Optic tract

At a glance

English

Optic tract

Latin (TA2)

Tractus opticus

System

Brain and spinal cord

Region

Head

Side

Paired, left and right

About the optic tract

Each optic tract continues the visual pathway beyond the optic chiasm. It leaves the back corner of the chiasm and passes backwards and laterally between the tuber cinereum and the anterior perforated substance, then winds round the cerebral peduncle of the midbrain, close to the uncus and the posterior cerebral artery. Near the thalamus it ends mainly in the lateral geniculate body, with a smaller bundle, the superior collicular brachium, passing on to the pretectal area and the superior colliculus.

Because of the partial crossing at the chiasm, each tract contains a mixture of fibres: the uncrossed ones from the temporal half of the retina on the same side and the crossed ones from the nasal half of the retina of the other eye. Together these represent the opposite half of the visual field. The left optic tract therefore carries the right half of what both eyes see, and the right tract the left half.

Most of its fibres relay in the lateral geniculate body, from which the optic radiation carries vision to the primary visual cortex of the occipital lobe. The fibres that go to the pretectal nuclei form the start of the pupillary light reflex. Pretectal cells project to the Edinger-Westphal nuclei of both sides, so light shone into one eye narrows both pupils. Other fibres reach the hypothalamus, where they help set daily rhythms and influence the widening of the pupils in dim light.

A lesion of one tract, from a stroke, tumour, infection, injury or surgery, causes homonymous hemianopia, in which both eyes lose the same half-field, on the side opposite the damage. Because the tract also carries pupillary fibres, it can produce a relative afferent pupillary defect in the opposite eye. People with a lasting field loss may be helped by prism devices or training. After the corpus callosum has been cut, information arriving through one tract stays in one hemisphere, so an object seen only in the left half-field cannot be named aloud.

Function and supply

Function

Carries visual information about the opposite half of the visual field from both eyes to the lateral geniculate body, and supplies the pretectal pupillary reflex pathway and the superior colliculus.

Blood supply

The anterior choroidal artery mainly, with branches of the posterior communicating artery.