Depth and fusion tests: how the eyes' teamwork is measured

A visual acuity test measures one eye's level of detail. But in amblyopia the real question is whether the eyes work together, and that cannot be answered by looking at a chart.

This article covers which tests assess two-eyed vision at an exam and what their results mean.

The logic of stereo tests is simple: through special glasses, each eye is shown a slightly different image. If depth perception is working, some shapes appear to lift off the page.

Booklet-based versions are the most common; the child is asked to point at the shape that stands out or to say which circle floats. Gamified versions exist for younger children.

The result is expressed in seconds of arc. The smaller the number, the finer the depth difference being distinguished, so a lower number means better depth perception.

In fusion tests each eye is shown a different image and the question is whether they combine into one perception. One eye may be given part of a figure and the other its complementary part.

If the child sees both parts together, fusion is working. Seeing only one part can mean the other eye's image is being suppressed. Seeing both parts separately and displaced points toward an alignment problem.

These tests are painless and usually look like a game to the child.

Visual acuity and depth perception do not always move together. A child with good acuity in both eyes can struggle markedly on a stereo test, because the two eyes' signals are not being merged.

The reverse also happens: while acuity has not yet shifted much under treatment, progress can show up in how the eyes work together.

So follow-up is not done by chart alone. If the real target of treatment is two-eyed vision, the measurement has to include it.

A single measurement is not a verdict on its own; a child's tiredness, attention and familiarity with the test all affect it. Doctors therefore watch the trend, and change over time means more than one day's number.

Two questions are worth asking as you leave an exam: what level did depth perception measure at today, and what will we be looking at next time?