A blind spot is a small, physiologic, slightly elongated-oval visual field defect caused by the location of the optic disc, the exit gate of the collected optic nerve fibers. In the area of the optic disc, the retina is interrupted so that no light stimuli can be perceived at the site. Normally, the blind spot is not perceived because the brain has an ingenious “image processing program” and adds the blind spot according to the environment and logic.
What is the blind spot?
The blind spot is physiological because the bundled optic nerve fibers derived from each photoreceptor (rods and cones) leave the eye in the optic disc area to carry the signals to the visual center. In addition, blood vessels are also passed through. In the area of the optic disc, the retina is pierced, so that no photoreceptors can be located at the site. The optic disc is located about 15 degrees nasally outside the visual axis, which opens directly into the yellow spot, the zone of sharpest and best color vision. The magnitude of the visual field loss due to the papilla is about 5 – 6 degrees horizontally and about 7 – 8 degrees vertically. The optic discs of both eyes are arranged so that the blind spots in the visual field do not overlap. This ensures that when seeing with both eyes, no object is in the blind spot of the left eye and the right eye at the same time, which would mean that it could no longer be perceived.
Anatomy and structure
The optic disc (papilla nervi optici), the passage through the retina that causes the blind spot in the field of vision, is about 1.6 to 1.7 mm in diameter. Individually, the optic discs can also deviate significantly from the above mean, so that if the size is appropriate, we speak of macropapillae or micropapillae. During an ophthalmoscopy, an optical inspection of the ocular fundus, the papilla with the bundled nerve fibers is clearly visible. Their pale red color contrasts clearly with the dark red color of the retina. The bundled, purely sensory, afferent nerve fibers of the individual photoreceptors leave the eye through the optic disc as the optic nerve, also called the second cranial nerve, and pass the signals on to specific areas in the brain. In addition, the optic disc is used by the central artery of the eye (Arteria centralis retinae) as an entrance port and by the central ophthalmic vein as an exit passage. Thus, the optic disc is not only available to the photoreceptors for the discharge of their action potentials, but also serves the central supply and disposal of the eye by the blood circulation.
Function and Tasks
The blind spot itself has no function or tasks; rather, it should be considered a problem or necessary evil that exists because of the way the eye is constructed. The papilla, which causes the blind spot, has the task of minimizing the disadvantages of the blind spot while still fulfilling its function of smoothly guiding the bundled nerve fibers of the photoreceptors and the necessary blood vessels out of or into the posterior wall of the eye. In principle, there is a conflict of goals to make the papilla as narrow as possible to keep the blind spot as small as possible, but this may lead to functional impairment of the nerve fibers and performed blood vessels due to pressure lesions. Since the visual field loss (scotoma) of the blind spot cannot be prevented physiologically-mechanically, the visual center in the brain has developed, by evolutionary means, a virtual image processing program that allows, when seeing with both eyes (binocular vision), to supplement the visual field loss with visual impressions from the other eye, so that the blind spots cannot be consciously perceived. Even when seeing with only one eye (monocular vision), the blind spot is not perceived because the visual center virtually supplements the scotoma analogously to the surrounding visual impressions. The process is called filling-in. This can be verified in a simple experiment. If we look monocularly at a regular pattern that has a small gap such that the gap coincides with the blind spot, the pattern suddenly appears to fill in. We no longer see the gap in the pattern because the visual center is unaware of it due to the real blind spot and logically completes the visual field with the surrounding pattern. We see something seemingly real that exists only virtually.
Diseases
Diseases and conditions that may be associated with the blind spot necessarily relate to the optic disc and possible dysfunction of the nerve fibers and blood vessels that pass through it. The most common disorder of the optic disc is papilledema, also called congestive papilledema. The disorder usually occurs bilaterally and in advanced stages leads to pressure lesions on the optic nerves and blood vessels. Papilledema is easily visible when looking at the back of the eye. The color of the optic disc changes from pale pink to red or grayish red, and grayish white deposits become visible. The margins of the optic disc are severely swollen at this stage. In the even more advanced atrophic phase, in which the tissue dies, the optic disc appears very pale and optic nerve fibers die irreversibly. This results in typical visual field deficits. Usually, secondary diseases such as increased intracranial pressure due to cerebral hemorrhage or brain tumors are the triggers of optic disc edema. However, they can also be caused by optic neuritis due to nerve toxins (neurotoxins) or diseases such as Lyme disease, multiple sclerosis and others. Chronic high blood pressure and diabetes mellitus can cause fatty deposits in the optic artery and lead to optic disc infarction.
