Relationship between laminar topology and retinotopy in the rhesus lateral geniculate nucleus: results from a functional atlas.

The primary focus of this paper is the abrupt transition that occurs midway through the rhesus lateral geniculate nucleus (LGN) from six layers posteriorly (conventionally numbered 1-6, ventral to dorsal) to four layers anteriorly. At this transition, layers 4 and 6 fuse into a single layer, as do layers 3 and 5, requiring an inversion of the stacking order of the cell categories making up layers 4 and 5. To understand the topology of this transition and its relationship to geniculate retinotopy, we have created a functional atlas of a rhesus LGN that affords three-dimensional views of morphology and retinotopy at a resolution of 25 microm. The projection of the path of the transition into visual space is highly biased toward lower visual fields, intersecting the upper vertical meridian at 6.4 degrees , the horizonal meridian at 15.4 degrees, and the lower vertical meridian at 25.0 degrees. Between inclinations of -31 degrees and 55 degrees, layers 3 and 5 merge through an elongated tear in layer 4 that subsumes the optic disk gap and extends medially and laterally; elsewhere, layers 4 and 6 merge through a tear in layer 5. These tears cause substantial violations of retinotopy and laminar integrity, so the inversion of layers 4 and 5 requires that the forces establishing retinotopy and grouping by cell class be locally overcome during morphogenesis. The transition and associated tears are evaluated in the context of recent computational models of geniculate morphogenesis. We have also used the atlas to estimate the borders of the binocular (55 approximately 62 degrees) and monocular (91 approximately 97 degrees) visual fields. Files containing the atlas are made publicly available on a website.