What is "microsaccade"

n. A small, jerky eye movement that is a part of normal vision when fixate on an image, which seems to play a role in visual perception.

Microsaccades are a kind of fixational eye movement. They are small, jerk-like, involuntary eye movements, similar to miniature versions of voluntary saccades. They typically occur during prolonged visual fixation (of at least several seconds), not only in humans, but also in animals with foveal vision (primates, cats, etc.). Microsaccade amplitudes vary from 2 to 120 arcminutes. The first empirical evidence for their existence was provided by Robert Darwin, the father of Charles Darwin.