How Vision Works: From Eye to Brain

By Stuart Mathieson · Independent web developer in Scotland with an interest in all things science. · Last updated 2026-10-14 · Editorial policy

Who Does What

From Light to Signals

Seeing starts in the retina at the back of the eye, where around 120 million rods (for dim light) and 6 million cones (for colour and fine detail) turn light into electrical signals. Before those signals even leave the eye, layers of retinal cells start processing them, enhancing edges and contrast. About a million nerve fibres then carry the result out of each eye along the optic nerve.

Where the two optic nerves meet, at the optic chiasm, fibres from the inner half of each retina cross to the other side. The result is that everything you see to your left - from both eyes - ends up in the right hemisphere, and everything to your right ends up in the left. That's why a stroke in one occipital lobe can wipe out the same half of vision in both eyes.

The Journey Through the Brain

Most fibres travel to the lateral geniculate nucleus of the thalamus, which relays the signals onward along a fan of fibres (the optic radiations) to the primary visual cortex in the occipital lobe. Some of these fibres loop forward through the temporal lobe on the way, which is why temporal lobe surgery can sometimes cause a small loss in the upper part of vision. A smaller set of fibres goes to the superior colliculus in the midbrain (part of the brain stem), which helps turn the eyes and head toward sudden movement.

The primary visual cortex is just the start. From it, information flows into a series of specialised areas, and then along two broad streams. The ventral stream runs down into the temporal lobe and works out what you're looking at. The dorsal stream runs up into the parietal lobe and works out where things are and how to act on them. Psychologists David Milner and Melvyn Goodale described a patient, D.F., who couldn't say whether a slot was horizontal or vertical but could post a card through it perfectly - her "how" stream still worked when her "what" stream didn't.

Landmark Discoveries

In 1962, David Hubel and Torsten Wiesel recorded from single neurons in the visual cortex and found that each responded best to a line or edge at a particular angle. Building up perception from such simple features became a foundation of neuroscience, and of computer vision.

Damage to specialised areas shows how divided the work is. In 1983, Josef Zihl and colleagues described a woman who, after damage on both sides of her brain, lost the ability to see motion. Pouring tea looked to her like a frozen glacier, and she couldn't tell when to stop pouring. Damage to a face-selective area on the underside of the temporal lobe - the fusiform face area, identified with brain imaging by Nancy Kanwisher in 1997 - can cause prosopagnosia, or face blindness.

Perhaps strangest of all is "blindsight". In 1974, Lawrence Weiskrantz and colleagues studied a man who was blind in part of his vision after occipital surgery. Asked to guess where a light was in his blind area, he pointed to it far more accurately than chance, while insisting he saw nothing - evidence that routes bypassing the primary visual cortex can still guide behaviour without awareness.

Common Myths

"We see with our eyes."

The eyes collect light, but seeing is built by the brain. It fills in the blind spot where the optic nerve leaves each eye, smooths over blinks, and uses expectations to interpret what's in front of us - which is why optical illusions work.

"The brain flips the upside-down image from the eye."

The image on the retina is upside down, but there's no picture inside the head that needs turning round. Visual areas simply learn which signals correspond to which directions in the world.

"In blind people, the visual cortex does nothing."

In people who have been blind from early life, the "visual" cortex is taken over for other jobs. A 1996 study found it became active when blind participants read Braille by touch.

When It Goes Wrong

These conditions affect vision in different ways, depending on which part of the network is involved:

• Stroke & Aphasia - Stroke occurs when blood supply to part of the brain is interrupted; when it damages language centres, it can cause aphasia - difficulty producing or understanding language.
• Migraine - A neurological condition causing recurrent, often severe headaches, frequently accompanied by nausea and sensitivity to light or sound, and in some cases preceded by visual or sensory 'aura' symptoms.
• Multiple Sclerosis - A chronic autoimmune condition in which the immune system attacks myelin, the insulating sheath around nerve fibres in the brain and spinal cord, disrupting communication between the brain and the rest of the body.

Key Terms

• Occipital lobe
• Thalamus
• Contralateral
• Cerebral cortex
• Neuroplasticity

More on How the Brain Works

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Sources & Further Reading

• How the Eyes Work - National Eye Institute
• Receptive fields, binocular interaction and functional architecture in the cat's visual cortex - Hubel & Wiesel (1962), The Journal of Physiology
• Separate visual pathways for perception and action - Goodale & Milner (1992), Trends in Neurosciences
• Selective disturbance of movement vision after bilateral brain damage - Zihl et al. (1983), Brain
• The fusiform face area: a module in human extrastriate cortex specialized for face perception - Kanwisher et al. (1997), The Journal of Neuroscience
• Visual capacity in the hemianopic field following a restricted occipital ablation - Weiskrantz et al. (1974), Brain
• Activation of the primary visual cortex by Braille reading in blind subjects - Sadato et al. (1996), Nature

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