Fixation (visual)
The species in which fixational eye movement has been verified thus far include humans, primates, cats, rabbits, turtles, salamanders, and owls.
Regular eye movement alternates between saccades and visual fixations, the notable exception being in smooth pursuit, controlled by a different neural substrate that appears to have developed for hunting prey.
[1][2] To maintain visibility, the nervous system carries out a procedure called fixational eye movement, which continuously stimulates neurons in the early visual areas of the brain responding to transient stimuli.
The field remained quiet until the 2000s, when key neurological properties of fixational eye movement were discovered and a new wave of research began.
Poletti et al. propose using a threshold based on the amplitude of sustained fixations and give a cutoff of 30 arcminutes or 0.5 degrees.
By this definition regular saccades are produced during the active and intentional exploration of the eye, during non-fixation tasks such as free viewing or visual search.
Because they shift the retinal image, microsaccades overcome adaption[8] and generate neural responses to stationary stimuli in visual neurons.
Some neuroscientists believe that microsaccades are potentially important in neurological and ophthalmic diseases since they are strongly related to many features of visual perception, attention, and cognition.
[15] The development of non-invasive eye-movement-recording devices, the ability to record single-neuron activity in monkeys, and the use of computational processing power in the analysis of dynamic behavior led to advancements in microsaccade research.
It has been shown that when fixation is not explicitly enforced as it often occurs in vision research experiments, microsaccades precisely shift gaze to nearby locations of interest.
[19][20] Microsaccades are also being explored as diagnostic measures for Progressive supranuclear palsy, Alzheimer's disease, autism spectrum disorder, acute hypoxia, and other conditions.
[21] Ocular drifts are altered by some neurologic conditions[20] including Tourette syndrome[28] and autism spectrum disorder[29] Ocular microtremors (OMTs) are small, quick, and synchronized oscillations of the eyes occurring at frequencies in a range of 40 to 100 Hz, although they typically occur at around 90 Hz in the average healthy individual.
Although the function of ocular microtremors is debatable and not fully known, they seem to play a role in processing of high spatial frequencies, which allows for perception of fine detail.
