Homonymous hemianopia

Loss of the same half of the visual field in both eyes, the right half or the left. It is also spelled homonymous hemianopsia.

After a complete lesion of the right optic tract: left homonymous hemianopia. Fields as the patient sees them, central 30° with rings at 10° and 20°. Hatched areas are lost, and the dark ovals are the normal blind spots.

Lesion site: Optic tract or lateral geniculate nucleus
Left eye loses: Temporal half, 50% of the plotted field
Right eye loses: Nasal half, 50% of the plotted field
Pattern: Homonymous: the same half in both eyes, so the lesion is behind the chiasm
Lost to both eyes at once: 50% of the central 30°
Fibers cut: 4 of the 8 fiber groups the model draws

Try in: Visual Field Defects Explorer →

Put the lesion on the right optic tract, then step it back to the geniculate nucleus: the fields do not change.

Where is the lesion in homonymous hemianopia?

Behind the optic chiasm, on the side opposite the missing half of the field, so a right-sided lesion takes the left half of the world from both eyes. A complete lesion of the optic tract or the lateral geniculate nucleus gives the defect drawn here, split through fixation; an occipital lesion often spares the macula.

Why both eyes lose the same half

Behind the chiasm each optic tract carries the uncrossed temporal fibers of its own eye and the crossed nasal fibers of the other. Both sets see the opposite half of the world, so a complete right tract lesion takes the temporal half of the left eye’s field and the nasal half of the right eye’s, and the two losses line up on the same side of the chart.

Defects are named for the half of the field that is lost, never for the lesion, so the right-sided lesion drawn here gives a left homonymous hemianopia.

The fiber groups cut by a complete lesion of the right optic tract
Fibers from	See	Cross at the chiasm	Then run in
Left eye, nasal retina, upper half	Lower temporal quarter	Yes	Right optic tract, then the parietal radiation
Left eye, nasal retina, lower half	Upper temporal quarter	Yes	Right optic tract, then Meyer’s loop
Right eye, temporal retina, upper half	Lower nasal quarter	No	Right optic tract, then the parietal radiation
Right eye, temporal retina, lower half	Upper nasal quarter	No	Right optic tract, then Meyer’s loop

4 of the 8 fiber groups the model draws are cut; the other 4 reach the cortex.

Optic tract or geniculate nucleus

Most tract fibers end in the lateral geniculate nucleus of the thalamus, which has six layers: crossed fibers end in layers 1, 4 and 6 and uncrossed fibers in layers 2, 3 and 5. A complete lesion of the nucleus therefore gives exactly the defect of a complete tract lesion, and the model draws the two identically.

Partial lesions tell them apart. A partial tract lesion is usually incongruous, with differently shaped defects in the two eyes, because fibers from matching points of the two retinae are not yet side by side. A partial lesion of the nucleus gives wedge-shaped sectoranopias, whose shape depends on whether the anterior or the lateral posterior choroidal artery is blocked.

The pupils

The pupil fibers leave the tract for the pretectal area of the midbrain before it reaches the geniculate nucleus. A tract lesion can therefore cause a relative afferent pupillary defect in the eye opposite the lesion, the eye that loses its temporal half; from the nucleus back the pupil reactions are normal.

Usual causes

In the optic tract: tumors near the chiasm, demyelination, trauma, and strokes in the territory of the anterior choroidal artery. In the lateral geniculate nucleus: infarcts in the territory of the anterior choroidal or lateral posterior choroidal artery, and tumors. Further back, a lesion through the whole optic radiation gives a complete hemianopia too, and a lesion of the visual cortex often gives a hemianopia with macular sparing.

Left-sided and right-sided lesions

The same site on the other side gives the mirror image, named for the half of the field it takes.

The defect from a complete lesion at this site on each side
Lesion	Defect	Left eye loses	Right eye loses
Left optic tract	Right homonymous hemianopia	nasal half	temporal half
Right optic tract	Left homonymous hemianopia	temporal half	nasal half

A complete lesion of the right lateral geniculate nucleus gives the same fields, side for side.

Tools for the visual pathway

Visualiser · Biology Visual Field Defects Explorer Place a lesion anywhere from optic nerve to visual cortex and see each eye’s field loss, named and explained. Visualiser · Biology Blind Spot Mapper Watch a dot vanish, as Mariotte’s paper did in 1668, then map your own blind spot in degrees and see it as the optic disc on the retina. Visualiser · Biology Brain Explorer 101 brain structures, gyri to ventricles, separable.

Visual Field Defects Explorer puts a lesion anywhere on the pathway and draws every fiber it cuts.
Blind Spot Mapper maps your own blind spot, the dark oval in each chart, in degrees.
Brain Explorer has the optic nerves, chiasm, tracts and geniculate bodies as 3D models.

Sources: Kandel, Principles of Neural Science, 6th edition, chapter 22, for the pathway and its lesions; Purves, Neuroscience, 6th edition, chapter 12, for the pathway and its lesions; Kupfer, Chumbley and Downer, Journal of Anatomy 1967;101:393, for the 53 percent of optic nerve fibers that cross at the chiasm. A teaching diagram of complete lesions: real fields are read with the history, the other eye, the optic discs and imaging.