Cortical connections of dorsal cortex rostral to V II in squirrel monkeys

Abstract

A region of dorsal cortex along the rostral border of V II has been described as comprising a visual area or areas separate from more lateral cortex in both New and Old World primates. To evaluate these possibilities in squirrel monkeys, we studied patterns of cortical connections by injecting Fast Blue, Fluoro‐Gold, horseradish peroxidase, and wheat germ agglutinin conjugated to horseradish peroxidase into the dorsal region and related results to distinctions in myeloarchitecture. Our major conclusions are as follows. (1) The dorsal region (D) has distinctly different connections from the area found laterally, the caudal subdivision of the dorsolateral area (DL_C). These include major connections with the rostral subdivision of the dorsolateral area (DL_R), ventral posterior parietal cortex in the Sylvian fissure, the middle temporal area (MT), the medial superior temporal area (MST), ventral cortex just rostral to V II, and cortex in the inferior temporal sulcus. Weaker connections are with V I, V II, DL_C, the fundal superior temporal area (FST), and the frontal lobe. In contrast, DL_C, has strong connections with V II and inferior temporal (IT) cortex, weaker connections with DL_R, and lacks connections with ventral posterior parietal cortex (Steele et al: J Comp Neurol 306:495–520, 1991). (2) Caudal and rostral aspects of dorsal cortex differ in the magnitude of connections with V I, V II, DL_R, and FST. These differences are consistent with the previous proposal that at least two visual areas, caudal and rostral, occupy the dorsal region in squirrel monkeys (Krubitzer and Kaas: Visual Neurosci 5:165, 1990), but they could also reflect regional differences in the connections of a single visual area. (3) The dorsal region is more densely myelinated than surrounding cortex; however, rostral aspects of dorsal cortex are less myelinated than caudal aspects, again suggesting the existence of at least two areas. (4) The distinctiveness of connections between dorsal cortex and rostral as compared to caudal dorsolateral cortex provides further evidence for dividing the region of DL into two visual areas, DL_C and DL_R (Cusick and Kaas: Visual Neurosci 1:211, 1988; Steele et al: J Comp Neurol 306:495–520, 1991).