Smooth pursuit

What it measures

Smooth pursuit is the eye movement that keeps a moving target on the fovea. It needs a moving target: a patient cannot produce it voluntarily on a still background.

Pursuit quality is expressed as gain: eye velocity divided by target velocity. A gain of one means the eye matches the target. Below one, the eye falls behind and the visual system corrects the position error with a small fast movement, a catch-up saccade. Frequent catch-up saccades mean pursuit is not keeping up.

A sinusoidal trajectory is used because its velocity changes smoothly and repeats exactly, so the same demand can be presented again later. Pursuit is normally better horizontally than vertically, and gain falls as target velocity rises.

How it is administered

The patient sits at a comfortable distance with habitual correction worn, head still. A still head is the requirement of the test. A patient who turns the head is using the vestibulo-ocular reflex instead of pursuit.

The screen sits at eye level, with steady ambient light and no reflection on the panel. The target moves along the selected trajectory at the selected speed. The patient follows it with the eyes only, blinking as little as is comfortable and without running ahead of it.

Horizontal tracking is run first, then vertical, then oblique or circular. Start slow and increase only while tracking stays smooth.

The clinician sits to the side and watches the eyes rather than the screen, noting whether both eyes track together, whether one lags, whether tracking breaks down in one direction, and whether the patient loses the target at the turning points.

Reading the result

VisReed presents the target and records nothing. The result is what the clinician sees, written into the notes with the trajectory, the speed and the target size used, because the same settings are needed to compare two sessions.

Watch how closely the eyes follow. Reduced pursuit looks like eyes that trail the target and then jump forward to it with a catch-up saccade. Occasional catch-up saccades at high target velocity are expected. Frequent ones at low velocity are not.

Where tracking breaks down matters: breakdown in one direction only suggests a directional problem, while breakdown at the turning points, where velocity is lowest, often reflects attention rather than the pursuit system.

Compare eyes, directions and sessions, and compare a patient with their own earlier result rather than a published figure. Pursuit degrades with inattention, fatigue and many medications, so one poor result in a tired patient means little.

When it is used

Pursuit testing is used when oculomotor dysfunction is suspected in a child, alongside saccade and convergence testing. It is used after concussion, where pursuit deficits are common, and as a baseline repeated during orthoptic training.

It also suits a patient who reports that text moves on the page, and neurological follow-up where pursuit is one of several ocular motor signs. A pursuit abnormality is a sign. Read it with the rest of the examination.

Limitations

This test is in beta. Treat the output as indicative and not as a settled clinical measure.

Nothing is recorded. There is no eye tracking and no timing, so pursuit quality is judged by the clinician watching the eyes. That judgement varies between observers, and a small reduction in gain that an eye tracker would detect is not visible by eye.

Viewing distance is not fixed. Target velocity in degrees per second depends on distance and screen size, so one speed setting on a phone at 30 cm and on a monitor at one metre present very different demands.

Screen refresh rate limits how smooth the target really is. On a low refresh display it advances in steps, which can provoke saccades by itself.

Head movement and ambient light are not controlled. The test does not replace laboratory video-oculography where a quantitative gain value is needed.