By Sodaba Khatab, ophthalmologist
Part 1: The Visual System
The visual system consists of the eye, the optic nerve, and the neural pathways in the brain involved in vision. It is the most complex sensory system in the human body. We need the visual system for our social development, in which eye contact and the recognition of facial expressions and body language are important. We also use it for more basic functions such as spatial awareness and danger detection, as well as more complex tasks like reading and appreciating art, etc. The figure above illustrates the visual system. Light enters the eye and, via the photoreceptors in the retina, is transmitted as a signal through the optic nerve to the visual cortex at the back of the brain (the area colored orange). From the visual cortex, the information travels to other areas of the brain for further processing. The visual system functions as part of the overall sensory system. All sensory systems—such as hearing, touch, balance, taste, and smell—together form an integrated system in the brain.
Part 2, Development of the Visual System
The primitive eye and visual pathways begin to form between 23 and 24 weeks of pregnancy. At that point, the eyelids are still closed. Between 24 and 28 weeks of pregnancy, the eyelids open, and the fetus may be able to perceive bright light. It is not until the last trimester of pregnancy that visual functions truly develop. In the womb, there is little to no light, which is why the visual system is the least developed of all the sensory systems during pregnancy. There are many sounds in the womb—such as the mother’s heartbeat and bowel sounds—which is why hearing is particularly dominant at birth. At birth, a baby can perceive light, contrast, and movement. The ability to determine “where” something is located develops in particular. By 2 months, a baby begins to perceive more details, and the ability to recognize “what” something is develops. By 3 months, the eyes work together effectively, and vision becomes more dominant than hearing. The baby recognizes faces or their bottle and begins to touch and explore everything within their field of vision, dropping objects to learn how to estimate distances. By 8 months of age, the number of connections in the brain’s visual system has doubled. Between the ages of 1 and 6, vision continues to improve, and there is still a great deal of neuroplasticity. This is the ability of our nervous system to adapt to (changes in) the environment and sensory perception. In this age group, therefore, a lazy eye can still be treated optimally. We believe that neuroplasticity begins to decline in the 7- to 10-year-old age group. We observe that the number of neural connections begins to decline again around that age, with only the connections that are actively used persisting. Starting around age 12, “higher-order visual systems”—such as spatial awareness—begin to develop.
Part 3: Eye Removal and Its Impact on the Brain
Little research has been conducted on the effects of eye removal on the human brain. However, there are animal studies and hypotheses based on research involving blind people.
The Effect of Eye Removal on the Brain in Children
Nerve connections from a removed eye largely disappear, and the connections from the remaining eye to the brain increase. There is also a reorganization of the visual cortex and a strengthening of the other sensory systems. Visual acuity and contrast sensitivity in the remaining eye are often higher, and sound localization is often improved. Perception of motion and speed appears to be reduced, as are higher-level visual functions such as facial recognition. Due to children’s neuroplasticity and greater adaptability, eye removal appears to have less of an impact on functioning than it does in adults.
The Effect of Eye Removal on the Brain in Adults
In the short term, there is an increased signal from the removed eye to the brain. This is likely because sensitivity to this eye increases, since initially very little information from this eye reaches the brain. In the long term, the nerve connections to the removed eye degenerate. The visual cortex in the brain is then recruited for other functions. The connection between the visual system and the other sensory systems is strengthened. Finally, the other sensory systems are enhanced. After an eye removal in adults, there is a small risk of “phantom eye syndrome.” This can include phantom pain, phantom sensations, or phantom vision. Phantom pain often occurs in the first few days after surgery and typically subsides afterward. It can last from seconds to hours. Phantom sensations may include itching or the feeling that an eye is still open. Phantom vision can occur in the “simple” form, in which no clear shape or figure is perceived, or in the “complex” form, in which people, animals, buildings, or landscapes are perceived. This, too, often begins in the first few days after surgery and then subsides; it usually lasts a few minutes. For a small percentage of people with this syndrome, the symptoms do not go away on their own. Psychological support and medication prescribed by a pain management team can help in such cases.
