Definition (Blind spot):
A blind spot, scotoma, is an obscuration of the visual
field. A particular blind spot known as the blind spot, physiological
blind spot, "blind point", 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
of the retina
where the optic
nerve passes through the optic disc. Since there
are no cells to detect light on the optic disc, a part of the field of vision is not perceived. The brain
interpolates the blind spot based on surrounding detail and information from
the other eye,
so the blind spot is not normally perceived.
Although all vertebrates
have this blind spot, cephalopod eyes, 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 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.
Instructions:
Close one eye and focus the other on the appropriate letter (R for
right or L for left). Place your eye a distance from the screen
approximately equal to 3× the distance between the R and the L.
Move your eye towards or away from the screen until you see the other letter
disappear. For example, close your right eye, look at the "L" with
your left eye, and the "R" will disappear.
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