In vertebrate eyes, the nerve fibers route
before the
retina, blocking some light and creating a
blind spot where the fibers pass through the retina and out of the eye. In
octopus eyes, the nerve fibers route
behind the
retina, and do not block light or disrupt the retina. In the example,
4 denotes the vertebrate blind spot, which is notably absent in the octopus eye. In vertebrates,
1 denotes the
retina and
2 the nerve fibers, including the optic nerve (
3), whereas in octopuses,
1 and
2 denote the nerve fibers and retina respectively.
A blind spot, also known as a scotoma, is an obscuration of the visual field. 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 of the retina where the optic nerve passes through it.[1] 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.[2]
[edit] Blind spot test
| Demonstration of the blind spot |
|
| O |
|
X |
|
|
| 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. O, the X will disappear over a similar range. |
[edit] See also
[edit] References
- ^ Gregory, R., & Cavanagh, P. (2011). "The Blind Spot". Scholarpedia. Retrieved on 2011-05-21.
- ^ MIL-STD-1472F, Military Standard, Human Engineering, Design Criteria For Military Systems, Equipment, And Facilities (23 Aug 1999) PDF
[edit] External links
