Idiopathic Bilateral Vestibulopathy: Peripheral Vestibular Disorders and Their Repercussions

This assessment, designed to evaluate the impact of the vestibular deficit on everyday tasks, will include questionnaires and standardised clinical tests.

The questionnaires are the Fall Efficacy Scale; the Dizziness Handicap Inventory; the Oscillopsia Severity Questionnaire; the Hospital Anxiety and Depression Scale; the Hamilton Anxiety Scale; the Global Physical Activity Questionnaire; the Motion Sickness Susceptibility Questionnaire part B; and the Cambridge Depersonalization Scale.

In addition, three standardised clinical tests will be carried out: Five Times Sit to Stand Test; Tandem walking test; and Fukuda step test (50 steps).

Five specific clinical tests will also be carried out: Sit-to-stand-with-walk-and-turn; Perception of active body rotation; Perception of walking distance; Triangle Completion Task; and Gait assessment.

Four cognitive tests from the French Focus Group on Executive Functions Assessment (GREFEX) battery will be carried out: the Stroop test; the Trail Making Test; the Baddeley's dual task; and a test of Corsi's Blocks.

This test, also known as the unilateral centrifugation test, provides a quantitative assessment of unilateral utricular otolith function, whereas the tests currently used (vestibular evoked myogenic potential, VEMP) only provide a qualitative assessment.

To assess the participants' bone mineral density, the reference technique of densitometry using two-photon absorptiometry will be used.

Participants' ability to maintain their balance will be assessed using measurements taken on the Synapsys platform. To standardise their position, they will have to stand on the platform with their feet apart, arms at their sides and look straight ahead. Volunteers will be asked to sit down between each trial. The surface area and length of the centre of pressure will be measured under open-eye balance conditions with and without image reading/exploration and dynamics. During this test, 9 balance assessment situations lasting approximately 1 minute each will be performed.

Participants will be seated on a rotating chair in the dark. Using a virtual reality headset, three visual stimuli will be presented: (A) no visual stimulus (darkness), (B) visual stimulus corresponding to a displacement of the participant along a cylindrical trajectory, (C) visual stimulus corresponding to a displacement of the participant along a conical trajectory. These 3 types of visual stimuli will be presented alone or during an Off-Vertical Axis Rotation (OVAR) with the chair axis inclined at an angle of 10° to the vertical and a rotation speed of 60°/s.

During each sequence, blood pressure, heart rate and end-tidal carbon dioxide (CO2) will be continuously recorded using standard medical equipment.

Participants will have to complete five standardised clinical questionnaires: Pittsburgh Sleep Quality Index; Spiegel questionnaire assessing sleep quality; Morningness-Eveningness questionnaire; Insomnia Severity Scale; and Epworth Sleepiness Scale.

Moreover, the activity/rest rhythm will be measured continuously by actimetry over a period of 11 days.

The volunteer will also have to wear a Somno-Art® bracelet all night long during the eleven days of monitoring to collect the volunteer's actimetry and heart rate in order to specify which sleep stage the volunteer is in and thus monitor changes in sleep stages over the course of the night.

Finally, participants will have to complete a sleep diary.

• Classic geometric illusions (inverted T, Mueller-Lyer, Ponzo, Poggendorff, Zoellner, Hering) which generate systematic distortions will be used.
• Time perception task: Participants will wear a virtual reality headset in which instructions will appear for 6 consecutive tasks each repeated 10 times with different durations.
• Perception of rotation amplitude and duration: Participants will be seated on a rotating chair, in complete darkness with noise-cancelling headphones. Participant will have to estimate the duration and the amplitude of rotations of the chair.
• Perceptual time constant: Participants will be seated on a rotating chair, in complete darkness, with a mask over their eyes and noise-cancelling headphones. After each chair rotation, participants will have to turn a crank every time the chair stops and reproduce their sensation of rotation in terms of direction (left or right) and intensity.

Participants will wear a virtual reality headset which allows them to be immersed in a virtual room similar in appearance to the one they are in. They will sit on a stool and hold a joystick in their hand. An avatar seen from behind, also sitting on a stool, will be presented in the centre of the virtual room approximately 2 m from them. The experimenter will touch several areas of the participant's back, over his clothes, with the end of the joystick. This movement will be reproduced by the virtual joystick in contact with the avatar's back, in two conditions: the synchronous condition, and the asynchronous condition.

After 2 min of stimulation, participants will perform a mental imagery task: a ball in the background of the virtual scene will roll towards them. After 3'', a black screen will appear and participants will have to imagine that the ball keeps moving towards them at the same speed. They will have to press the trigger when they think the ball has reached their level.

The imaging evaluation will include an acquisition of anatomical images of the brain in its entirety and centred on the hippocampus.; and T2*-weighted images sensitive to the blood-oxygen-level-dependent (BOLD) effect to assess functional brain activation during cognitive tasks and functional brain connectivity during rest.

For the functional activation sequences, three activation tasks will be performed, each lasting approximately 5 minutes: a mental rotation task, a time estimation task, and a prediction task.

These acquisitions will be combined with a collection of cardiorespiratory variables: respiratory movements and plethysmography. These signals will be used in the pre-processing of the functional MRI to remove physiological noise from the BOLD signal.