Areal, modular, and connectional organization of visual cortex in a prosimian primate, the slow loris (Nycticebus...

Slow lorises (Nycticebus coucang) are nocturnal prosimian (i.e. strepsirhine) primates, closely related to bushbabies (Galago spp.). We examined the organization of visual cortex in four hemispheres from two slow lorises, using connectional and architectonic techniques. All hemispheres were flattened and sections stained for myelin and cytochrome oxidase (CO). Our results indicate, first, that the primary visual area (V1) in slow lorises has a system of small CO-dense blobs, as has been described in most other anthropoid and prosimian primates examined to date. The second visual area (V2) is characterized by broad, stripe-like zones of dense CO staining separated by zones of lighter staining. Loris V2 stripes are less distinct than those of anthropoid primates, and separate classes of thin and thick dark stripes are not apparent. However, V2 stripes are much better developed than in Galago, where they are virtually absent. Injections of wheat-germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) in area V1 revealed reciprocal connections with area V2, and the middle temporal (MT) and dorsolateral (DL) extrastriate areas. Area MT was also identified by its distinctive, dense myelination. As has been reported in anthropoids, DL can be divided into separate caudal and rostral divisions, which differ in myelin and CO staining, and in the strength of their connections with V1. Taken together, our results suggest that many of the features that characterize visual cortex organization in anthropoid primates are present in prosimians and thus probably evolved early in primate history, prior to the diversification of modern primate groups.