The blind spot is the name given to the scotomatous area of each eye’s visual field that lacks visual input, due to the photoreceptor-free region of the retina where the optic nerve exits the eye.
The phenomenon of the blind spot was first described in 1668 by the French Roman Catholic priest and scientist Edme Mariotte, who used small circles of white paper to locate the region of visual space in which they disappeared . Prior to this discovery, the role of the optic disc was not clear, and scientists such as Leonardo da Vinci (1452–1519) had thought that the visual image actually fell only on the head of the optic nerve itself . The vertically oval area of the blind spot subtends about 5 × 7° of visual angle and corresponds to the projection in visual space of the optic nerve head, which has average vertical and horizontal disc diameters of 1.88 and 1.77 mm, respectively . The physiological blind spot is centered about 15° temporally from the point of fixation, due to the nasal anatomical location of the optic nerve head in relation to the fovea. One is normally not aware of the area of the visual field which corresponds to the blind spot in binocular vision, due to the fellow eye’s retinal and cortical representation of the corresponding area of the visual field. Even in monocular vision, this region is perceptually filled in at a cortical level using the brightness, color, and texture detected in the visual field adjacent to the blind spot . This filling in of the scotomatous area of visual field caused by the optic nerve head has also been shown to be an active process involving spatial integration, wherein the pattern and color of not just the area immediately adjacent to the scotoma but also from the more remote surround are used to perceptually fill in the scotomatous region .
The size of the blind spot strongly correlates with the area of the optic disc, as well as other features such as the region around the nerve head where the retina has wasted away (i.e. parapapillary chorioretinal atrophy)  and the surface topography of the disc, with the more prominent nasal part of the disc appearing less “blind” than the shallow temporal part, probably due to a higher degree of light scattering by the more prominent nasal disc region . In certain pathological conditions, most of which affect the optic nerve head, such as inflammation (i.e. optic neuritis) or optic disc swelling due to raised intracranial pressure, the area of the blind spot can become enlarged, and this can be detected clinically using automated visual field tests or, in more gross abnormalities, confrontational estimation of the blind spot size using a small target such as red hat pin in direct comparison with a normal observer’s blind spot. Such pathological conditions of the optic nerve are often associated with defects in color vision, most classically that of red desaturation. These defects may be further evident on testing using pseudoisochromatic plates, typically demonstrating more severe red-green deficits with milder blue-yellow losses . Only the vertebrate eye has a physiological blind spot, on account of light having to pass through the anterior retinal nerve fiber layer (which forms the optic nerve head as it exits the eye) in order to reach the photoreceptor layer beneath; cephalopods, such as the squid and octopus, have retinae wherein the nerve fiber layer lies behind the photoreceptor layer, and so have no physiological blind spot .
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