n = 3), distal toes (n = 2), toes and digits (n = 1), proximal forelimb (n = 1), proximal hindlimb (n = 1), trunk (n = 2), and the face and nose (n = 1). Reconstruction of the pattern of retrograde labeling in somatosensory area II revealed dense, heavily labeled patches of cells in regions that were precisely homotypical to the injection site as determined by electrophysiological recordings. This dense, homotypical patch of labeled cells was usually surrounded by a less densely populated fringe of labeled cells that bordered, but did not appear to enter, heterotypical zones. In two animals, however, some retrogradely labeled cells were found in the cortex representing somatotopic zones adjacent to the sites injected with horseradish peroxidase. These results indicated that somatosensory area II primarily sends homotypical projections to somatosensory area I. In a few cases, however, some retrogradely labeled cells may represent either homo- or heterotypical projections depending on how receptive field sizes and the areal extent of labeling in somatosensory areas I and II are interpreted.
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