The Five Senses and Beyond: The Encyclopedia of Perception - Jennifer L. Hellier 2017

Blind Spot

Each vertebrate animal has a natural blind spot or scotoma in its vision. The term scotoma is derived from the Greek word meaning “darkness.” Thus, a scotoma is an obstruction of the visual field surrounded by a field of normal vision. The field of vision is everything that a person sees either straight ahead or peripherally. Abnormal or pathological scotomata (plural form of scotoma) can develop from disease processes that affect the retina or the optic nerve.

History

The first documented recording of the eye’s blind spot was by French physicist and priest Edme Mariotte (ca. 1620—1684). While at the French royal court, Mariotte showed this phenomenon by placing a small coin at a certain distance from a person’s right eye. He asked the volunteer to close his left eye and with his right eye to look left. The coin instantly disappeared from the person’s vision, which made people think it was a “magic coin.” For his research in physics, particularly for acknowledging Boyle’s law and its inverse relationship for gases, Mariotte was asked to join the French Academy of Sciences.

Anatomy and Physiology

In vertebrates, there is an anatomical blind spot that is caused by the lack of photosensitive receptors (rods and cones) and cells in one location of the retina. This area is called the optic disc, which can be viewed using specific tools such as an ophthalmoscope. In most humans, the optic disc is found in each eye about three to four millimeters toward the nasal side of the fovea. The fovea is the center of the macular region of the retina, which is the section responsible for precise and high-resolution vision. The optic disc is an oval shape that is thinner vertically and about 0.2 millimeter wider horizontally. The optic disc is the beginning of the optic nerve, or cranial nerve II, and it is where the optic nerve exits the eye. Anatomically, the output of the retina, called the retinal ganglion cell axons, all come together at the optic disc while blood vessels enter and exit the eye at the same spot. Thus, there are no light-sensitive receptors or neurons within the optic disc to receive and interpret signals. This makes a true anatomical blind spot.

However, the blind spot is generally not perceived during vision, especially when both eyes are open. This is because the signals of the visual fields have an overlap or are slightly different. The blind spot in one eye will not be noticed because the visual cortex of the opposite eye will take over that portion of the entire visual field. In general, the brain takes the surrounding information around the blind spot in each eye as well as information from the opposite eye to compensate for the blind spots. The blind spots are not perceived and normal vision is the result. It is only in certain circumstances, as in the “magic coin” experiment performed by Mariotte, that the blind spot can be tested. To test your own blind spot, see http://faculty.washington.edu/chudler/chvision.html.

There are some animals that do not have an anatomical blind spot, such as octopuses or other cephalopods. In their eyes, the optic nerve fibers all group together behind the retina. This is different from vertebrates whose optic nerve fibers group together in front of the retina. Not only does this anatomical orientation in vertebrates make a scotoma, but it also blocks some of the light that should “hit” the retina.

Diseases, Disorders, and Treatments

Some diseases or disorders that affect the retina or the optic nerve can result in a pathological scotoma that can be temporary or permanent depending on whether permanent damage occurs to the retina or the optic nerves. These scotomata are abnormal and can be seen in any part of the visual field, can have any shape, and can be any size. Additionally, these pathological scotomata can enlarge the normal anatomical blind spot. If the scotoma affects the macula, it can cause severe vision loss, as high-resolution vision will be compromised. On the other hand, if the scotoma affects peripheral vision, people may not even notice it because they can turn their heads to see what they are missing. This adjustment does not help with scotomata in the macular region.

The most common causes of pathological scotomata are multiple sclerosis, high blood pressure, drinking methyl alcohol, nutritional deficiencies, and tumors that block or damage the retina and/or the optic nerve. In most patients, a scotoma affects only one eye. However, in rare cases, bilateral scotomata can occur if there is a tumor of the pituitary gland. This is because the pituitary gland is inferior to the optic chiasm. When the tumor enlarges, it compresses the optic chiasm, causing a bitemporal paracentral scotoma, which can lead to bitemporal hemianopsia that extends to the periphery. If a person presents with bitemporal hemianopsia, it is usually the first sign of a pituitary tumor. Temporary pathological scotomata can occur (1) during migraines, when a bright light during the aura causes the blind spot; and (2) during pregnancy, especially if the woman is having severe hypertension (also called severe preeclampsia).

Patricia A. Bloomquist

See also: Occipital Lobe; Optic Nerve; Retina; Sensory Receptors; Tunnel Vision; Visual Fields; Visual Perception; Visual System

Further Reading

Bear, Mark F., Barry W. Connors, & Michael A. Paradiso. (2007). Neuroscience exploring the brain (3rd ed.). Baltimore, MD: Lippincott Williams & Wilkins.

Chudler, Eric H. (n.d.). Neuroscience for kids: The blind spot. Retrieved from http://faculty.washington.edu/chudler/chvision.html

Tasman, William, & Edward Jaeger. (2012). Duane’s ophthalmology (13th ed.). Philadelphia, PA: Lippincott Williams & Wilkins.