Areal and laminar distribution of some pulvinar cortical efferents in rhesus monkey

Abstract

The areal and laminar distribution of the cortical efferents of the medial, lateral and inferior pulvinar nuclei (PM, PL and PI respectively) were determined in rhesus monkey using autoradiography and Horseradish Peroxidase (HRP). The autoradiographic data indicated that: areas 8a, 45 and 46 on the convexity and 11 and 12 on the orbital surface of the frontal lobe received projections from PM; areas 20, 21 and 22 in temporal lobe received projections from PM primarily with caudal‐medial parts of PM projecting to more rostral‐dorsal parts of temporal lobe and rostral‐lateral parts of PM projecting to more caudal‐ventral parts of temporal lobe but PL also sends some efferents to caudal temporal lobe; areas 5 and 7 in parietal lobe and 18 and 19 in occipital lobe received projections primarily from the region in pulvinar comprising PL and PI with the more ventral parts of this region projecting to the ventral‐lateral parts of occipital lobe and the more dorsal parts of this region projecting to the more dorsal‐lateral and medial parts of parieto‐occipital cortex and with PM contributing slightly to these projections rostrally. The autoradiographic information on the pulvinar projections to frontal lobe and temporal pole was supplemented by data derived from cortical HRP injections. These indicated that although only PM of the pulvinar subnuclei projected to these regions, three other caudal thalamic structures, i.e., medial dorsal nucleus, nucleus limitans and suprageniculate nucleus also projected to these regions raising some questions about the identity of the densocellular part of the medial dorsal nucleus which has also been considered to be part of pulvinar. The laminar distribution of pulvinar cortical efferents was uniformly similar regardless of the pulvinar recipient area examined. Elevated numbers of silver grains were observed over all cortical layers, but the silver grains were densest over the deep parts of layer III. The thalamic reticular nucleus was the only diencephalic structure observed to receive projections from pulvinar and it did so from PM, PL and PI. The pulvinar's efferents are to homotypical rather than heterotypical cortex and its connections are most extensive with cortex rather than with subcortical structures.