Homonymous hemianopia with macular sparing

Loss of the same half of the field in both eyes, with a small island of central vision kept on the blind side.

After a complete lesion of the right primary visual cortex: left homonymous hemianopia with macular sparing. 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. The dashed circle is the spared center, 5° in radius.

Lesion site: Primary visual cortex (V1), occipital lobe
Left eye loses: Temporal half, center spared, 48.6% of the plotted field
Right eye loses: Nasal half, center spared, 48.6% 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: 48.6% of the central 30°
Fibers cut: 4 of the 8 fiber groups the model draws

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Put the lesion on the right primary visual cortex to see the occipital pole left intact.

Where is the lesion in hemianopia with macular sparing?

In the primary visual cortex of the occipital lobe, on the side opposite the missing half-field, with the occipital pole intact. The cortex along the calcarine sulcus maps the opposite half of the field, and the pole maps the macula, so a lesion that spares the pole leaves the center of that half-field.

Why the occipital pole survives

Most occipital strokes are in the territory of the posterior cerebral artery. The pole is often also supplied by branches of the middle cerebral artery, so it can survive when the rest of the calcarine cortex does not.

The upper bank of the sulcus, the cuneus, maps the lower field, and the lower bank, the lingual gyrus, maps the upper field. The macula takes far more cortex at the pole than its size in the field suggests.

The fiber groups cut by a complete lesion of the right primary visual cortex
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. Each cut group keeps the share that serves the central 5°, which ends at the occipital pole.

How much the spared center is

The model spares a central disc of 5° radius, a modeling choice inside the range seen in patients, whose sparing varies. On the plotted 30° field that disc is (5/30)² = 1/36 of the area, so the spared half of it is 1/72, about 1.4% of each eye’s field, and each eye loses 48.6% rather than half. That island holds fixation and the central vision used for reading and for recognizing faces.

A narrow strip of apparent sparing can also be an artifact of the eye drifting toward the blind side during testing, so on its own it does not prove the macula is spared.

Other occipital patterns

A lesion of the occipital pole alone gives a central homonymous scotoma instead, and damage to both occipital lobes gives cortical blindness with normal pupil reactions. Occipital defects are congruous, matching closely in the two eyes, and the far temporal crescent of the field, seen by one eye only and mapped at the front of the calcarine cortex, can be spared as well.

Usual causes

Posterior cerebral artery stroke, by far the most common, then tumors and trauma. A complete optic tract lesion gives the same half-field loss without the spared center, split right through fixation.

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 primary visual cortex	Right homonymous hemianopia with macular sparing	nasal half, center spared	temporal half, center spared
Right primary visual cortex	Left homonymous hemianopia with macular sparing	temporal half, center spared	nasal half, center spared

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.