Optic chiasm

At a glance

English

Optic chiasm

Latin (TA2)

Chiasma opticum

System

Brain and spinal cord

Region

Head

Side

Paired, left and right

About the optic chiasm

The optic chiasm is a flattened, roughly X-shaped band of nerve fibres on the underside of the brain. The two optic nerves run back from the orbits through the optic canals and meet here in the midline. From its back corners the two optic tracts continue backwards round the cerebral peduncles. The chiasm lies just below the floor of the third ventricle and the hypothalamus, in front of the infundibulum (pituitary stalk), and above the diaphragma sellae and the pituitary gland. The internal carotid arteries lie on either side and the anterior cerebral and anterior communicating arteries above and in front.

Within the chiasm the fibres sort themselves. Axons from the nasal (medial) half of each retina cross to the opposite side, while those from the temporal (lateral) half stay on their own side. In humans a little under half the fibres stay uncrossed. Because the lens inverts the image, the nasal retina sees the temporal visual field, so each optic tract ends up carrying information about the opposite half of the visual world from both eyes. Fibres from the lower nasal retina cross in the front part of the chiasm and those from the upper nasal retina in its back part. This partial crossing allows the visual cortex of each hemisphere to compare the same half-field from the two eyes and so to build binocular depth perception.

During development the crossing is steered by guidance molecules at the midline. Some repel axons from the temporal retina so that they stay on their own side, while others attract the axons that cross. The proportion left uncrossed reflects how much the two eyes' fields overlap, and it is upset in albinism, where too many fibres cross.

The chiasm is clinically important because of its close relation to the pituitary. A pituitary adenoma growing upwards presses on the crossing nasal fibres first and causes loss of both temporal visual fields, bitemporal hemianopia. Craniopharyngiomas and aneurysms of the anterior communicating artery can do the same.

Function and supply

Function

Redistributes the optic nerve fibres so that each optic tract carries the opposite half of the visual field from both eyes, the basis of binocular vision.

Blood supply

Small branches of the internal carotid, anterior cerebral, anterior communicating and superior hypophysial arteries.