fixational

Fixational is an adjective relating to fixation of gaze, especially the small involuntary eye movements that occur even when a person tries to keep their eyes still on a fixed target. In vision science, fixational processes are typically described in terms of fixational eye movements, including microsaccades, drift, and tremor. Microsaccades are brief, involuntary saccades that correct minor gaze errors; drift is a slow, smooth displacement of the eye; tremor is a high-frequency, low-amplitude motion.

Although small, these movements are important: they help maintain visibility by refreshing images on the retina and preventing perceptual fading, and they may contribute to high-acuity sampling of spatial detail. Their precise role is studied in psychophysics, reading, and visual perception experiments.

Neural control involves brainstem oculomotor circuits and cortical areas such as the superior colliculus and frontal eye fields, with feedback from visual areas. Eye-tracking methods measure fixation stability through metrics such as microsaccade rate, amplitude, and drift velocity.

Fixational movements can vary with attention, fatigue, and task demands. Abnormalities in fixational eye movements have been observed in several neurological and ophthalmic conditions, and studying them informs understanding of normal vision as well as clinical diagnosis and rehabilitation.

See also: fixation, microsaccade, ocular motor control.