Definition
A blind spot is the area in the field of vision of each eye where there are no sensory cells that can receive light. This is a naturally occurring defect in the visual field (scotoma) – i.e. an area in which we are blind.
Structure of the blind spot
Anatomically, the blind spot corresponds to the optic nerve papilla (papilla nervi optici), where the optic nerve leaves the eye. Due to the development of the eye, the conductive fibres of each light-sensitive sensory cell are located further in the centre of the eye than the sensory cells themselves. In addition to a slight deterioration in the resolving power of our eye, this leads to the problem that when the fibres leave the eye, they have to penetrate the layer of sensory cells.
This happens in the optic nerve papilla, which therefore cannot contain any sensory cells and is not sensitive to light. The blind spot is located in the visual field of each eye, shifted 15° towards the nose. Because of the refraction of light through the lens of the eye, the area in the field of vision is 15° off-center on each side of the visual axis. The fact that the healthy person is not aware of the lack of visual information at this point is due to the excellent performance of our brain to deduce the image in the blind spot from the surrounding areas, the information from the other eye and by calculating different images from different eye movements.
How big is the blind spot?
The blind spot has a diameter of about 1.6-1.7 mm. It is a passageway (papilla) through which both nerve fibres and the corresponding blood vessels leave the eyeball. It is kept as small as possible by the body, but must also be large enough for the number of fibres passing through. If it is too small, it would crush the vessels and the eye could be damaged. The above-mentioned size is an average value, which can also vary slightly up or down in individual cases.
What function does the blind spot have?
A blind spot is the physiological exit point of the optic nerve from the eyeball. This point itself has no function. Here, the nerve fibres of the optic nerve leave the eye as a bundle on their way to the brain.
There are no photoreceptors at this point. As a result, no visual performance can be recorded here either and the person cannot see anything there. The blind spot is kept as small as possible by the body in order to create only the smallest possible loss of visual field.
However, it must also be large enough to allow the nerves and blood vessels to pass on without bruising. The loss of the field of vision is compensated by optical impressions of the other eye in the brain so that the blank spot is not noticeable. The brain is able to compensate for the missing spot and thus ensures that everyone can naturally perceive an overall picture of the surroundings.
