Blind spot (vision)
A blind spot, also known as a scotoma
A scotoma is an area of partial alteration in one's field of vision consisting of a partially diminished or entirely degenerated visual acuity which is surrounded by a field of normal - or relatively well-preserved - vision.Every normal mammalian eye has a scotoma in its field of vision, usually...

, is an obscuration of the visual field
Visual field
The term visual field is sometimes used as a synonym to field of view, though they do not designate the same thing. The visual field is the "spatial array of visual sensations available to observation in introspectionist psychological experiments", while 'field of view' "refers to the physical...

. A particular blind spot known as the blindspot, or physiological blind spot, or punctum caecum in medical literature, is the place in the visual field that corresponds to the lack of light-detecting photoreceptor cells on the optic disc
Optic disc
The optic disc or optic nerve head is the location where ganglion cell axons exit the eye to form the optic nerve. There are no light sensitive rods or cones to respond to a light stimulus at this point. This causes a break in the visual field called "the blind spot" or the "physiological blind spot"...

 of the retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

 where the optic nerve
Optic nerve
The optic nerve, also called cranial nerve 2, transmits visual information from the retina to the brain. Derived from the embryonic retinal ganglion cell, a diverticulum located in the diencephalon, the optic nerve doesn't regenerate after transection.-Anatomy:The optic nerve is the second of...

 passes through it. Since there are no cells to detect light on the optic disc, a part of the field of vision is not perceived. The brain
Human brain
The human brain has the same general structure as the brains of other mammals, but is over three times larger than the brain of a typical mammal with an equivalent body size. Estimates for the number of neurons in the human brain range from 80 to 120 billion...

 fills in with surrounding detail and with information from the other eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

, so the blind spot is not normally perceived.

Although all vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

s have this blind spot, cephalopod eye
Cephalopod eye
Cephalopods, as active marine predators, possess sensory organs specialized for use in aquatic conditions. They have a camera-type eye, which consists of a lens projecting an image onto a retina. Unlike the vertebrate camera eye, the cephalopods' form as invaginations of the body surface , and...

s, which are only superficially similar, do not. In them, the optic nerve approaches the receptors from behind, so it does not create a break in the retina.

The first documented observation of the phenomenon was in the 1660s by Edme Mariotte
Edme Mariotte
Edme Mariotte was a French physicist and priest.- Biography :Edme Mariotte was the youngest son of Simon Mariotte, administrator at the district Til-Châtel , and Catherine Denisot . His parents lived in Til-Châtel and had 4 other children: Jean, Denise, Claude, and Catharine...

in France. At the time it was generally thought that the point at which the optic nerve entered the eye should actually be the most sensitive portion of the retina; however, Mariotte's discovery disproved this theory.

The blind spot is located about 12–15° temporal and 1.5° below the horizontal and is roughly 7.5° high and 5.5° wide.

Blind spot test

! colspan="5" style="border:solid 1px #AAAAAA; " | Demonstration of the blind spot
| colspan="5" height="50px" |
| align="right" style="font-size: 0.7cm; width:0.4cm;"| O
| style="max-width: 8cm; width: 8cm" |
| align="left" style="font-size: 0.7cm; width:0.4cm;" | X
| colspan="5" height="50px" |
| colspan="5" style="border:solid 1px #AAAAAA;" | Instructions: Your face should be a few fingers away from the screen. Close the right eye and focus the left eye on the X. Now move away from the screen slowly, and at one point the O will disappear. This will happen approx. when the screen is about 25cm away (3x the width of this diagram). As you move further away, the O will reappear. If you close your left eye and look at the O, the X will disappear over a similar range.>

External links

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