Primary visual cortex
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At the time the article was created Daniel Loh had no recorded disclosures.View Daniel Loh's current disclosures
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The primary visual cortex (Brodmann area 17 ) is also known as the calcarine cortex, striate cortex, or V1. It is the main site of input of signals coming from the retina. It is located on the medial aspect of the occipital lobe, in the gyrus superior and inferior to the calcarine sulcus. Most of the cortex lies within the deep walls of the calcarine sulcus.
The term striate cortex comes from its histological appearance in which the white-matter tracts of afferent fibers within lamina IV (the outer line of Baillarger) is just visible to the naked eye and is known as the line of Gennari 1,2.
Extending from the primary visual cortex is the visual association (extra striate) cortex which occurs throughout the rest of the occipital lobe and in certain sections of the parietal and temporal lobes. It is divided anatomically into Brodmann area 18 (V2 and V3) and area 19 (V3A, V4, and V5). These areas are functionally organized into two main pathways:
- ventral pathway, leading from V1 to V2 to V4 and into the inferior temporal lobe; thought to be responsible for color perception, object recognition and high-resolution perception
- dorsal pathway, leading from V1 to V2 to V5/MT (middle temporal) and into the parietal lobe; thought to be responsible for spatial vision such as motion and positioning
Corticotectal fibers extend from the primary visual cortex (and visual association cortex) to the superior colliculus in the tectum. These function in the control of eye movements and directing gaze.
Input from the retina is mapped to the primary visual cortex in a specific pattern:
- afferents from the temporal (lateral) half of the ipsilateral retina and from the nasal (medial) half of the contralateral retina corresponding to the same side of the visual field innervate the visual cortex on the same side; in simpler terms: the right side of the visual field is represented by the left occipital hemisphere and vice versa
- afferents from the inferior half of the retina (upper field of vision) innervate the gyrus inferior to the calcarine sulcus
- afferents from the macula project to the occipital pole while those from the peripheral retina travel more anteriorly
Lesions in the primary visual cortex result in a complete loss of visual awareness although some patients retain a degree of functional vision called 'blindsight'. These patients are able to discriminate the presence, location and movement of objects under forced-choice conditions despite reporting no awareness of them 4.