Direct W-like geniculate projections to the cytochrome oxidase (CO) blobs in primate visual cortex: axon morphology.

The primate lateral geniculate nucleus (LGN) is composed of large, medium, and small cells located, respectively, in magnocellular (M), parvocellular (P), and specialized layers (intercalated and S-layers in simians, koniocellular (K) layers in prosimians). Several studies have examined the physiology and connections of M and P LGN cells and have concluded that they provide separate contributions to visual perception via separate pathways. Less is known about the structure and contributions of the small LGN cells. This study examined the distribution and structure of K LGN cell axons in the cortex of the prosimian, Galago crassicaudatus. Wheat germ agglutinin conjugated to horseradish peroxidase, or Phaseonlus vulgaris leucoaglutinin, was injected into the LGN K layers to demonstrate the overall axon projection pattern and the details of individual axons, respectively. Location of axons within striate cortex was specified relative to boundaries determined by Nissl or cytochrome oxidase (CO) stains on the same or adjacent sections. Our results show that K LGN axons end as single complex arbors within one CO blob zone in layer III; they never terminate in interblob zones. These axons also emit a collateral in layer I that arborizes more broadly and spans both CO blob and interblob zones. These data, together with data on K cell physiology and intralaminar cortical connections, suggest that the LGN small cell pathway could modulate the activity of the other two pathways in striate cortex and contribute directly to visual perception.