Saccade - Timing and Kinematics

Timing and Kinematics

Saccades are the fastest movements produced by the human body. The peak angular speed of the eye during a saccade reaches up to 1000°/sec in monkeys (900°/sec in humans). Saccades to an unexpected stimulus normally take about 200 milliseconds (ms) to initiate, and then last from about 20–200 ms, depending on their amplitude (20–30 ms is typical in language reading). Under certain laboratory circumstances, the latency of, or reaction time to, saccade production can be cut nearly in half (express saccades). These saccades are generated by a neuronal mechanism that bypasses time-consuming circuits and activates the eye muscles more directly. Specific pre-target oscillatory (alpha rhythms) and transient activities occurring in posterior-lateral parietal cortex and occipital cortex also characterise express saccades.

The amplitude of a saccade is the angular distance the eye travels during the movement. For amplitudes up to about 60°, the velocity of a saccade linearly depends on the amplitude (the so-called "saccadic main sequence"). In saccades larger than 60°, the peak velocity starts to plateau (nonlinearly) toward the maximum velocity attainable by the eye. For instance, a 10° amplitude is associated with a velocity of 300°/sec, and 30° is associated with 500°/sec.

Saccades may rotate the eyes in any direction to relocate gaze direction (the direction of sight that corresponds to the fovea), but normally saccades do not rotate the eyes torsionally. (Torsion is clockwise or counterclockwise rotation around the line of sight when the eye is at its central primary position; defined this way, Listing's law says that when the head is motionless, torsion is kept at zero.)

Head-fixed saccades can have amplitudes of up to 90° (from one edge of the oculomotor range to the other), but in normal conditions saccades are far smaller, and any shift of gaze larger than about 20° is accompanied by a head movement. During such gaze saccades, first the eye produces a saccade to get gaze on target, whereas the head follows more slowly and the vestibulo-ocular reflex causes the eyes to roll back in the head to keep gaze on the target. During these head movements Listing's law is no longer obeyed.

Saccades, as well as microsaccades, can be distinguished from other eye movements (ocular tremor, ocular drift, smooth pursuit) using their ballistic nature: their top velocity is proportional to their length. This property can be used in algorithms for saccade detection in eye tracking data.