The human brain manages countless functions, including vision. Understanding how the brain controls vision and eye movement reveals a complex and surprising system.
The Brain’s Crossover Control
The brain operates using contralateral control, meaning one side of the brain governs functions on the opposite side of the body. For example, the left hemisphere controls movements and sensations on the body’s right side.
This contralateral organization extends to visual information processing. The right side of the brain interprets visual input from the left visual field, while the left side processes information from the right visual field. This ensures each brain half receives a complete picture of the opposite visual world.
The Visual Pathway: How Information Travels
Visual information begins when light enters the eyes and strikes the retina. Electrical signals are then sent from each retina along the optic nerve towards the brain.
The optic chiasm is a key point in this pathway, where optic nerves from both eyes meet. Here, fibers carrying information from the nasal (inner) halves of each retina cross to the opposite side of the brain. Fibers from the temporal (outer) halves remain on the same side. This partial crossing enables the brain’s contralateral processing of visual fields.
After the optic chiasm, the re-sorted fibers form the optic tracts, carrying visual signals to brain regions like the lateral geniculate nucleus (LGN) in the thalamus. From the LGN, signals are sent to the primary visual cortex in the occipital lobe. This is where visual input is processed and interpreted.
Understanding Visual Fields and Eye Control
When discussing brain control, it is important to distinguish between an “eye” and a “visual field.” Each eye captures light from both the left and right sides of our environment, meaning both eyes receive input from both the left and right visual fields.
The brain processes these visual fields contralaterally: the left brain handles the right visual field, and the right brain handles the left visual field. This ensures both hemispheres receive a complete representation of the visual world. While visual information processing is contralateral, eye movements are controlled by a complex network of brain areas, not solely by one side of the brain.
When Brain Control of Vision is Affected
Damage to parts of the visual pathway or brain’s visual processing centers can disrupt vision. For example, damage to the primary visual cortex in the right hemisphere might cause difficulty seeing in the entire left visual field, a condition known as homonymous hemianopia.
These impairments show how specific brain regions are linked to aspects of sight. The precise routing of visual information through the optic chiasm and to the occipital lobes means injury to a specific location can result in a corresponding loss of vision in a particular visual field.
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Citations:
Homonymous hemianopia. Cleveland Clinic. Retrieved from https://my.clevelandclinic.org/health/diseases/22216-homonymous-hemianopia (Accessed August 23, 2025).