Title:

What are the characteristics of visual fixation in people with motion sensitivity after vestibular insult compared to healthy adults when subjected to different visual environments?

Past Literature Review:

Postural control is the process by which our central nervous system processes sensory information to produce the appropriate motor output to maintain balance during standing upright and moving in space (Massion 1994). There are three main sensory systems that provide information about a person and his position within the environment to the central nervous system. These are: the visual system, the vestibular system and the somatosensory system (Massion 1994). The somatosensory system provides information regarding the position of joints with respect to surface and the vestibular system provides information about the linear and angular motion of the head with respect to gravity and one’s own body. Unlike these two systems which provide information about self-motion or position in space, the information from the visual system has role in differentiating self-motion from external motion (Redfern, Yardley et al. 2001). Postural control is dependent on the integration of information from these three systems. This differentiation between self-motion and external motion is dependent on perceiving whether an image motion on the retina is the result of an object moving relative to the person or the result of concurrent eye and head movements (Angelaki and Hess 2005, Fajen and Matthis 2013). This is achieved by a mechanism that compares retinal image motion (the retinal signal) and an estimate of the velocity of eyes in space (the reference signal). The reference signal is derived from an interaction between various kinds of information including optokinetic stimulation (which evokes a combination of a slow-phase and fast-phase eye movements where the eyes momentarily follow the moving object and then rapidly reset eyes back to the initial position), the vestibular afferents (encoding earth relative head velocity), the efference copy of the oculomotor-signal (encoding eye-velocity in the head); and, the proprioceptive feedback of the extraocular muscles (EOM), somatosensory kinaesthetic proprioception, and cognition. When the reference signal and the retinal signal equal each other the object is perceived as stationary, whereas when they differ, object motion is perceived (Wertheim 1994).Any discrepancy in identifying self-motion and external motion can challenge postural control, which would then require adjustments in these systems to determine the correct orientation in space. There can be difficulties in resolving these conflicts when a peripheral or central vestibular disorders exists due to visual- vestibular interaction that takes place while perceiving motion (Redfern and Furman 1993). These conflicts can alter the perception of motion and sometimes can lead to illusionary motion perception. The visual system along with the vestibular input has to achieve a balance to create a rich perceptual experience and provide the inputs to differentiate between self-motion and object motion which are then processed by the other systems to maintain postural control (Redfern, Yardley et al. 2001, Fajen and Matthis 2013). Thus, the information from the visual system can either enhance or degrade postural stability.

Visual Influences on Postural Control

The visual system helps us to interact with and move around in our environment. Postural stability is dependent, in part, on the visual input received from the environment. The visual system is divided in to the central visual system (fovea), and the peripheral visual system. The input from these two systems generates movement of the visual image of the environment on the surface of the retina, known as retinal slip (Guerraz and Bronstein 2008). The fovea specialises in object motion perception and object recognition, whereas peripheral vision is sensitive to motion in the scene and is thought to dominate both perception of self-motion and postural control. The perception of object motion and self-motion occurs when the postural control system detects and interprets the changes in the optic flow (Lappe, Bremmer et al. 1999). Optic flow provides sensory information from the environment. This information is provided when a person moves through a stationary environment and when an environment is moving against a person who is stationary (self-motion and external motion). This information is perceived as retinal slip by the visual system. The retinal slip along with the information from other sensory systems helps differentiates self-motion and external motion. The retinal slip is described as the motion of the visual image created by the optic field of the environment on the surface of the retina and is used as a feedback for compensatory sway which causes the image of the visual scene to move on the retina in the opposite direction to that of head sway (Strupp, Glasauer et al. 2003). In order maintain the body in a stable position this retinal slip has to be minimised (Gielen, Gabel et al. 2004). The nervous system receives information about the retinal slip by compensatory eye movements. These eye movements help in minimising the retinal slip thus providing cues to the central nervous system regarding the resultant retinal slip against which the compensatory postural sway is generated (Strupp, Glasauer et al. 2003, Angelaki and Hess 2005). These eye movements are generated by: a vestibular driven foveal stabilization reflex known as the translational vestibular-ocular reflex (the TVOR) and the ocular following reflex (OFRs). Both these reflexes generates compensatory eye movements that help maintain the image on fovea and minimise retinal slip (Angelaki, Zhou et al. 2003, Angelaki and Hess 2005).

Eye movements and postural control

Eye movements also contribute to postural control. There are three eye movements: saccades; quick eye movements from one fixation point to another, used to scan a stationary environment, visual fixation which maintain gaze on a single object, and smooth pursuits which are used to track a moving stimulus (Leigh and Zee 2015). Movement perception differs according to the eye movement.

The visual fixation and saccades contribute to spatial orientation and postural control, suppressing the visual field motion perception by maximising peripheral vision. This gives a stable image to enhance the visual signals of self-motion. Sensations of small body movements then facilitate the execution of compensatory postural reactions. The stabilising role of saccades has also been recently investigated (Rey, Lê et al. 2008, Ajrezo, Wiener-Vacher et al. 2013, Rodrigues, Aguiar et al. 2013, Thomas, Bampouras et al. 2016). The role of suppressing optic flow is primarily that of fixation. Fixation help reduce optic flow from the environment and enable visual stability to enhance postural control. Saccades on the other hand help maintain visual stability while shifting fixation targets from one fixed point to another.

This is not the case during smooth pursuits as they track and maintain an image on the fovea and register the movement of the field as caused by the movement of the eyes. This may impair the visual signals of self-motion which disturb the visual field stillness due to the depiction of surround motion (Thomas, Dewhurst et al. 2017). A series of experiments reported that saccades and visual fixation help maintain postural stability whereas smooth pursuits impede it (Warren and Hannon 1990, Webb and Griffin 2003, Rodrigues, Aguiar et al. 2013).

Fixational eye movements, Illusions and Postural Control

Fixational eye movements consist of small involuntary movements which are divided into microsaccades, ocular drifts and ocular tremors (Martinez-Conde, Macknik et al. 2004). fixation drive 80% of total visual experience (Martinez-Conde 2006) and are responsible for preventing peripheral fading while focussing on a target, and suppressing the optokinetic response (Pola, Wyatt et al. 1995).

Fixation drive our visual experience and have an important role in visual perception, however the signals that are produced during fixation can lead to illusory motion perception (Martinez-Conde 2006). Inability to fixate is termed as fixational instability, can make a person experience visual illusion and predispose them to develop motion sensitivity (Beer, Heckel et al. 2008, Poletti, Listorti et al. 2010, Otero-Millan, Macknik et al. 2012).A large number of studies have studied the relationship of fixational instability and strength of illusionary motion (Beer, Heckel et al. 2008, Poletti, Listorti et al. 2010, Otero-Millan, Macknik et al. 2012). Illusions vary in strength depending on the accuracy of fixation. Illusion strength is positively correlated with fixation instability. As gaze fluctuates the retinal image of a physically static figure fluctuates and these jittery motions on the retina lead to more vigorous impressions of illusory motion (Murakami, Kitaoka et al. 2006).

Difficulty in fixation leads to poor identification of unreferenced motion (absence of reference frame around motion). This is because motion is only noticed when it is compared to its surround and absence of a surrounding reference makes it harder to detect motion because poorer fixation increases internal velocity noise which leads to motion sensitivity (Murakami 2004). Fixational instability can be measured by frequency of refixation and saccades (Lencer and Clarke 1998, Winkler and Ciuffreda 2009). As suggested by literature, a person with fixational instability would have high frequency of saccades and re fixation. Thus, it could be inferred that fixational eye movements contribute to motion sensitivity and illusions. This is due to the increased fluctuation of the gaze that happens due to fixational instability.

Guerraz, Yardley et al. 2001 studied symptoms, spatial orientation and postural sway in 21 participants with visual motion hypersensitivity, compared with 16 people with labyrinthine deficits without visual motion hypersensitivity symptoms and 25 healthy controls. The experimental protocol assessed the participant’s report of subjective visual vertical under three visual conditions: total darkness, rod and frame test and in front of large disc rotating in the frontal plane. Body sway was measured with eyes open, eyes closed, rod and frame and during disc rotating stimulation. The results indicated that participants with visual motion hypersensitivity have abnormally large perceptual and postural responses to disorienting visual environments.

Recently, (Van Ombergen, Lubeck et al. 2016) investigated nine participants with visual vestibular mismatch (VVM) who experienced symptom provocation in complex visual environments, comparing them with normal healthy controls. They investigated whether optokinetic (roll motion) or a complex but stationary visual pattern evoked the symptoms to the same degree. Each participant completed two sessions (still and moving). Outcome measures included subjective visual vertical, postural sway, the simulator sickness questionnaire and the misery scale. They found that participants with VVM always swayed more than controls and had increased postural sway when deprived of visual cues, compared to controls. A comparison of groups showed that the symptoms were always significantly higher for participants with VVM than for controls. They concluded that participants with VVM differ from healthy controls in postural and subjective symptoms and motion is a crucial factor in provoking these symptoms.

Lencer and Clarke, 1998 discussed the importance of the ability to maintain fixation against a moving background. These investigators proposed a clinical test to assess fitness for driving in people with a vestibular disorder based on the ability to maintain stable fixation against a moving optokinetic background, thus simulating conditions that occur while driving. This was done in 35 healthy adults and 5 participants with unilateral vestibular loss. Healthy participants were tested with and without caloric-induced vestibular imbalance. Eye movements were recorded during a combination of standard pursuit tasks against an optokinetic background. Participants with unilateral vestibular dysfunction had significantly increased frequency of saccades against the optokinetic background. The authors concluded that the number of saccades in the presence of a moving background was a sensitive indicator of problems in visual function that were critical for driving. The authors felt that inadequate suppression of OKN through fixation would result in visual illusion and consequently in partial loss of orientation, a symptom that people with visual motion hypersensitivity have on a frequent constant basis, even when not driving (Lencer and Clarke 1998).

The relationship between fixation, vestibular dysfunction and visual motion hypersensitivity (VMH) has been investigated (Winkler and Ciuffreda 2009). It was seen that people with visual motion hypersensitivity/ visual vertigo exhibit unsteady fixation, characterised by an increase in the number of refixational eye movements while attempting to maintain fixation in a visually stimulating environment when compared with a participant with vestibular dysfunction without visual vertigo or normal healthy controls. It was found that people with VMH exhibit poorer fixation, impaired fusional vergence and higher DHI scores.

Summary

The ability to perceive a stable world depends on the visual inputs derived from the environment. The visual input from the environment is provided by optic flow. Optic flow is the pattern of images sweeping across the retina that provide sensory information from the environment. This information is perceived as retinal slip by the visual system (Angelaki and Hess 2005). The retinal slip along with the information from other sensory systems helps differentiate self-motion from external motion (Angelaki and Hess 2005, Fajen and Matthis 2013). Motor responses of the eyes and postural system are generated to reduce overall optic flow, minimise retinal slip and maintain the body in a stable position (Fajen and Matthis 2013). The main motor response is ‘visual fixation’, which maintain the gaze on a single object are the main contributing factor to visual stability. Visual fixation contribute to 80% of the visual experience and are important to suppress optic flow from the environment and minimise retinal slip (Martinez-Conde, Macknik et al. 2004). An inability to fixate is termed fixational instability, which may induce visual illusion and predispose a person to develop motion sensitivity (Beer, Heckel et al. 2008, Poletti, Listorti et al. 2010, Otero-Millan, Macknik et al. 2012). Motion sensitivity has been given different names such as visual motion hypersensitivity, visual vertigo, visual vestibular mismatch and space motion discomfort. Studies have shown that people with motion sensitivity after vestibular disorders have abnormally large perceptual and postural responses to complex visual environments and exhibit fixational instability (Lencer and Clarke 1998, Winkler and Ciuffreda 2009, Otero-Millan, Macknik et al. 2012, Van Ombergen, Lubeck et al. 2016).

This literature review has identified the lack of investigation of visual fixation in people with motion sensitivity after vestibular disorder. This research aims to investigate visual fixation and postural responses in complex visual environments using a mobile eye tracker (SMIBegaze)to record visual fixation and a 3D motion analysis system (Qualisys) with a force plate (AMTI) to record body sway in people with motion sensitivity after a vestibular disorder. The findings will provide insight into motion sensitivity and will be the next step towards developing a diagnostic tool using virtual reality goggles and postural sway measurements. It is hoped that the results of this study will guide the development of rehabilitation programmes for people with motion sensitivity.

Research Question:

What are the characteristics of visual fixation and body sway in people with motion sensitivity after vestibular insult compared to healthy adults when subjected to different visual environments?

Hypothesis:

Complex and moving backgrounds will lead to higher variability in visual fixation and increased body sway in people with motion sensitivity as compared to healthy adults.

Participants:

· Healthy adults

· People with motion sensitivity.

Measures will be taken to recruit people across spread of ages between 18-60 years.

Sample Size:

· Healthy adults: 20

· People with motion sensitivity: 20

There is no evidence in the literature of any confounding factors on the measurements of interest, and until the study is commenced there is minimal information on population variation of these measures, therefore an arbitrary sample size 20 in each group is selected. An interim analysis will be undertaken to investigate differences and to create subgroups or increase sample size if required.

Inclusion Criteria (Healthy adults)

Healthy participants will be recruited through the neurorehab research team research networks and community advertising. Participants will be included in the healthy control group if they are aged between 18-60 years with independent level of mobility. However, people having reading glasses will be included in the study, as the design of the study require focusing on a letter at a distance. Also, the size of the letter has been chosen after piloting initial work with fellow research team members.

Inclusion Criteria (motion sensitivity)

Participants with motion sensitivity will be recruited through the New Zealand Dizziness and Balance Centre.

· 18-60 years of age (Agrawal, Carey et al. 2009, Bisdorff, Bosser et al. 2013)

· History of Vestibular disorder (to be confirmed with NZDBC)

· No signs of deficits of vestibular system

· History of motion sensitivity symptoms as reported by visual vertigo analogue scale.(score >5) (Dannenbaum, Chilingaryan et al. 2011, Sharon and Hullar 2014, Silva, Ferreira et al. 2016)

· Level of dizziness as recorded by Dizziness Handicap Inventory. (Score >40)(Silva, Ferreira et al. 2016)

Exclusion Criteria:

This information will be gathered by asking the participant. In case of any queries, participant’s medical practitioner will be consulted.

· History neurological conditions that could interfere with eye movements.

· History of any previous eye surgery

· Medical condition which may influence the eye movements results such as sarcoidosis, Lyme disease, diabetes mellitus etc

Experimental Procedure:

Written informed consent will be obtained from all participants prior to participation in the study. Each participant will attend the Motion Analysis laboratory for a single session. The experimental procedure will be explained to each participant. The experimental set up will consist of letters projected on a screen with different backgrounds with the screen 3.5 metres in front of the participant. Participants will be standing with feet 20 cm apart on a force plate to estimate centre of pressure. Each participant will wear a mobile eye tracker device with markers attached to determine eye movements and head kinematics respectively. This will then be repeated in a virtual reality environment. Data will be collected for all tasks on the same day. The motion analysis system will be calibrated prior to the participant arriving in the laboratory.

protocol:

Experimental session includes following steps:

1. Lab/technical set up prior to participant arriving

2. Static calibration procedure for 3D kinematics

3. Participant Introduction and consent process

4. Preparing participant

5. Data collection

1) Lab/technical set up prior to participant arriving:

Turn on computer/cameras and load programme

· Turn on cameras at wall switch (white plug). Check they are all on and that they are displaying a number. If not, then turn off and on again. 9 cameras in total.

· Open Qualysis programme.

· Load Project (BCI stroke).

· Open ‘Project options’ > Input Devices > double click on USB (this is where the camera/force plate info comes in) > Change selection from external sync, to simultaneous start > Apply, OK.

2) Calibration of 3D motion system

· Click on “New File”   from the toolbar

· Click on “Calibrate” from the top toolbar. Check “Linerization parameters” shows 9 of 9 cameras. If not, click > Load > Load Automatically > OK.

· Place the “L” shaped bar on floor inside the force plate margins. The long edge of the bar needs to be in direction of movement. Click OK.

Force Plate and force plate control panel window

· Pending delay (usually set at 7 seconds) wave the “T” shaped wand for 30sec in the area that is being used. Note: make sure that you walk around the region- so that all cameras are able to “see” the wand, and slower rotations- rather than spinning.

· Check that all 9 cameras were seen and “Calibration passed”. Average resolution should be <0.6mm (new software Nov 07) version 2.2. If higher, then recalibrate > new measurement. If ok, click OK.

Generate force plate orientations

· Click on new file.

· Set up 4 reflective markers on FP1.

Force plate dimensional axis

· Click on the red “Capture” button.

· Set the Capture period to 2 seconds.

· Check Marker frames at “120Hz”.

· Save - set folder

· Remove tick from counter.

· Click START. Push trigger.

· Load and Assign Marker List

· Top right corner > right click on Trajectory > load label list > Select “force plate” (under D: gait clinic\marker label) > OPEN

· You will see 4 markers in top right hand corner. Assign markers by clicking on red markers on lower right side and dragging up to green markers. (There might be more than 4 red markers but don’t worry about extras).

· Assign the markers as outlined in picture above OR Rotate the view as below (x axis vertical), with the cable (under the floor) coming out at the bottom of the FP1 (cable is green). Then start at top right with 1, then number in clockwise direction. This technique works for FP2 also (keep cable at bottom of square). SAVE.

Assigning markers

· Generate Solution

· Go to “Tools” > Workspace/Project Options > Force Data (under input devices > processing)

· Double click on Force Plate 1 or Force Plate 2 (depending on which you have just labelled).

· Click Generate. It will offer you two solutions. Chose correct solution (so that the markers are in the correct location). OK.

· Another screen comes up that shows the distance from each marker to you zero point. OK.

· Apply. OK. Apply. OK. (Note: you don’t need to save it again).

Repeat for FP2.

Zero force plate

· Push zero on amp 1 and amp 2.

· To check – Click View > Data Info > Right click on x > Display Force data > tick which force plate. Right click on Force > Plot > parameters.

· Check vectors are zeroed (blue will be higher than yellow and red). If not, push zero on amp 1 and 2 again.

Check force plate is working

· View > Data info > Right click on x > display force data > tick which force plate. Right click on Force > Plot > parameters.

Now get someone to walk over force plate and check you see a reading

3) Participant Introduction and consent process

Following recruitment participants will be scheduled for data collection in lab at a time convenient to them. Following orientation and lab set up participants will perform six tasks during a single session. The session will last approx. 1.5-2 hours to allow enough rest time and avoid provocation of any symptoms. The following protocol will be followed for each participant.

Have you read the ‘Participant information sheet’?

We will be recording your eye movements and balance using an eye tracker. These are glasses with markers attached on each side to record your head movement and eye movements.

This eye tracker records your eye movements by using two cameras placed on the glasses. We will also be attaching a reflective marker to your head to record your head movements. You will be standing on a force plate to record your balance. You will be asked to look at letters projected on a screen. There are 6 tasks in the session and with each new task the background behind the letters will get more complex. You will have a break of at least 30 seconds between each task, longer if you need it. Let me know if you require a break at any time in middle of task or if you feel dizzy.

Eye tracker SMIBe Gaze

I would like to ask you some personal information for study records and to ensure that you meet the criteria of the study.

Complete screening checklist

Complete DHI and visual vertigo analogue scale.

This document is a consent form saying that you are happy to participate in the research and outlining your rights as a participant. Please read the form.

Is there anything else you would like me to explain or clarify?

Do you have any other questions?

You should also know that at any time you may withdraw from the study, no questions asked. You will have a choice whether you want us to use your data that has been collected or discard it immediately. Would you like to receive a summary of the research?

Would you like to be contacted about the possibility of participating in future studies undertaken by the neurophysiology laboratory of the Health and Rehabilitation Research Institute of AUT University?

Are you happy to take part in the study? Please sign this consent form

4) Preparing participant

The participant will be asked to stand on the force plate, feet 20 cm apart. After attaining position, eye tracker will be worn by the participant and reflective head marker will be attached to centre of forehead.

The following steps will be followed for each participant:

· Position your feet here on this force plate and try to keep them still for the complete session. (check distance with measuring tape; should be 20 cm).

· The glasses (show glasses) and a marker on your forehead will allow the computer to record your head movement while you are viewing the screen.

· Please put these glasses on and adjust them comfortably. They should be tight enough to ensure that they don’t move during the session. Once you put these glasses on I will calibrate the eye tracker. That means that eye tracker will adapt to your eye before recording eye movements. Try not to frown or move your eyebrows as this will move the glasses and we may get incorrect recordings. If you want to take of the glasses anytime between the task, let me know. We will take them off and then recalibrate them for your comfort.

· Ok now I am going to place this marker on your head.

· Shall we start the calibration process? Please try and maintain your focus on the points that I ask you to look at.

5) Data Collection

This step will start data collection for the session. In this phase letters with different backgrounds will be projected on the screen in front of the participant. There are a total of 6 tasks, each task lasting approximately 100 seconds. The letters on the screen will appear for 10 seconds each at different position each time. The specific tasks with instructions are as follows:

Now we will be starting with the tasks. I will be giving you instructions before each task. For each task, try not to blink too much.

Shall we start the tasks.

Task 1: Random letters with no background: In this task, random letters will be projected with no background.

Instructions: For the first task, there will be letters appearing on the screen, look at the letters and try to keep looking at them.

After completing each task: You can look away from the screen if you want to, give your eyes a bit of rest or blink before we move on to the next task (Repeat this after each task).

We are about to start the second task.

Task 2: Random letters with busy background: In this task, random letters will be projected on busy background.

Instructions: There will be letters appearing on the screen again. Try to keep looking at them.

After completing task: You can look away from the screen if you want to, give your eyes a bit of rest or blink before we move on to the next task.

Task 3: Multiple letters with no background: In this task random multiple letters will be projected on the screen. Participants will be asked to find and focus on letter E each time.

Instructions: For the third task, there will be multiple letters appearing on the screen. Look for letter ‘E” and maintain you focus on it. Let me know if you can’t find ‘E’ anytime.

After completing task: You can look away from the screen if you want to, give your eyes a bit of rest or blink if you want to before we move on to the next task (Repeat this after each task).

We are about to start with the fourth task.

Task 4: Multiple letters on busy background: In this task random multiple letters will be projected with busy background on the screen. The participant will be asked to focus on letter E each time.

Instructions: For this task, there will be multiple letters appearing on the screen again. Look for letter ‘E” and maintain you focus on it. Let me know if you can’t find ‘E’ anytime.

You can look away from the screen if you want to, give your eyes a bit of rest or blink if you want to before we move on to the next task.

Are you comfortable? Shall we move on to the next one?

Task 5: Random letters on complex background: In this task, random letters will be projected on a complex 2d moving background.

Instructions: For the next task, there will be single letter appearing on the screen. Keep looking at the letter for the whole time it is on the screen as you previously did.

You can look away from the screen if you want to, give your eyes a bit of rest or blink if you want to before we move on to the next task.

We are now on the last task.

Task 6: Random letters on moving background: in this task, random letters on a moving background will be projected.

Instructions: For this task, there will be single letter appearing on the screen again. Keep looking at each letter as they appear as you previously did.

The session will end after this task. All the gear: eye tracker glasses, markers will be taken off the participant.

Data Analysis

Data analysis will be performed to determine the intergroup differences (Healthy adults and people with motion sensitivity after vestibular disorder) for:

1. Fixational characteristics: No. of fixation and re- fixation, Duration of fixation, Fixation dispersion

2. Centre of pressure in A-P axis and lateral axis.

Eye movement, head position and body sway will be recorded using the eye tracker, body sway using an AMTI force plate, and kinematics of the sway using 3D motion analysis (Qualisys). This will then be repeated in a virtual reality environment and allow comparison between a laboratory environment and a virtual reality environment.

Statistical Analysis

The groups (healthy adults and people with motion sensitivity after vestibular disorder) will be compared using linear mixed modelling if normality is justified; otherwise an appropriate generalized linear mixed model will be selected on the advice of a biostatistician.

References

Agrawal, Y., et al. (2009). "Disorders of balance and vestibular function in US adults: data from the National Health and Nutrition Examination Survey, 2001-2004." Archives of internal medicine 169(10): 938-944.

Ajrezo, L., et al. (2013). "Saccades improve postural control: a developmental study in normal children." PLoS One 8(11): e81066.

Angelaki, D. E. and B. J. Hess (2005). "Self-motion-induced eye movements: effects on visual acuity and navigation." Nature reviews. Neuroscience 6(12): 966.

Angelaki, D. E. and B. J. Hess (2005). "Self-motion-induced eye movements: effects on visual acuity and navigation." Nature Reviews Neuroscience 6(12): 966.

Angelaki, D. E., et al. (2003). "Foveal versus full-field visual stabilization strategies for translational and rotational head movements." Journal of Neuroscience 23(4): 1104-1108.

Beer, A. L., et al. (2008). "A motion illusion reveals mechanisms of perceptual stabilization." PLoS One 3(7): e2741.

Bisdorff, A., et al. (2013). "The epidemiology of vertigo, dizziness, and unsteadiness and its links to co-morbidities." Frontiers in neurology 4: 29.

Dannenbaum, E., et al. (2011). "Visual vertigo analogue scale: an assessment questionnaire for visual vertigo." Journal of Vestibular Research 21(3): 153-159.

Fajen, B. R. and J. S. Matthis (2013). "Visual and non-visual contributions to the perception of object motion during self-motion." PLoS One 8(2): e55446.

Gielen, C., et al. (2004). "Retinal slip during active head motion and stimulus motion." Experimental brain research 155(2): 211-219.

Guerraz, M. and A. Bronstein (2008). "Ocular versus extraocular control of posture and equilibrium." Neurophysiologie Clinique/Clinical Neurophysiology 38(6): 391-398.

Lappe, M., et al. (1999). "Perception of self-motion from visual flow." Trends in cognitive sciences 3(9): 329-336.

Leigh, R. J. and D. S. Zee (2015). The neurology of eye movements, Oxford University Press, USA.

Lencer, R. and A. Clarke (1998). "Influence of optokinetic and vestibular stimuli on the performance of smooth pursuit eye movements: implications for a clinical test." Acta oto-laryngologica 118(2): 161-169.

Martinez-Conde, S. (2006). "Fixational eye movements in normal and pathological vision." Progress in brain research 154: 151-176.

Martinez-Conde, S., et al. (2004). "The role of fixational eye movements in visual perception." Nature Reviews Neuroscience 5(3): 229-240.

Massion, J. (1994). "Postural control system." Current opinion in neurobiology 4(6): 877-887.

Murakami, I. (2004). "Correlations between fixation stability and visual motion sensitivity." Vision research 44(8): 751-761.

Murakami, I., et al. (2006). "A positive correlation between fixation instability and the strength of illusory motion in a static display." Vision research 46(15): 2421-2431.

Otero-Millan, J., et al. (2012). "Microsaccades and blinks trigger illusory rotation in the “rotating snakes” illusion." Journal of Neuroscience 32(17): 6043-6051.

Pola, J., et al. (1995). "Visual fixation of a target and suppression of optokinetic nystagmus: effects of varying target feedback." Vision research 35(8): 1079-1087.

Poletti, M., et al. (2010). "Stability of the visual world during eye drift." Journal of Neuroscience 30(33): 11143-11150.

Redfern, M. S. and J. Furman (1993). "Postural sway of patients with vestibular disorders during optic flow." Journal of vestibular research: equilibrium & orientation 4(3): 221-230.

Redfern, M. S., et al. (2001). "Visual influences on balance." Journal of anxiety disorders 15(1): 81-94.

Rey, F., et al. (2008). "Saccades horizontal or vertical at near or at far do not deteriorate postural control." Auris Nasus Larynx 35(2): 185-191.

Rodrigues, S. T., et al. (2013). "Effects of saccadic eye movements on postural control stabilization." Motriz: Revista de Educação Física 19(3): 614-619.

Sharon, J. D. and T. E. Hullar (2014). "Motion sensitivity and caloric responsiveness in vestibular migraine and Meniere's disease." The Laryngoscope 124(4): 969-973.

Silva, A. M., et al. (2016). "Dizziness handicap inventory and visual vertigo analog scale in vestibular dysfunction." International archives of otorhinolaryngology 20(3): 241-243.

Strupp, M., et al. (2003). "Eye movements and balance." Annals of the New York Academy of Sciences 1004(1): 352-358.

Thomas, N. M., et al. (2016). "Eye movements affect postural control in young and older females." Frontiers in aging neuroscience 8.

Thomas, N. M., et al. (2017). "Smooth pursuits decrease balance control during locomotion in young and older healthy females." Experimental Brain Research: 1-8.

Van Ombergen, A., et al. (2016). "The effect of optokinetic stimulation on perceptual and postural symptoms in visual vestibular mismatch patients." PLoS One 11(4): e0154528.

Warren, W. H. and D. J. Hannon (1990). "Eye movements and optical flow." JOSA A 7(1): 160-169.

Webb, N. A. and M. J. Griffin (2003). "Eye movement, vection, and motion sickness with foveal and peripheral vision." Aviation, space, and environmental medicine 74(6): 622-625.

Wertheim, A. H. (1994). "Motion perception during selfmotion: The direct versus inferential controversy revisited." Behavioral and Brain Sciences 17(2): 293-311.

Winkler, P. A. and K. J. Ciuffreda (2009). "Ocular fixation, vestibular dysfunction, and visual motion hypersensitivity." Optometry-Journal of the American Optometric Association 80(9): 502-512.