Where it is
- The visual cortex is in the occipital lobe, at the back of the head, and both hemispheres have one. [1]
- The primary visual cortex (V1) lies in and around the calcarine sulcus, also called the calcarine fissure, and at the posterior pole of the occipital lobe. [1]
- Human V1 lies on the medial (inner) side of the occipital lobe within the calcarine sulcus, and its full extent often continues onto the occipital pole. [4]
- The calcarine sulcus runs along the inner face of the occipital lobe, with the cuneus above it and the lingual gyrus below it. [2]
- Around V1 lie the extrastriate areas, V2 to V5, which make up Brodmann area 18 and all of area 19. [1]
- Its size varies a lot: in a study of 25 hemispheres from 15 adults, V1 averaged about 2,400 mm² of surface per hemisphere, and the sizes of V1, V2 and V3 can differ three-fold between people, a difference that is partly inherited. [1]
What it does
- V1 is the earliest and simplest visual area of the cortex: it is the area that receives the visual input relayed by the lateral geniculate nucleus of the thalamus. [1]
- Each side works on the opposite half of what you see: the left visual cortex receives signals from the right visual field, the right one from the left visual field. [1]
- Its map of the visual field is very precise, down to the blind spots of the retina; the small centre of gaze served by the fovea takes up a large share of V1, which is called cortical magnification. [1]
- Its neurons are tuned to simple features such as edge orientation, spatial frequency and colour, most prefer one eye (ocular dominance), and neurons with similar tuning cluster in columns. [1]
- From V1 the information splits into two streams: the ventral “what” stream through V2 and V4 to the inferior temporal cortex, for recognising forms and objects, and the dorsal “where” or “how” stream through V2 and MT (V5) to the posterior parietal cortex, for motion, location and guiding the eyes and arms. [1]
Connections
- Signals from the retina run through the optic tracts to the lateral geniculate bodies, from which the optic radiations continue to the visual cortex. [4]
- Input: each hemisphere’s V1 receives the signals from the eyes directly from the lateral geniculate nucleus of the thalamus on its own side. [1]
- V2 receives strong feedforward connections from V1, directly and through the pulvinar, and sends strong feedback connections back to it. [1]
- V1’s output also reaches the superior colliculus in the midbrain, which on one hypothesis reads it to steer shifts of gaze. [1]
- Its blood comes mainly from the calcarine branch of the posterior cerebral artery. [1]
Other names
- The primary visual cortex is also called visual area 1 (V1) and the striate cortex, approximately Brodmann area 17; “striate” refers to the line of Gennari, a stripe of myelinated fibres visible to the naked eye. [1]
- The extrastriate areas V2, V3, V4 and V5 correspond to Brodmann area 18 and all of area 19. [1]
Size
- The model has no shape for the visual cortex itself, so it is not measured here. The whole left occipital lobe that holds it measures about 63 mm front to back, 54 mm top to bottom and 44 mm side to side on this model (the box it fits in, not its volume). [3]