A horseradish peroxidase-autoradiographic study of parietopulvinar connections in Saimiri sciureus.

Descending connections from parietal cortex to pulvinar in squirrel monkey were investigated with the autoradiographic method. Somatosensory areas I (SI) and II (SII) were found to project to the oral (PuO) and medial (PuM) subdivisions of the pulvinar. Projections from the posterior parietal region were recorded in circumscribed areas of PuM and the lateral (PuL) and inferior (PuI) subdivisions of pulvinar. Retrograde labeling with horseradish peroxidase (HRP) demonstrated that rostral parietal cortex including the lateral cortex of SI and the rostral part of area 5 received reciprocal projections from PuO and rostral PuM. Injections of HRP into medial and lateral regions of SI also resulted in labeled cells in PuO and PuM. Within the limitations of the HRP technique, the latter results suggest a direct pathway from pulvinar to primary sensory cortex. The experimental results confirm the accepted view of projections from parieto-temporo-occipital "association" cortex to PuM, PuL and PuI. In addition, reciprocal connections of rostral parietal cortex with PuO and PuM were demonstrated.