The Human Brain : From Neurone to Nervous System
VISUAL PATHWAY and CORTEX
- The primary visual cortex (VI) is located above and below the calcarine sulcus (Brodmann's area 17) of each occipital lobe. It is sometimes called the striate cortex, because of a visible stripe (the line of Gennari) that runs parallel to the surface.
Area 17 in each hemisphere contains a map of the contralateral visual field, using inputs from the lateral half of the retina of the ipsilateral eye, and the medial half of the retina of contralateral eye. These inputs are relayed by lateral geniculate nuclei of the thalamus.
The fovea has a disproportionately large area of visual cortex surrounding the anterior end of the calcarine sulcus and the peripheral visual field is represented by smaller areas posteriorly .
The primary visual cortex (V1) analyses its input in terms of linear orientation, contrast, and to some extent colour: the cortical columns respond to the orientation of linear features in the visual field. Adjacent columns deal with the inputs from the two eyes separately. A comparison of inputs to adjacent columns is thought to contribute to an assessment of depth within the visual field, and contribute to convergent movements of the eyes.
Other areas of the occipital lobe are involved in higher level processing of visual images, and can be divided into areas V2 thru V6, each of which has multiple functions.
Colour is processed partly in V1, but also by adjacent areas, particularly the V4 area of the lingual gyrus; it had been thought that V4 was the major area concerned with colour, but other areas are known to be involved in colour processing.
Key Words: Visual Cortex; Colour Vision; Cortical Columns