The Role of Visual Fixations in Vision and Motion Sensitivity
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Motion sensitivity is a commonly reported symptom in people with chronic dizziness and is described as a lingering symptom that persists after recovery from an acute vestibular disorder. Adults with motion sensitivity typically report discomfort instigated by environments with rich visual information. There are no effective diagnostic tools or treatments for motion sensitivity and people report being frustrated due to a failure in symptom resolution. Little has been reported in the literature regarding the aetiology of motion sensitivity, making the treatment of motion sensitivity challenging. The reporting of symptoms primarily in environments with rich visual information necessitates an increased understanding of the visual system’s contribution to motion sensitivity. Thus, the overall aim of this thesis was to investigate the visual system’s role in motion sensitivity and identify underlying factors that could guide the development of an intervention. A narrative review of the literature was undertaken to understand the visual system’s role in postural control. Information from the visual system is necessary to estimate movements of self and objects in relation to the environment. This information is essential for spatial orientation and postural control while navigating in an environment. Spatial orientation within the environment requires the integration of visual and vestibular information. The vestibular system contributes to visual stability by generating reflexive eye movements during head movements (the vestibulo-ocular reflex). The vestibulo-ocular reflex compensates for the head movement by moving the eyes in the opposite direction and is essential for gaze stabilisation during head movements. The moment the eyes are focused on a single location, the stabilisation of gaze occurs via visual fixation. Visual fixation consists of three small eye movements. These are microsaccades, ocular drifts and ocular tremors. These small eye movements during visual fixation are essential for visual processing, to acquire information about the scene and to prevent visual fading. Visual fixations plays a significant role in maintaining a stable image by interacting with the environment and the vestibular system. The importance of visual fixations has been highlighted in the literature; however, there is a lack of evidence regarding visual fixations in adults with motion sensitivity. An observational exploratory study was undertaken to investigate visual fixations in adults with motion sensitivity. The observational exploratory study exposed people to six conditions with increasing levels of visual complexity to investigate the characteristics of visual fixations and postural parameters. Healthy adult participants were compared to adults with motion sensitivity. This study demonstrated the presence of fixational instability and an increase in head and postural sway in adults with motion sensitivity, particularly in conditions with complex and moving backgrounds. This study identified two parameters that could differentiate adults with motion sensitivity from healthy adults. The findings from the narrative review and the observational exploratory study suggested visual fixations are a key factor for people with motion sensitivity and that they may be ameliorable by an intervention. Study findings were mapped onto the Medical Research Council’s framework for intervention development. The iterative cycles led to the development of a treatment theory. The treatment theory outlined the physiological basis for fixational instability which posits that the presence of an increased number of microsaccades predisposes an individual to motion sensitivity. Microsaccades increase the transient motion signals rendering an image unstable on the retina, leading to fixational instability. Future work will explore the theory by testing and developing an intervention. The current work has identified parameters that can be used to diagnose motion sensitivity. This will allow a feasible and valid diagnostic tool to be developed for use in clinical settings.