Smooth pursuit follows a continuously moving target and needs the target itself to be moving

What it is

When a continuously moving target appears in the visual field, the eyes rotate smoothly and continuously to follow its trajectory, keeping the target near the fovea — this is smooth pursuit. Its key difference from the saccade is what triggers it: smooth pursuit can barely be produced voluntarily. Without a real, continuously moving visual target, a person can hardly rotate the eyes with the same smoothness to "chase" an imagined path across a static scene — attempting this instead produces a series of saccades, not a truly continuous smooth movement.

Why it happens

Smooth pursuit depends on continuous visual feedback about the target's motion: the retina detects the target's slip speed relative to the eye (retinal slip), and this velocity-error signal is used in real time to adjust the eye's following speed so it gradually matches the target's speed, driving retinal slip toward zero. This is a closed-loop negative-feedback process, entirely unlike the saccade's open-loop, pre-programmed ballistic execution with no mid-flight feedback correction. Pursuit onset itself has a latency of a few tens to over a hundred milliseconds, and when a target suddenly accelerates or changes direction, pursuit speed briefly lags behind the target, with a small catch-up saccade closing the remaining position error afterward — pure smooth pursuit rarely achieves zero error throughout, and real recorded trajectories are usually a mixture of smooth pursuit and small corrective saccades.

Studying it

A classic paradigm has participants fixate a target moving at constant velocity or following a sinusoidal pattern, recording eye velocity and computing pursuit gain (the ratio of eye velocity to target velocity, with values closer to 1 indicating more accurate tracking), then comparing how gain and catch-up-saccade frequency change with target speed and predictability (steady linear motion versus sudden direction changes). Common independent variables are target speed, motion predictability, and target contrast; common dependent variables are pursuit gain, catch-up saccade frequency and amplitude, and pursuit onset latency.

Where it stops holding

Pursuit gain declines as target speed rises — beyond roughly tens of degrees per second, the eye can no longer keep up with smooth movement alone and must rely on increasingly frequent saccades to catch up intermittently, at which point "tracking" has effectively degenerated into an approximate trajectory stitched together from saccades. Smooth pursuit also depends almost entirely on a genuinely continuous visual motion signal — interface attempts to simulate a "sense of continuous movement" using static elements or discrete jumps may look like motion, but the eye movement actually driving it may already contain more saccadic content than genuine smooth pursuit.

Applying it

• For interface elements that require users to keep visually following them (marquee cues, gesture-linked animated feedback, dynamic curves that need visual tracking), keep the motion speed within a range where most users can maintain a reasonably high pursuit gain, avoiding speeds so high that users are forced to catch up with intermittent saccades and lose detail during the gaps.
• Designs where the target's motion path suddenly changes direction or accelerates (non-uniform emphasis animations) interrupt an established smooth pursuit and create a brief window of lost information; if that information matters, avoid placing it at the moment of the sudden direction change.
• Verification: record users' actual eye-movement trajectories and compare their velocity profile against the target's motion profile (pursuit gain); a markedly low gain or frequent catch-up saccades indicate the motion design has exceeded users' smooth-pursuit capacity.

Related

• Same group: A1.22.1 The saccade is a fast, ballistic eye movement used to shift the point of fixation · A1.22.3 The vestibulo-ocular reflex counter-rotates the eyes during head movement to stabilize the retinal image · A1.22.4 Vergence movements turn the two eyes in opposite directions to align their visual axes
• Nearby: A1.14 Motion perception
• Site search: smooth pursuit · pursuit gain · retinal slip · catch-up saccade