Optic chiasm · Grey Matter

The optic chiasm is the X-shaped crossing under the front of the brain where the two optic nerves meet and exchange half their fibres, so that each hemisphere receives the opposite half of the visual field from both eyes.

Optic chiasm. The optic chiasm is the X-shaped crossing under the front of the brain where the two optic nerves meet and exchange half their fibres, so that each hemisphere receives the opposite half of the visual field from both eyes.

Each optic nerve (cranial nerve II) carries the axons of the ganglion cells of one retina, itself brain tissue that grew out of the embryonic diencephalon. At the chiasm, which lies just above the pituitary gland and in front of the hypothalamus, fibres from the nasal half of each retina cross to the other side, and fibres from the temporal half stay on their own side (about 53 % crossed to 47 % uncrossed). Because the lens inverts the image, the nasal retina sees the outer (temporal) part of the visual field. Behind the chiasm each optic tract therefore carries the opposite visual field from both eyes to the lateral geniculate nucleus of the thalamus, and from there the optic radiation reaches the visual cortex.

Bringing both eyes' views of the same point together is what makes stereopsis possible. Neurons in the visual cortex compare the small differences between the two images, and depth is seen in the central, binocular part of the field (about 120 degrees wide).

The crossing makes lesions readable. A defect in one eye only points to the eye or nerve, a loss of both outer fields to the chiasm (bitemporal hemianopia), and a loss of the same half-field in both eyes to the tract, radiation or cortex behind it (homonymous hemianopia).

Pressure on a closed eye produces phosphenes, spots of light seen without light, because mechanical stimulation of the retina sends signals down the same pathway.

Questions: Why does each hemisphere's visual cortex see the opposite half of the world with both eyes? Each retina is split down the middle: its nasal half sees the outer part of the field on its own side, its temporal half the inner part toward the other side. At the chiasm the nasal fibres cross and the temporal ones stay, so the left optic tract collects everything both eyes see of the right half of the world, and the right tract the left half. That arrangement brings the two eyes' views of the same point to the same hemisphere, which is what the visual cortex needs to compare them and see depth. Why does the nasal half of the retina see the outer half of the world? The cornea and lens act like a camera lens and project an inverted, reversed image onto the retina. Light from an object far out to the side enters the pupil at an angle and lands on the opposite part of the retina, which is the half nearest the nose. The nasal retina therefore serves the temporal field, and since its fibres are the ones that cross at the chiasm, damage at the crossing removes the outer field of each eye. How does daylight set the brain's clock, and why does the clock sit on top of the optic chiasm? The master circadian clock is the suprachiasmatic nucleus, a small pair of nuclei in the front of the hypothalamus resting just above the optic chiasm, as its name says. Its neurons keep a rhythm of about 24 hours on their own, and it is reset each day by light through the retinohypothalamic tract, fibres that leave the optic nerves near the chiasm and come from a special class of light-sensitive retinal ganglion cells. The clock then times sleep, body temperature, cortisol and melatonin across the day, which is why light in the evening shifts sleep later. Where do the signals of the two eyes first meet in a single neuron? Although the chiasm sends both eyes' views of one half-field into the same optic tract, the lateral geniculate nucleus of the thalamus keeps them apart, in separate layers for each eye. The first neurons that respond to both eyes are in the primary visual cortex, beyond its input layer, where the inputs from the two eyes arrive in alternating bands (ocular dominance columns) and then converge. Neurons there are tuned to small differences between the two retinal images, the raw material of stereoscopic depth in the central binocular field, about 120 degrees wide.