VS142 Visual Neuroscience Striate Cortex: Anatomy

-> In primates (almost completely) all visual information that gets to cortex must first go through primary visual cortex. Four main synonyms: -> Primary Visual Cortex -> Area V1 -> Brodmann’s area 17 -> Striate cortex: Heavy band of myelinated axons in layer 4, the stria of Gennari (also band, path), about the only landmark you can see in cortex without special stains.

Cortical Magnification Factor! Chiasm crosses fibers from left and right, no crossing up/down.

Retinotopy of human V1, From Wong and Sharpe 1999

V1 is Brodmann’s cytoarchitectonic area 17 (the 17th area Brodmann named) These areas are defined using Nissl staining and looking at how cell bodies etc. vary in size and distribution from place to place. In some cortical regions this technique mis-classifies multiple areas as one area (such as “area 19”, actually multiple functionally distinct areas). But area 17 is pretty accurately the same as V1.

Gray vs. White matter (macaque). In any complex information processing system, the connections take a lot of space. Warning: this is NOT striate cortex, the bands here are due to a specific stain

Generic cell layers and arrangement of ‘generic’ human cerebral cortex. Granule layer (IV), Supragranular (I,II,III), and Infragranular (V, VI). Note: Cortex is packed solid with cell bodies, dendrites, axons, etc. Need differential stains to see anything. What you see depends on the stain!

V1 is also termed “koniocortex”, because it has a very well -developed layer IV. Layer IV is the (internal) granular layer, and is also the main input part of cortex, and as V1 is all about getting visual inputs, it has a granule layer that is relatively larger than other areas of the cerebral cortex. It’s so big that it is subdivided into multiple sub-laminae (A, B, C-alpha and C-beta). Other examples of koniocortex are primary somatosensory and primary auditory cortex.

No crossing over of fibers up/down, so the inversion of the image caused by the optics in the eye is preserved in primary visual cortex

From Sherbondy et al Following the Optic Radiations (OR) in living humans using a tract-tracing MRI algorithm.

Anatomical Variability: there is a great deal of variation in the size and extent of V1 between different people. Total surface area between different people varies as about 2.5 to 1. That is, there are normal people with 2.5 times the V1 surface area as other normal people. This has implications for things like interpreting the visual cortical evoked potential (VEP) or the magneto-encephalogram (MEG).

There is even a fair amount of variability between left and right V1 in the same person! This also means that you cannot tell where the borders of V1 (or other visual cortical areas) are in a standard x-ray, MRI, or CAT scan. So neurologists just talk about “the occipital lobe” as a single unit.