Laminar organization of geniculostriate projections. A common organizational plan based on layers rather than...

The types of laminar segregation of neuronal classes found in the lateral geniculate nucleus (GL) of mammals are almost as varied as the groups themselves which exhibit lamination. This essay deals with the question of whether, in spite of the differences in which classes are segregated by layer in different species, one rule might apply to the laminar distribution of projections from the GL to the striate cortex. Until recently, the weight of experimental evidence suggested that the laminar or sublaminar distribution of such projections was organized so as to segregate functional classes (i.e. W, X, Y, on center, off center). While this model appears to be consistent with the organization of geniculostriate projections in many species (especially primates), recent studies on species other than primates suggest that the laminar distribution of geniculostriate projections is not based on a segregation of functional classes per se, but on the segregation of projections from whole GL layers (or layer pairs) including all of their constituent cell classes.