Neuropsychology · Simplified
Where the Cut Is
The shape of the blind area is not a detail. It is the evidence.
The pathway
Break the pathway at one point
Signals from the left half of what you are looking at end up in the right hemisphere, and the other way round. The crossing happens at one place, and that is why where you cut it matters so much.
Key terms
- Visual field
- Everything one eye can see while looking straight ahead. Not the same as the eye itself.
- Optic chiasm
- The junction where fibres carrying the inner half of each eye's view cross to the opposite side.
- Homonymous
- The same side of the field is missing in both eyes.
- Quadrantanopia
- A quarter of the field missing rather than a half.
Above, the pathway from the two eyes to the two hemispheres, with the break marked. Below, what each eye can still see: the dark areas are gone. Both circles are drawn as the person sees the world, so the left of each circle is the left of their view.
What this shows
One crossing, and everything follows from it
Every pattern here follows from a single fact of wiring: the fibres carrying the inner half of each eye's view cross at the chiasm, and nothing crosses anywhere else.
Key idea: Before the crossing, a break affects one eye only, because that is all the nerve carries. At the crossing, a break catches the crossing fibres from both eyes, which are the ones carrying the outer edges of each eye's view, so the person loses the outside of both fields and keeps the middle. After the crossing, every fibre in the bundle carries the same side of the world from both eyes, so a break takes the same side out of both eyes together: that is what homonymous means. Further back still, the bundle spreads out, upper and lower parts separate, and a break can take a quarter rather than a half. The shape of the hole therefore locates the damage, which is why a field test is worth doing before any scan.
This is a schematic. Real field defects are rarely as sharp-edged as these circles: they shade off at their borders, they are often incomplete, and damage seldom respects the tidy boundaries drawn here. A great many other things cause field loss, from the retina to the eye itself, and the pattern narrows the possibilities rather than naming a diagnosis. Some people are unaware of a substantial field loss, so the absence of a complaint is not evidence of an intact field. Nothing here describes a real patient or replaces a proper perimetry test.
The longer version adds the inference run backwards from a defect to the sites that could have caused it, macular sparing, and further sites along the pathway. It is at Visual Field Defect Mapper in the main collection.