Stage 08 · in depth
Beneath the brain, the two optic nerves meet and swap half their traffic. It looks like a pointless complication until you work out what it buys, and then it turns the visual pathway into the best-localised system in clinical neurology: the shape of what a person cannot see identifies where along ten centimetres of brain the damage sits.
Fibres from the nasal half of each retina cross to the opposite side. Fibres from the temporal half stay where they are. Half the traffic changes lanes and half does not, which is the detail that makes the whole arrangement work.
The eye's optics invert the image, so the nasal retina views the temporal field and the temporal retina views the nasal field. Trace it through and the effect of a partial crossing is this: everything to the left of where you are looking, seen by either eye, ends up together in the right hemisphere, and everything to the right ends up in the left.
After the chiasm the brain is not organised by eye. It is organised by half of the world. That is what makes it possible to compare the two eyes' views of the same scene in one place, which is the raw material for stereoscopic depth.
It also means the two nerves have to meet, and where they meet matters. The chiasm sits directly above the pituitary gland. An anatomical coincidence, and the reason an endocrine tumour presents at an optician.
Because the fibres are sorted geographically all the way from retina to cortex, damage anywhere along the route removes a predictable piece of the visual field. Six sites, six signatures. Shaded areas are what the person cannot see, drawn as their own field, with the left of each circle being their left.
Right optic nerve: Loss in that eye only. Left eye normal
Right optic nerve
In front of the chiasm nothing has crossed yet, so the damage is confined to one eye and the other eye is untouched.
Typical causes: Optic neuritis, ischaemic optic neuropathy, trauma, compression in the orbit.
The chiasm itself: Bitemporal hemianopia.
The chiasm itself
Only the crossing fibres run through the midline, and those carry the nasal retina, which views the temporal field. Compression from below or within therefore takes the outer half of each eye's field and leaves the inner halves.
Typical causes: Classically a pituitary adenoma growing upward out of the sella, which sits directly beneath the chiasm.
Right optic tract: Left homonymous hemianopia.
Right optic tract
Behind the chiasm the bundle carries the whole left half of the world from both eyes, so the same side is lost in each. Defects here are often noticeably incongruous: the two eyes' losses do not match each other exactly.
Typical causes: Stroke, tumour, demyelination.
Temporal radiation (Meyer's loop), right: Left superior quadrantanopia.
Temporal radiation (Meyer's loop), right
Fibres carrying the upper field loop forward through the temporal lobe before turning back, so a temporal lesion takes the upper quadrant of the opposite side and leaves everything else.
Typical causes: Temporal lobectomy, temporal lobe stroke or tumour.
Parietal radiation, right: Left inferior quadrantanopia.
Parietal radiation, right
The lower field is carried by the more direct parietal fibres, so the mirror lesion produces the mirror defect.
Typical causes: Parietal stroke or tumour.
Right occipital cortex: Left homonymous hemianopia, macula spared.
Right occipital cortex
The further back the lesion, the more precisely the two eyes' defects match. The very centre often survives, because the foveal representation occupies an enormous share of the occipital pole and can be supplied from more than one direction.
Typical causes: Posterior cerebral artery stroke.
The rule in one line
One eye affected means in front of the chiasm. Both outer halves means the chiasm itself. The same side in both eyes means behind it, and the more precisely the two defects match, the further back the lesion sits.
You cannot localise your own lesion from it, and it would not be informative to try. Real defects are mapped by perimetry under controlled conditions and then read alongside examination and imaging, because the same shape can arise more than one way and because perception is a poor witness to its own gaps.
That last point is not rhetorical. As the blind spot demonstration shows, the visual system fills in missing regions and gives the result the same confidence as everything else. People with substantial hemianopia frequently report only that they keep bumping into door frames.
What this page is for is understanding why the map works. If you have noticed something missing from your vision, that is a reason to be examined, not to read further.
So that each hemisphere receives the whole opposite half of the visual world from both eyes at once. Fibres from the nasal half of each retina cross; fibres from the temporal half do not. Combining the two eyes' views of the same hemifield in one place is what makes binocular depth comparison possible.
Loss of the outer half of the visual field in both eyes. Only the crossing fibres pass through the midline of the chiasm, and those fibres come from the nasal retina, which views the temporal field. The pituitary gland sits directly beneath the chiasm, so a pituitary tumour growing upward compresses precisely those fibres and nothing else.
The same side of the visual field is lost in both eyes. It indicates a lesion behind the chiasm, where the fibres are already sorted by hemifield rather than by eye. The more closely the two eyes' defects resemble each other, the further back along the pathway the lesion sits.
Survival of the central few degrees of vision despite a hemianopia. It points to the occipital cortex, where the foveal representation occupies a very large share of tissue at the occipital pole and can receive blood supply from more than one arterial territory.
Because the fibres carrying the upper visual field take a detour forward into the temporal lobe before turning back toward the occipital cortex, a route called Meyer's loop. A temporal lesion catches them and leaves the lower-field fibres, which run a more direct parietal course, intact.
No, and the attempt would not be informative. Perimetry maps a field under controlled conditions and even then is interpreted alongside examination and imaging. What this page gives you is the logic of why the map works. If you have noticed anything missing from your vision, that is a reason to be examined rather than to read further.
Back to stage 08 in the atlas, or read how a photon becomes a signal.