Lamination of primary visual cortex in the macaque: Layer 5 subdivisions

Human and non-human primates (NHPs) have a distinct lamination pattern of their primary visual cortex (V1) where layer 4 can be separated into four well-defined sub-laminae. Brodmann (1909) classified these into layers 4A, 4B, and 4C. Layer 4C can be further divided into 4Cα and 4Cβ based on inputs from the lateral geniculate magnocellular and parvocellular layers, respectively (Hubel Lund, 1973). We have examined the lamination pattern of layer 5 and have found that this layer has a trilaminar structure and have denoted the sublayers as layers 5Aα, 5Aβ, and 5B. This trilaminar division is prominent within the opercular region of the occipital cortex but is usually absent from V1 surrounding the calcarine fissure. In addition, with age we find the trilaminar arrangement of layer 5 within the opercular region interdigitates with a bilaminar (denoted as layers 5Aβ, 5B) pattern. We also show that in a bilaterally enucleated macaque, at embryonic age of 63 days, postnatally, layer 5 is also trilaminar around the occipital cortex and bilaminar around the calcarine fissure. These facts imply that the interaction of the geniculo-cortical input and local cortical development takes place in the absence of any visual experience, and we propose, functionally that this trilaminar arrangement of layer 5 may play a part in processing of central vision.