Claustro-neocortical projections in the rhesus monkey (projections to area 6).

The distribution of claustral neurons projecting to area 6 on the convexity and medial wall of the hemisphere was studied in four macaque monkeys. In all cases, HRP-positive neurons were found almost throughout the antero-posterior extent of the claustrum in its dorsal half. Following HRP injections in area 6 on the medial wall of the hemisphere most of the labelled neurons were found in the dorsal and dorsolateral portions of the claustrum. After injections in area 6 on the hemispheral convexity, labelled neurons predominated in about the middle third of the vertical extent of the claustrum. Perikaryal polymorphism and considerable differences in perikaryal size are features characteristic of claustral neurons projecting to area 6. There is a preponderance of labelled neurons whose perikarya are noted for their multipolar or pyramidal (triangular) shape. Oval and spindle-shaped neurons were found less frequently. The size of the perikarya ranged from 11 to 26 microns while most of the labelled neurons had perikarya ranging form 14 to 18 microns in size.