NEUROSCIENCE: VISION -- Visual Cortex: Primary Visual Cortex

VISUAL CORTEX: Primary Visual Cortex

A. Primary Visual Cortex (synonyms: V1, striate cortex, area 17)

1. Located in occipital cortex (calcarine fissure/sulcus); mainly on medial aspect

2. Visual field mapped on V1, with cortical area proportional to number of optic nerve fibers -- that is, relatively large area devoted to fovea/macula; map preserves spatial relations (inverted) of the visual fields; the left visual field is represented on the right occipital cortex and the right visual field on the left occipital cortex (==>)

3. Primary visual cortex composed of six layers of cells, layers 1 (superficial) to 6 (deep), parallel to the cortical surface, and several million columns perpendicular to the surface; each column represents a functional unit; input from the lateral geniculate synapses with neurons in layer 4, which then project to both more superficial and deeper layers; axons from various layers then project to other CNS areas (==>)

4. Each column represents a particular property (e.g. form, color) from a particular region of the retina of a given eye; some columns detect line orientation, some detect color, etc. (==>)

5. Columns are arranged in groups representing all line orientations and all colors ("blob") from a particular retinal region in a given eye; adjacent groups from the two eyes are arranged in pairs (sometimes termed ocular dominance pairs), one representing an area on the retina of the right eye and the other the corresponding area of the retina of the left eye

Note phenomenon of ocular dominance: when the images from the two eyes are not consistent, only the image from the dominant eye is perceived

Amblyopia: chronic suppression of image from one eye

6. Hypercolumns: columns representing properties (line orientation, color or "blobs") from both eyes of a particular area of the visual field are grouped together in a small area of the visual cortex. Each of these regions is termed an hypercolumn

7. Hypercolumns are interconnected to extract properties

a. fuse images from the two eyes
b. detect color by comparing input from adjacent cones with differing visual pigments (blobs)
c. enhance contrast and detect edges
d. respond to movement
e. detect line orientation (e.g., horizontal or vertical) at a specific position ("simple" cells)
f. detect line orientation independent of position ("complex" cells)
g. detect lines of specific shapes or specific lengths ("hypercomplex" cells)