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.
The role of microsaccades in visual perception has been a highly debated topic that is still largely unresolved. It has been proposed that microsaccades correct displacements in eye position produced by drifts, although non-corrective microsaccades also occur. Some work has suggested that microsaccades enhance vision of fine spatial detail. Microsaccades were also believed to prevent the retinal image from fading, but they do not occur often enough for that purpose, considering that perfectly stabilized images can disappear from perception in a few seconds or less. The current consensus is that all fixational eye movements are important for the maintenance of visibility.
Microsaccades are tied to complex visual processing like reading. The specific timing pattern of microsaccades in humans changes during reading based on the structure of the word being read.
Experiments in neurophysiology from different laboratories showed that fixational eye movements, particularly microsaccades, strongly modulate the activity of neurons in the visual areas of the macaque brain. In the lateral geniculate nucleus (LGN) and the primary visual cortex (V1), microsaccades can move a stationary stimulus in and out of a neuron's receptive field, thereby producing transient neural responses. Microsaccades might account for much of the response variability of neurons in visual area V1 of the awake monkey.
Current research in visual neuroscience and psychophysics is investigating how microsaccades relate to fixation correction, control of binocular fixation disparity and attentional shifts. Recent research has found a direct correlation between illusory motion and microsaccades.
ADHD subjects fail to suppress eye blinks and microsaccades while anticipating visual stimuli but recover with medication. Microsaccade movements might therefore be used as a diagnostic test for ADHD, although this has not been clinically validated. The assessment of microsaccades can also help in the diagnosis of multiple other neurological and ophthalmological conditions.
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