White matter
Optic chiasm
Also called Optic chiasma
The X-shaped crossing where half of each eye's fibers swap sides.
The optic chiasm is where the two optic nerves meet and form an X, just above the pituitary gland. Here, about half the fibers from each eye cross to the other side of the brain. The result is that each half of the brain gets a view of the opposite side of the world from both eyes. This neat trick is key to seeing in depth.
Where it is
On the underside of the brain, behind the eyes and roughly in the middle of the head. It sits just above the pituitary gland, at the front edge of the hypothalamus.
What it does
A half crossing
Fibers from the inner, nose-side half of each retina cross over. Fibers from the outer half stay on the same side.
One world, both eyes
After the crossing, the right brain gets the left side of your view from both eyes, and the left brain gets the right side.
Depth vision
Bringing both eyes' views of the same spot together on one side lets the brain compare them and judge distance.
Think of it as a highway interchange where half the lanes from each side cross over, so all the traffic about the left side of the world ends up in the right half of the brain, and the other way around.
Works with
- Optic nerve (II)The two optic nerves arrive here from the eyes.
- Pituitary glandThis gland sits directly below the chiasm, so a growing pituitary tumor can press on it.
- HypothalamusThe body clock in the hypothalamus sits just above the chiasm and picks up light signals from it.
- Lateral geniculate nucleusAfter the crossing, most fibers travel on to this relay station in the thalamus.
- Superior colliculusSome fibers go to this midbrain center, which turns your eyes and head toward sudden movements.
- Optic radiationFrom the thalamus, the sorted signals fan out through the optic radiation to the back of the brain.
When it goes wrong
Pituitary tumors
A growth of the pituitary gland pushes up on the chiasm from below, pressing on the crossing fibers first. People slowly lose the outer half of vision in both eyes, called bitemporal hemianopia, a bit like wearing horse blinders.
Easy to miss
Because side vision fades slowly, people may only notice when they start bumping into doorframes or have a near miss while driving. A side vision test at the eye doctor can catch it early.
For learning only, not medical advice. If you have a health concern, talk to a doctor.
Surprising facts
- In ocular albinism, a form of albinism that mainly affects the eyes, the optic nerves are often wired differently, and many people have trouble combining the two eyes' views to judge depth.
- The brain's master body clock sits right above the chiasm and is named after it: the suprachiasmatic nucleus, meaning "above the chiasm".
By the numbers
Where the name comes from
From the Greek letter chi, which is shaped like an X. Chiasma means "two things placed crosswise", an X-shaped crossing.
Learn it in a lesson
- SeeingFrom the eye to the back of the brain. A short guided lesson in 3D.
Quick quiz
3 questions. Your answers are saved and come back in your daily review.
Which of these is a job of the optic chiasm?
Show the answer
A half crossing. Fibers from the inner, nose-side half of each retina cross over. Fibers from the outer half stay on the same side.
A half crossing. Fibers from the inner, nose-side half of each retina cross over. Fibers from the outer half stay on the same side.
The optic chiasm is part of which larger structure?
Show the answer
White matter pathways. It belongs to the white matter pathways.
White matter pathways. It belongs to the white matter pathways.
Which part fits this description? "Like a highway interchange where half the lanes from each side cross over, so all the traffic about the left side of the world ends up in the right half of the brain, and the other way around."
Show the answer
Optic chiasm. It is the optic chiasm. The X-shaped crossing where half of each eye's fibers swap sides.
Optic chiasm. It is the optic chiasm. The X-shaped crossing where half of each eye's fibers swap sides.
These parts now come back in your daily review, just before you would forget them.
Practice it and its neighbors in 3DSources
- 14.2 Central Processing, OpenStax, Anatomy and Physiology 2e
- 6.4 The Thalamus and Primary Visual Cortex, OpenStax, Introduction to Behavioral Neuroscience
- 15.2 Where Are Rhythms in the Brain?, OpenStax, Introduction to Behavioral Neuroscience
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Explore the optic chiasm in 3D
Turn it around, zoom inside and see how it connects to the rest of the brain, then test yourself with quick quizzes.
Open it in 3D