Organization of the feedback pathway from striate cortex (V1) to the lateral geniculate nucleus (LGN) in the owl monkey (Aotus trivirgatus)

The function of the corticogeniculate feedback pathway from the striate cortex (VI) to the lateral geniculate nucleus (LGN) in primates is not well understood. Insight into possible function can be gained by studying the morphology and projection patterns of corticogeniculate axons in the LGN. The goal of this research was to examine how corticogeniculate axons innervate the functionally specific (e.g., parvocellular [P], magnocellular [M], and koniocellular [K]) and eye-specific layers of the LGN. Pressure injections of biotinylated dextran were made into owl monkey V1, and the resulting labeled axons were reconstructed through serial sections of the LGN. All of the corticogeniculate axons, regardless of termination pattern, were thin with boutons en passant or at the ends of small stalks, as described in cats. Axons were found in all layers of the LGN, and two main patterns of innervation were observed. In the first pattern, axons terminated in individual M or P LGN layers. In the second pattern of innervation, axons terminated in pairs of functionally matched layers. Examples of this type were seen within pairs of M, P, or K layers. In most cases, both classes of axons contain arbors focused within the P or M layers but also had collateral side branches in neighboring K layers. Unlike corticogeniculate axons seen in the cat, corticogeniculate axons in the owl monkey maintained topographic innervation in the LGN layers that was consistent with receptive field sizes represented in V1. The patterns of layer projections along with the retinotopic match of corticogeniculate axons within the LGN suggest that in primates VI can modulate activity in the LGN through functionally specific projections in a more tightly tuned retinotopic fashion than previously believed.