View source for Biology:Fixation (visual)

{{short description|Maintaining one's gaze on a single location}}
[[File:Two Types of Fixational Eye Movement.png|thumb|Microsaccades and Ocular Drifts]]

'''Fixation''' or '''visual fixation''' is the maintaining of the [[Philosophy:Visual perception|gaze]] on a single location. An animal can exhibit visual fixation if it possess a [[Medicine:Fovea centralis|fovea]] in the anatomy of their [[Biology:Eye|eye]]. The fovea is typically located at the center of the [[Biology:Retina|retina]] and is the point of clearest vision. The species in which fixational eye movement has been verified thus far include humans, primates, cats, rabbits, turtles, salamanders, and owls. Regular [[Biology:Eye movement|eye movement]] alternates between [[Biology:Saccade|saccade]]s and visual fixations, the notable exception being in [[Biology:Smooth pursuit|smooth pursuit]], controlled by a different [[Biology:Neural substrate|neural substrate]] that appears to have developed for hunting prey. The term "fixation" can either be used to refer to the point in time and space of focus or the act of fixating. Fixation, in the act of fixating, is the point between any two saccades, during which the eyes are relatively stationary and virtually all visual input occurs. In the absence of retinal jitter, a laboratory condition known as [[Biology:Stabilized images|retinal stabilization]], perceptions tend to rapidly fade away.
<ref>{{cite journal | author1 = Pritchard R.M. |author2=Heron W. |author3=Hebb D.O. | year = 1960 | title = Visual Perception Approached by the Method of Stabilized Images | journal = Canadian J. Psych | volume = 14 | issue = 2| pages = 67–77 | doi = 10.1037/h0083168 |pmid=14434966 }}</ref><ref>{{cite journal |last1=Coppola |first1=D |last2=Purves |first2=D |title=The Extraordinarily Rapid Disappearance of Entoptic Images |journal=Proceedings of the National Academy of Sciences of the USA |date=1996 |volume=93 |issue=15 |pages=8001–8004 |doi=10.1073/pnas.93.15.8001 |pmid=8755592 |pmc=38864 |bibcode=1996PNAS...93.8001C |doi-access=free }}</ref>
To maintain visibility, the [[Biology:Nervous system|nervous system]] carries out a procedure called '''fixational eye movement,''' which continuously stimulates neurons in the early visual areas of the [[Biology:Brain|brain]] responding to transient [[Biology:Stimulus (physiology)|stimuli]]. There are three categories of fixational eye movement: 
microsaccades,
ocular drifts,
and ocular microtremor. At small amplitudes the boundaries between categories become unclear, particularly between drift and tremor.<ref>{{cite journal | author = Rucci M., Poletti M. | year = 2015 | title = Control and function of fixational eye movements | journal = Annual Review of Vision Science | volume = 11 | pages = 499–518 | doi = 10.1146/annurev-vision-082114-035742 | pmid = 27795997 | pmc = 5082990}}</ref><ref name="Alexander2019">{{cite journal |last1=Alexander |first1=R. G. |last2=Martinez-Conde |first2=S. |title=Fixational Eye Movements |journal=Eye Movement Research. Studies in Neuroscience, Psychology and Behavioral Economics. |series=Studies in Neuroscience, Psychology and Behavioral Economics |date=2019 |pages=73–115 |doi=10.1007/978-3-030-20085-5_3|isbn=978-3-030-20083-1 }}</ref>

== History ==

In 1738, James Jurin made the first known reference to a "trembling of the eye" that was presumably caused by fixational eye movements.<ref name=Alexander2019/> Robert Darwin noted in 1786 that the jiggling of color after-effects was presumably the consequence of small eye movements. [[Eye tracking]] with sufficient resolution to record fixational eye movements was developed in the 1950s. Retinal stabilization, the ability to project [[Biology:Stabilized images|stabilized images]] on the retina, showed that retinal motion was necessary for visual perception, also in the 1950s. The field remained quiet until the 2000s, when key neurological properties of fixational eye movement were discovered and a new wave of research began.<ref>{{cite journal | author = Rucci M., Poletti M. | year = 2015 | title = Control and function of fixational eye movements | journal = Annual Review of Vision Science | volume = 1 | pages = 499–518 | doi = 10.1146/annurev-vision-082114-035742 | pmid = 27795997 | pmc = 5082990|quote=It is now known that these movements modulate neural responses in various cortical areas. }}</ref><ref name="Rucci 1–4">{{Cite journal|last1=Rucci|first1=Michele|last2=McGraw|first2=Paul V.|last3=Krauzlis|first3=Richard J.|date=2016-01-01|title=Fixational eye movements and perception|journal=Vision Research|volume=118|pages=1–4|doi=10.1016/j.visres.2015.12.001|issn=1878-5646|pmid=26686666|s2cid=5510697 |doi-access=|quote=After a period of quiescence at the end of last millennium, the study of fixational eye movements has now gained wide popularity among vision scientists.}}</ref>

== Microsaccades ==
[[File:Saccades and Microsaccades.jpg|thumb|The lines on this image display the saccadic and microsaccadic movements of a person's eye while they looked at this face. The involuntary, micro-saccadic movement is not steady when the person's eyes are concentrated at the eyes of the woman, while the voluntary, saccadic movement goes around the periphery of the face once at any give point.]]
A microsaccade, also known as a "flick", is a type of [[Biology:Saccade|saccade]]. Microsaccades are the largest and fastest of the fixational eye movements. Like saccades in general, microsaccades are usually binocular, and conjugate movements with comparable amplitudes and directions in both eyes. However, the definition of microsaccade varies from study to study and no common definition has emerged.<ref name=Poletti2016/>

In the 1960s, scientists suggested the maximum amplitude for microsaccades should be 12 arcminutes to distinguish microsaccades and saccades.<ref name="Collewijn2008">{{Cite journal|last1=Collewijn|first1=Han|last2=Kowler|first2=Eileen|date=2008-01-01|title=The significance of microsaccades for vision and oculomotor control|journal=Journal of Vision|volume=8|issue=14|pages=20.1–21|doi=10.1167/8.14.20|issn=1534-7362|pmc=3522523|pmid=19146321}}</ref> However, further studies have shown that microsaccades can certainly exceed this value.<ref>{{Cite journal|last1=Troncoso|first1=Xoana G.|last2=Macknik|first2=Stephen L.|last3=Martinez-Conde|first3=Susana|date=2008-01-01|title=Microsaccades counteract perceptual filling-in|journal=Journal of Vision|volume=8|issue=14|pages=15.1–9|doi=10.1167/8.14.15|issn=1534-7362|pmid=19146316|doi-access=free}}</ref> Newer studies have used a threshold of up to 2° to categorize microsaccades, expanding the definition by an order of magnitude. The distribution of saccade amplitudes is unimodal, giving no empirical threshold to distinguish microsaccades and saccades. 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.<ref name=Poletti2016>{{cite journal | author = Poletti M., Rucci M. | year = 2016 | title = A compact field guide to the study of microsaccades: Challenges and functions | journal = Vision Research | volume = 118 | pages = 83–97 | doi = 10.1016/j.visres.2015.01.018 | pmid = 25689315 | pmc = 4537412}}</ref>

Another way to distinguish microsaccades from saccades is by the intention of the subject when they happen. 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. Microsaccades are defined as the "involuntary saccades that occur spontaneously during intended fixation". The subjectivity of this definition has drawn criticism.<ref>{{cite journal | author = Poletti M., Rucci M. | year = 2016 | title = A compact field guide to the study of microsaccades: Challenges and functions | journal = Vision Research | volume = 118 | pages = 83–97 | doi = 10.1016/j.visres.2015.01.018 | pmid = 25689315 | pmc = 4537412 |quote=[...] this definition implicitly comes with drawbacks: its dependence on the subject's intention (note the terms: “involuntary”, “spontaneously”, “intended”) makes it little objective and prone to different interpretations}}</ref>

=== Mechanism ===
Moving in a straight-line-fashion, microsaccades have the ability to carry the retinal image from several dozen to several hundred [[Biology:Photoreceptor cell|photoreceptor]] widths. Because they shift the retinal image, microsaccades overcome adaption<ref name="Collewijn2008"/> and generate neural responses to stationary stimuli in visual neurons.<ref name="MartinezConde2000">{{Cite journal|last1=Martinez-Conde|first1=S.|last2=Macknik|first2=S. L.|last3=Hubel|first3=D. H.|date=2000-03-01|title=Microsaccadic eye movements and firing of single cells in the striate cortex of macaque monkeys|journal=Nature Neuroscience|volume=3|issue=3|pages=251–258|doi=10.1038/72961|issn=1097-6256|pmid=10700257|s2cid=8846976}}</ref> These movements might serve the function of maintaining visibility during fixation,<ref name="Collewijn2008"/>  or might be related to [[Philosophy:Attention|attention]]al shifts to objects in the visual field<ref>{{cite journal |last1=Laubrock |last2=Engbert |last3=Kliegl |title=Microsaccade dynamics during covert attention |journal=Vision Research |volume=45 |issue=6 |pages=721–730 |doi=10.1016/j.visres.2004.09.029|pmid=15639499 |year=2005 |s2cid=374682 |doi-access=free }}</ref> or in memory,<ref>{{cite journal |last1=Martinez-Conde |first1=S |last2=Alexander |first2=R |title=A gaze bias in the mind's eye |journal=Nature Human Behaviour |volume=3 |issue=5 |pages=424–425 |date=2019 |doi=10.1038/s41562-019-0546-1 |pmid=31089295 |s2cid=71148025 }}</ref> might help limit binocular [[Medicine:Fixation disparity|fixation disparity]],<ref>{{cite journal |last1=Valsecchi |first1=Matteo |last2=Gegenfurtner |first2=Karl R. |title=Control of binocular gaze in a high-precision manual task |journal=Vision Research |volume=110 |issue=Pt B |pages=203–214 |doi=10.1016/j.visres.2014.09.005|pmid=25250983 |year=2015 |doi-access=free }}</ref> or may serve some combination of those functions.

=== Medical application ===
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.<ref name=":1"/> Research aimed at finding the purpose of microsaccades began in the 1990s.<ref name=":1">{{Cite journal|last1=Martinez-Conde|first1=Susana|last2=Macknik|first2=Stephen L.|last3=Troncoso|first3=Xoana G.|last4=Hubel|first4=David H.|date=2009-09-01|title=Microsaccades: a neurophysiological analysis|journal=Trends in Neurosciences|volume=32|issue=9|pages=463–475|doi=10.1016/j.tins.2009.05.006|issn=1878-108X|pmid=19716186|citeseerx=10.1.1.493.7537|s2cid=124353}}</ref> 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.<ref name="MartinezConde2000"/>{{primary source inline|date=November 2018}} Today, there is growing interest in research on microsaccades. Research on microsaccades includes investigating the perceptual effects of microsaccades, recording the neural responses they induce, and tracking the mechanisms behind their oculomotor generation. 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.<ref>{{cite journal | author1 = Ko H.-K. |author2=Poletti M. |author3=Rucci M. | year = 2010 | title = Microsaccades precisely relocate gaze in a high visual acuity task | journal = Nat. Neurosci. | volume = 13 | issue = 12| pages = 1549–1553 | doi = 10.1038/nn.2663 |pmid=21037583 |pmc=3058801 }}</ref> This behavior compensates for non-uniform vision within the foveola.<ref>{{cite journal | author1 = Poletti M. |author2=Listorti C. |author3=Rucci M. | year = 2013 | title = Microscopic eye movements compensate for nonhomogeneous vision within the fovea | journal = Curr. Biol. | volume = 23 | issue = 17| pages = 1691–1695 | doi = 10.1016/j.cub.2013.07.007 |pmid=23954428 |pmc=3881259 }}</ref>

Some studies suggest the use of microsaccades as a diagnostic method for ADHD.<ref>{{cite journal |last1=Panagiotidi |first1=M |last2=Overton |first2=P |last3=Stafford |first3=T |title=Increased microsaccade rate in individuals with ADHD traits. |journal= Journal of Eye Movement Research|date=2017 |volume=10 |issue=1 |pages=1–9 |doi=10.16910/10.1.6|pmid=33828642 |pmc=7141051 |doi-access=free }}</ref><ref name="Fried2014">{{cite journal |last1=Fried |last2=Tsitsiashvili |last3=Bonneh |last4=Sterkin |last5=Wygnanski-Jaffe |last6=Epstein |title=ADHD subjects fail to suppress eye blinks and microsaccades while anticipating visual stimuli but recover with medication. |journal=Vision Res |volume=101 |pages=62–72 |date=2014 |doi=10.1016/j.visres.2014.05.004|pmid=24863585 |doi-access=free }}</ref> Adults diagnosed with ADHD but with no medication treatments tend to blink more and make more microsaccades.<ref name="Fried2014"/><ref name="Alexander2018"/> Microsaccades are also being explored as diagnostic measures for [[Medicine:Progressive supranuclear palsy|Progressive supranuclear palsy]], [[Medicine:Alzheimer's disease|Alzheimer's disease]], Autism Spectrum Disorder, acute hypoxia, and other conditions.<ref name="Alexander2018">{{cite journal |last1=Alexander |first1=Robert |last2=Macknik |first2=Stephen |last3=Martinez-Conde |first3=Susana |title=Microsaccade Characteristics in Neurological and Ophthalmic Disease |journal=Frontiers in Neurology |date=2018 |volume=9 |issue=144 |pages=144 |doi=10.3389/fneur.2018.00144|pmid=29593642 |pmc=5859063 |doi-access=free }}</ref>

== Ocular drifts ==
Ocular drift is the fixational eye movement characterized by a smoother, slower, roaming motion of the eye when fixed on an object. The exact movement of ocular drift is often compared to [[Brownian motion]], which is the random motion of a particle suspended in fluid as a result of its collision with the atoms and molecules that comprise that fluid. The movement can also be compared to a [[Random walk|random walk]], characterized by random and often erratic changes in direction.<ref name=":0" /> Ocular drifts occur incessantly during intersaccadic fixation. Although the frequency of ocular drifts is usually lower than the frequency of ocular microtremors (from 0 to 40&nbsp;Hz compared to from 40 to 100&nbsp;Hz), it is problematic to distinguish ocular drifts and ocular microtremors. In fact, microtremors might reflect the Brownian engine underlying the drift motion.<ref name=":3">{{Cite journal|last1=Ahissar|first1=Ehud|last2=Arieli|first2=Amos|last3=Fried|first3=Moshe|last4=Bonneh|first4=Yoram|date=2016-01-01|title=On the possible roles of microsaccades and drifts in visual perception|journal=Vision Research|volume=118|pages=25–30|doi=10.1016/j.visres.2014.12.004|issn=1878-5646|pmid=25535005|s2cid=12194501 |doi-access=}}</ref> Resolution of intersaccadic eye movements is technically challenging.<ref name="Rucci 1–4"/>
[[File:Brownian motion large.gif|thumb|Brownian Motion]]

=== Mechanism ===
The motion of ocular drift is related to the processing and encoding of space and time.<ref name="ahiarieli2001">{{cite journal | author = Ahissar E., Arieli A. | year = 2001 | title = Figuring space by time | journal = Neuron | volume = 32 | issue = 2| pages = 185–201 | doi = 10.1016/S0896-6273(01)00466-4 | pmid = 11683990 | doi-access = free }}</ref> It is also related to acquiring minute visual details of objects that are stationary, in order for these details to be further processed.<ref>{{Cite journal|last1=Kuang|first1=Xutao|last2=Poletti|first2=Martina|last3=Victor|first3=Jonathan D.|last4=Rucci|first4=Michele|date=2012-03-20|title=Temporal encoding of spatial information during active visual fixation|journal=Current Biology|volume=22|issue=6|pages=510–514|doi=10.1016/j.cub.2012.01.050|issn=1879-0445|pmc=3332095|pmid=22342751}}</ref><ref name="ahiarieli2012">{{cite journal | author = Ahissar E., Arieli A. | year = 2012 | title = Seeing via miniature eye movements: A dynamic hypothesis for vision | journal = Frontiers in Computational Neuroscience | volume = 6 | page = 89 | doi = 10.3389/fncom.2012.00089 | pmid = 23162458 | pmc = 3492788 | doi-access = free }}</ref>  Recent results have shown that ocular drift reformats the input signal to the retina equalizing (whitening) spatial power at non-zero temporal frequencies across a broad spatial frequency range.<ref name="D. Victor 2015">{{cite journal | author = Rucci M., Victor J. D. | year = 2015 | title = The unsteady eye: an information processing stage, not a bug | journal = Trends in Neurosciences | volume = 38 | issue = 4| pages = 195–206 | doi = 10.1016/j.tins.2015.01.005 | pmid = 25698649 | pmc = 4385455 }}</ref>

=== Medical application ===
Ocular drift of one type was first found to be caused by an instability of the ocular motor system.{{citation needed|date=March 2019}} However, more recent findings suggest that there are actually a number of hypotheses as to why ocular drifts occur. First, ocular drifts can be caused by the uncontrollable random movements driven by neuronal or muscular noise.<ref>{{cite book |last1=Carpenter |first1=R. H. S. |title=Movements of the Eyes |date=1988 |publisher=Pion. |location=London |edition=2nd}}</ref> Second, ocular drifts can occur to counter controlled motor variables, namely a faulty motor negative feedback loop.{{citation needed|date=March 2019}} When the head is not immobilized, as in daily life and as is often true in eye movement recordings in the laboratory, ocular drifts compensate for the natural fixational instability of the head.<ref name=":0">{{cite journal | author1 = Poletti M. |author2=Aytekin M. |author3=Rucci M. | year = 2015 | title = Head-eye coordination at a microscopic scale | journal = Current Biology | volume = 25 | issue = 24| pages = 3253–3259 | doi = 10.1016/j.cub.2015.11.004 |pmid=26687623 |pmc=4733666 }}</ref> Ocular drifts are altered by some neurologic conditions<ref name="Alexander2018"/> including [[Medicine:Tourette syndrome|Tourette syndrome]]<ref>{{cite journal |last1=Shaikh |doi=10.1007/s10072-017-3069-4|pmid=28815321|pmc=6246774|title=Fixational eye movements in Tourette syndrome|journal=Neurological Sciences|volume=38|issue=11|pages=1977–1984|year=2017}}</ref> and autism spectrum disorder<ref>{{cite journal |doi=10.1111/ejn.12243|pmid=23692590|pmc=4587666|title=Atypical cortical representation of peripheral visual space in children with an autism spectrum disorder|journal=European Journal of Neuroscience|volume=38|issue=1|pages=2125–2138|year=2013|last1=Frey|first1=Hans-Peter|last2=Molholm|first2=Sophie|last3=Lalor|first3=Edmund C|last4=Russo|first4=Natalie N|last5=Foxe|first5=John J}}</ref>

== Ocular microtremors ==
Ocular microtremors (OMTs) are small, quick, and synchronized oscillations of the eyes occurring at frequencies in a range of 40 to 100&nbsp;Hz, although they typically occur at around 90&nbsp;Hz in the average healthy individual.{{citation needed|date=March 2019}} They are characterized by their high frequency and minuscule amplitude of just a few [[Minute and second of arc|arcseconds]]. 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.<ref name="D. Victor 2015"/><ref>{{Cite journal |doi = 10.1038/nature05866|pmid = 17568745|bibcode = 2007Natur.447..852R|title = Miniature eye movements enhance fine spatial detail|year = 2007|last1 = Rucci|first1 = Michele|last2 = Iovin|first2 = Ramon|last3 = Poletti|first3 = Martina|last4 = Santini|first4 = Fabrizio|journal = Nature|volume = 447|issue = 7146|pages = 852–855|s2cid = 4416740}}</ref><ref>{{Cite journal|last1=Otero-Millan|first1=Jorge|last2=Macknik|first2=Stephen L.|last3=Martinez-Conde|first3=Susana|date=2014-01-01|title=Fixational eye movements and binocular vision|journal=Frontiers in Integrative Neuroscience|volume=8|pages=52|doi=10.3389/fnint.2014.00052|issn=1662-5145|pmc=4083562|pmid=25071480|doi-access=free}}</ref> Studies show that ocular microtremors have some promise as a tool for determining the level of consciousness in an individual,<ref>{{cite journal |last1=Bolger |first1=C |last2=Sheahan |first2=N |last3=Coakley |first3=D |last4=Malone |first4=J |title=High frequency eye tremor: reliability of measurement |journal=Clinical Physics and Physiological Measurement |date=1992 |volume=13 |issue=2|pages=151–9 |doi=10.1088/0143-0815/13/2/007 |pmid=1499258 |bibcode=1992CPPM...13..151B }}</ref> as well as the progression of some degenerative diseases including [[Medicine:Parkinson's disease|Parkinson's disease]]<ref name="Bolger 528–531">{{Cite journal|last1=Bolger|first1=C.|last2=Bojanic|first2=S.|last3=Sheahan|first3=N. F.|last4=Coakley|first4=D.|last5=Malone|first5=J. F.|date=1999-04-01|title=Ocular microtremor in patients with idiopathic Parkinson's disease|journal=Journal of Neurology, Neurosurgery, and Psychiatry|volume=66|issue=4|pages=528–531|issn=0022-3050|pmc=1736284|pmid=10201430|doi=10.1136/jnnp.66.4.528}}</ref> and multiple sclerosis.<ref name="Bolger 639–642">{{Cite journal|last1=Bolger|first1=C.|last2=Bojanic|first2=S.|last3=Sheahan|first3=N.|last4=Malone|first4=J.|last5=Hutchinson|first5=M.|last6=Coakley|first6=D.|date=2000-05-01|title=Ocular microtremor (OMT): a new neurophysiological approach to multiple sclerosis|journal=Journal of Neurology, Neurosurgery, and Psychiatry|volume=68|issue=5|pages=639–642|issn=0022-3050|pmc=1736931|pmid=10766897|doi=10.1136/jnnp.68.5.639}}</ref>
[[File:Tremblement oculaire.jpg|thumb|Ocular microtremor tracing with burst sections underlined]]

=== Mechanism ===
Although originally thought to stem from spontaneous firing of motor units, the origin of ocular microtremors is now believed to be in the oculomotor nuclei in the [[Biology:Reticular formation|reticular formation]] of the [[Biology:Brainstem|brainstem]].<ref>{{Cite journal|last1=Spauschus|first1=A.|last2=Marsden|first2=J.|last3=Halliday|first3=D. M.|last4=Rosenberg|first4=J. R.|last5=Brown|first5=P.|date=1999-06-01|title=The origin of ocular microtremor in man|journal=Experimental Brain Research|volume=126|issue=4|pages=556–562|issn=0014-4819|pmid=10422719|doi=10.1007/s002210050764|s2cid=2472268}}</ref> This new insight opened the possibility of using ocular tremors as a gauge for neuronal activity in that region of the [[Biology:Central nervous system|central nervous system]]. More research must be done, but recent studies strongly suggest that decreased activity in the brainstem correlates with decreased frequency of OMTs.<ref>{{Cite journal|last1=Bojanic|first1=S.|last2=Simpson|first2=T.|last3=Bolger|first3=C.|date=2001-04-01|title=Ocular microtremor: a tool for measuring depth of anaesthesia?|journal=British Journal of Anaesthesia|volume=86|issue=4|pages=519–522|issn=0007-0912|pmid=11573625|doi=10.1093/bja/86.4.519|doi-access=free}}</ref>

=== Medical application ===
Several methods of recording have been developed to observe these minuscule events, the most successful being the [[Engineering:Piezoelectric sensor|piezoelectric strain gauge method]], which translates eye movement through a latex probe in contact with the eye leading to piezoelectric strain gauge. This method is used in research settings; more practical adaptations of this technology have been developed for use in clinical settings to monitor the depth of anesthesia.<ref>{{Cite journal|last1=Bengi|first1=H.|last2=Thomas|first2=J. G.|date=1968-03-01|title=Three electronic methods for recording ocular tremor|journal=Medical & Biological Engineering|volume=6|issue=2|pages=171–179|issn=0025-696X|pmid=5651798|doi=10.1007/bf02474271|s2cid=29028883}}</ref> Despite the availability of these methods, tremor remains more difficult to measure than other fixational eye movements, and studies addressing medical applications of tremor movements are rare as a result.<ref name="Alexander2018"/> Some studies have, nevertheless, pointed to the possibility that tremor movements may be useful in assessing the progression of degenerative diseases including [[Medicine:Parkinson's disease|Parkinson's disease]]<ref name="Bolger 528–531"/> and multiple sclerosis.<ref name="Bolger 639–642"/>

== See also ==
*[[Biology:Rapid eye movement sleep|Rapid eye movement]]
*[[Biology:Microsaccade|Microsaccade]]
*[[Philosophy:Ocular tremor|Ocular tremor]]
*[[Biology:Saccade|Saccade]]

== References ==
{{Reflist}}

[[Category:Eye]]

{{Sourceattribution|Fixation (visual)}}