primary visual cortex

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The part of the visual cortex to which the photoreceptors in the retinas of the eyes are eventually projected, occupying the gyri on both sides of the deep calcarine sulcus in the occipital cortex, containing 200 million cells within a total area of 25 square centimetres and arranged in an inverted topographic map of the object or scene being viewed, with neighbouring cells having receptive fields close together and being stacked vertically in ocular-dominance columns and orientation columns. It contains six layers of cells numbered 1 to 6 from the top, layer 4 being subdivided into three sublayers 4A, 4B and 4C, layer 4C receiving inputs from the photoreceptors in the retina via the lateral geniculate nuclei, the upper part 4Cα receiving inputs from the magnocellular layer and the lower part 4Cβ from the parvocellular layer of the geniculate nuclei. All layers except layers 1 and 4A have neuronal connections outside the primary visual cortex: layers 1 and 2, which contain blobs for colour vision and are concerned with both form and colour, receive inputs from layers 4Cα and 4Cβ, and layer 2 projects to Area V2 of the visual cortex; layer 4B, concerned with movement and stereopsis, receives inputs from layer 4Cα and projects to Areas V2 and V5 (or MT) of the visual cortex, layer 4Cα (in the magnocellular pathway) projects to layer 4A, and layer 4Cβ (in the parvocellular pathway) projects to layers 2 and 3 and thence to Area V2. Layers 5 and 6 are less well understood but are known to project to deep structures within the brain, layer 5 to the superior colliculus and layer 6 back to the lateral geniculate nuclei. Also called the striate cortex and known technically as Area V1 or Brodmann area 17. See also retinotopic map, visual cortex.

Subjects: Psychology.

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