CONNECTIONS BETWEEN THE PULVINAR COMPLEX AND CYTOCHROME OXIDASE-DEFINED COMPARTMENTS IN VISUAL AREA V2 OF MACAQUE MONKEY

We examined the distribution of pulvinar afferents to visual area V2 o f macaque monkey cerebral cortex in relation to the distribution of th e metabolic enzyme cytochrome oxidase (GO). V2 contains three sets of stripelike subregions that are marked by differential staining for CO, and which have different corticocortical connections. The pulvinar pr ovides the major subcortical input to V2, and this input is known to b e patchy. We were interested to determine how the pattern of pulvinar afferents relates to the layout of the three stripelike compartments t hat characterize V2. We made large injections of WGA-HRP into the pulv inar (labelling both the inferior and lateral divisions) and mapped th e resulting orthograde terminal and retrograde cell label within V2. W e observed pulvinar terminal label mainly in lower layer 3 (at the lay er 4 border), with light label in layer 1 as well; terminal label in l ayers 3-4 was distributed in discrete patches with faint bridges of li ght label between. Comparison with adjacent sections stained for CO or Cat-301 showed that pulvinar terminal zones aligned precisely with re gions of increased CO staining, and targeted both ''thick'' (Cat-301()) and ''thin'' GO-rich stripes, avoiding the pale stripes (which alig ned with the faint bridges of terminal label). Retrogradely labelled c ells were found in layers 5A and 6, but the bulk of the feedback to pu lvinar arose from layer 6 rather than layer 5 (unlike V1, where feedba ck to pulvinar arises primarily from layer 5B). These results show tha t the increased CO staining in certain subregions of V2 is closely cor related with the presence of thalamic terminals from the pulvinar. Alt hough we cannot rule out the possibility that different sets of pulvin ar neurons project to different CO compartments in V2, the presence of a prominent thalamic input shared by the ''thick'' and ''thin'' CO st ripes (which receive different V1 afferents and make different feedfor ward projections to other visual cortical areas) could underlie the pr eferential intrinsic interconnections shown to exist between these V2 subregions and suggests another potential source of integration betwee n the two cortical visual streams.