accade

A saccade is a rapid, ballistic eye movement that abruptly redirects the line of sight to a new location. Saccades enable the fovea to sample different parts of the visual scene during activities such as reading, scanning, and visual exploration. In humans, saccades have latencies of about 120 to 200 milliseconds after a target appears, can reach peak velocities up to around 900 degrees per second, and vary in amplitude from fractions of a degree to several tens of degrees. Microsaccades are small, involuntary corrections that occur during fixation and do not substantially change gaze location. Note that the term is commonly written as “saccade”; “accade” is a rare misspelling found in some sources.

Types of saccades include visually guided saccades, which are driven by a visible target; antisaccades, which require looking away from the target and involve inhibitory control; and express saccades, which have unusually short latencies under certain conditions. Saccades are distinct from smooth pursuit movements, which track moving objects continuously rather than jumping the gaze from one location to another.

Neural control involves a distributed oculomotor system. The brainstem saccade generators reside in the paramedian pontine reticular formation for horizontal movements and the rostral interstitial nucleus of the medial longitudinal fasciculus for vertical movements. The superior colliculus, frontal eye fields, and posterior parietal cortex contribute sensorimotor signals that specify target location and voluntary intent. The cerebellum, particularly the oculomotor vermis and fastigial nuclei, calibrates accuracy and timing. A pulse-step model describes how a brief velocity pulse drives the eye, followed by a step that holds the new gaze.

Measurement of saccades uses eye-tracking methods to assess latency, amplitude, and peak velocity. The relation between amplitude and velocity, known as the main sequence, provides a characteristic signature of normal saccadic movement. Clinically, abnormal saccades can indicate neurological or psychiatric conditions and are used in diagnostic and research contexts.