How light becomes sight: rods and cones in the retina, the blind spot, the optic chiasm, the thalamus, the primary visual cortex and the 'what' and 'where' streams.
Senses & Perception
Intermediate
Vision: From Eye to Brain
The Editors
February 28, 2026
3 min read
In plain English
Your eye focuses light onto the retina at the back, where rods and cones turn it into electrical signals. These go through the optic nerve to the thalamus and then to the back of the brain. From there, two main pathways work out what you're looking at and where it is.
Intermediate level: Builds on the basics. Introduces the real vocabulary and the mechanisms behind it.
Concepts to know first
Vision is the most studied sense in neuroscience, and in primates a large share of the cortex is involved in it.
The retina
The cornea and lens focus an upside-down image onto the retina, a thin layer of neural tissue at the back of the eye — technically an outgrowth of the brain. It contains about 120 million rods, extremely sensitive to dim light but colour-blind, and about 6 million cones, which need brighter light and handle colour and fine detail. Cones are packed into the fovea, the small central pit giving the sharpest vision. Light-sensitive proteins (opsins) in the photoreceptors change shape when they absorb a photon, starting the electrical signal.
The retina already processes the image. Its circuits — bipolar, horizontal, amacrine and retinal ganglion cells — enhance edges and contrast. Ganglion cell axons, about a million per eye, bundle into the optic nerve. Where they leave the eye there are no photoreceptors: the blind spot, which the brain fills in.
Crossing over
At the optic chiasm, fibres from the inner (nasal) half of each retina cross to the other side. The result: the left half of the brain receives information about the right half of the visual field from both eyes, and vice versa.
Thalamus and cortex
Most fibres go to the lateral geniculate nucleus of the thalamus, then to the primary visual cortex (V1) in the occipital lobe, which detects edges, orientations, motion and colour. Some fibres go to the midbrain's superior colliculus to steer eye movements, and some to the hypothalamus to set the body clock.
Two streams
From V1, processing splits. The ventral stream into the temporal lobe identifies objects, faces and words ("what"). The dorsal stream into the parietal lobe works out location, motion and how to act ("where" or "how"). Patients with ventral damage may be unable to recognise an object yet still grasp it accurately; dorsal damage can produce the opposite.
Everyday examples & things to try
Try it
Find your blind spot
Draw a small cross and, about 8 cm to its right, a dot. Close your left eye, look at the cross with your right eye, and slowly move the paper toward and away from your face. At some distance the dot vanishes — it has fallen on your blind spot. Notice that you don't see a hole: your brain fills it in with the background.
By the numbers
A huge funnel
About 126 million photoreceptors feed only about a million optic nerve fibres per eye. The retina compresses and pre-processes the image before it ever reaches the brain.
Key takeaways
- Rods (~120 million) work in dim light; cones (~6 million) give colour and detail at the fovea.
- Optic nerve fibres partially cross at the chiasm; each hemisphere sees the opposite visual field.
- V1 extracts features; ventral and dorsal streams handle 'what' and 'where/how'.
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