SEGREGATION AND HETEROGENEITY OF THALAMIC CELL-POPULATIONS PROJECTING TO SUPERFICIAL LAYERS OF POSTERIOR PARIETAL CORTEX - A RETROGRADE TRACER STUDY IN CAT AND MONKEY

The thalamic neurons projecting to the superficial layers of areas 5 and 7 in the cat, and area 5 in the monkey, were investigated by using superficial deposits of either horseradish peroxidase or Fast Blue in one hemisphere. In the contralateral hemisphere injections of the same tracer involving the full cortical depth were made in homotopical locations, and the distribution and soma size of retrogradely labeled thalamocortical neurons in each side of the thalamus were compared. It was found that, in the cat, labeled neurons in the lateral posterior-pulvinar complex, and in paralaminar regions of the ventrolateral complex, were fewer in number and smaller in size in cases of superficial deposits than in cases of deep injection. In more lateral portions of the ventrolateral complex, however, there were no size differences. In the monkey, similar differences in number and size appeared in the caudal division of the ventrolateral complex and in the lateral posterior and pulvinar nuclei, whereas no such differences were found for neurons labeled in the oral and medial divisions of the ventrolateral complex, and in the ventral posteroinferior nucleus. In all cases the intralaminar and midline nuclei exhibited retrogradely labeled neurons only when deep layers were injected. These and previous findings point to the existence of a widely distributed layer I-projecting system of neurons which, in most nuclei, are interspersed among neurons projecting mainly to middle or deep layers. In some nuclei, however, as is the case with the ventromedial nucleus proper, layer I-projecting system neurons would make up the whole nucleus. The cell groups located in a paralaminar position, which would be but a part of this system, could provide through their projections to layer I in the posterior parietal and frontal cortical regions a final path for recruiting responses and spontaneous spindling activities.