Neuropsychology

Visual Field Defect Mapper

Pick a point on the pathway, predict which parts of which eye's field go dark, then find out - and then find out how much the pattern could have told you in reverse.

Simulated — a schematic pathway, textbook mappings, fictional cases

Learning objective

By the end you should be able to predict the field consequence of damage before, at and after the chiasm.

About 25 minutes. Nothing you do here is saved or sent anywhere.

Not an eye test

Nothing on this page examines your vision or anybody else's. The field diagrams are schematic quadrants, not perimetry, and no result here means anything about the person reading it. If you have concerns about your own vision, see an optometrist or a doctor.

Choose a point on the pathway.
Tick the quadrants you think go dark, in each eye, before revealing anything.
Reveal, read why, and try the next point.
Then answer the harder question in the challenge: given a field pattern, where could the damage be?

This part needs JavaScript

The pathway and the field charts are drawn and compared in your browser, so they cannot run with JavaScript switched off. The rest of this page is readable without it.

Which fibres cross at the chiasm, and what do they carry? The fibres from the nasal half of each retina, which see the outer (temporal) half of that eye's field The fibres from the temporal half of each retina, which see the inner (nasal) half of that eye's field All of them - each eye is processed entirely by the opposite hemisphere I would have to work it out

Key terms

Optic chiasm: Where fibres from the two eyes meet and some cross to the other side. Damage before it, at it and after it give different patterns.
Homonymous: Affecting the same side of the visual field in both eyes.
Nasal and temporal: Nasal is towards the nose, temporal towards the ear. A half of the retina and the half of the field it serves lie on opposite sides, which is what makes the two terms easy to swap by mistake.
Macular sparing: Central vision preserved inside a field loss that otherwise covers that side.

Your prediction so far

Quadrant by quadrant

Each quadrant of each eye's field, what you predicted and what the textbook mapping gives
Eye	Part of that eye's field	You predicted	Mapping gives

Every point on the pathway, side by side

Each point of damage and the field pattern it produces
Damage at	Left eye's field	Right eye's field	Usual description

Two rows in this table are identical in the first three columns. That is not a mistake.

The crossing is what makes the pattern informative

Fibres from the nasal half of each retina cross at the chiasm. The nasal retina sees the outer half of that eye's field, so after the chiasm each side of the brain carries the opposite half of the world from both eyes rather than one whole eye. That single fact does all the work. Damage before the chiasm affects one eye, damage at the chiasm affects the outer half of both, and damage after it affects the same half of the world in both.

Where the border falls tells you which side of the chiasm

A defect confined to one eye is in front of the chiasm. A defect that respects the vertical midline in both eyes is behind it. That distinction is genuinely diagnostic, it is available from a bedside examination, and it is one of the tidiest structure-to- function mappings anywhere in the nervous system - which is precisely why it is a bad model for the rest of neuropsychology.

The pattern cannot say which structure

Damage to the right optic tract and damage to the right occipital cortex both remove the left half of both fields. So does damage to both parts of the right optic radiation together, and so does damage to the right lateral geniculate nucleus. The field pattern locates the failure to one side and to somewhere behind the chiasm, and stops. Congruity - how closely the two eyes' defects match - and macular sparing are the traditional clues for going further, and both are unreliable enough that imaging settles it in practice.

Quadrants are a teaching fiction

Real defects have soft edges, vary in depth rather than being simply present or absent, and rarely align with a quadrant boundary. A field chart is a map of sensitivity, not a map of which squares are lit. Every diagram in this tool is a simplification chosen so that a prediction can be marked right or wrong, and the marking is a consequence of the simplification.

What the person notices is a third thing again

A homonymous hemianopia is not experienced as a black rectangle. Many people are unaware of it until they bump into something, and some report nothing missing at all. Field loss and awareness of field loss are separate, which is one of the reasons the previous tool in this module spends its time on the difference between not seeing and not attending.

What this does not demonstrate

Not a diagnosis. Field patterns in practice are interpreted alongside acuity, pupil responses, the rest of the neurological examination and imaging. Nothing here identifies a cause, and no cause is mentioned.
The mappings are idealised. Textbook one-to-one correspondences. Real lesions are partial, spread across boundaries, and produce incomplete and asymmetric defects far more often than the tidy patterns drawn here.
The diagram is schematic. A few boxes and lines. It is not an atlas, nothing is to scale, and the layout carries no spatial meaning.
Macular sparing is contested. It is shown because it appears in every textbook. Why it happens - dual blood supply, the size of the cortical representation, fixation during testing - is still argued about, and it is not reliable enough to localise anything on its own.
Nothing here is about the rest of the brain. The visual pathway is unusually orderly. Treating it as a model for how brain structure maps onto function is the mistake the rest of this module exists to correct.

Visual Field Defect Mapper

Learning objective

Not an eye test

First, the fact everything else rests on

The mapper

Choose a point on the pathway