The saccade is a fast, ballistic eye movement used to shift the point of fixation

What it is

To point the fovea, the high-resolution region of the retina, at whatever it wants to examine, the eye produces several distinct kinds of movement, collectively called eye movements. Broadly, these fall into: the saccade, which rapidly shifts gaze to a new target point; smooth pursuit, which continuously follows a moving target; the vestibulo-ocular reflex (VOR), which counter-rotates the eyes during head movement to keep the retinal image stable; and vergence movements, which turn the two eyes to align their visual axes on the same target in depth. The saccade is the most frequent and best-known of these: gaze jumps from one point to another with essentially no continuous path in between, landing on the new fixation point in a ballistic fashion — a waking person produces tens of thousands of these jumps a day.

Why it happens

These movements are classed separately because each is driven by a distinct signal and neural pathway: saccades are driven by burst neurons in the brainstem and, once triggered, execute in a pre-programmed, ballistic fashion with essentially no mid-flight correction from visual feedback; smooth pursuit requires a continuously moving visual target to supply the velocity signal that drives continuous following; the vestibulo-ocular reflex is driven directly by the inner ear's vestibular organs sensing head rotation, producing a compensatory counter-rotation with no dependence on visual input at all; vergence movements are driven by binocular disparity or defocus signals, turning each eye inward or outward to align on the same target in depth. Because these drive pathways are independent of one another, the movements can stack and occur simultaneously — a saccade during a head turn, for instance — sharing only the same pair of eyeballs as their final shared actuator.

Studying it

The first step in processing eye-tracking data is usually to classify the continuously recorded gaze trajectory into these different movement types before any further analysis — a common approach is a velocity-based classifier: segments exceeding an empirical velocity threshold (typically on the order of tens of degrees per second) are labeled saccades, segments with lower but sustained directional velocity are labeled smooth pursuit, and segments with near-zero velocity are labeled fixations. The accuracy of this classification step directly affects every downstream conclusion drawn from the eye-tracking data, which is why classification algorithms themselves are repeatedly scrutinized and compared in the eye-tracking methodology literature. Common independent variables are sampling rate and the velocity/acceleration thresholds used by the classifier; common dependent variables are the detection accuracy and misclassification rate for each movement type.

Where it stops holding

Velocity-threshold classification is sensitive to the eye tracker's sampling rate — at too low a rate, a fast saccade can be undersampled and misclassified as a series of smooth-pursuit-like points, requiring the classification parameters to be re-tuned for the specific device's sampling rate; thresholds tuned at one sampling rate cannot simply be reused on a different device sampling at a different rate.

Related

• Same group: A1.22.2 Smooth pursuit follows a continuously moving target and needs the target itself to be moving · 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: A1.01 Visual field extent and the effective viewing region · A5.04 Change blindness
• Site search: saccade · eye movement classification · velocity-based classifier · oculomotor system