Normal auditory functiona and without previous auditory or vestibular symptoms or complaints. Normal vestibular function documented with the video-head impulse test.

A cochlear implant (CI) is a device providing a sense of sound to a person who suffers from severe to profound sensorineural hearing loss. A CI comprises the following parts, a microphone (capturing the sound from the environment), a speech processor (receiving and encoding the sounds captured by the mircophone), a transmitter-receiver antenna pair (transmitting the information from the external to the implanted components), an implanted stimulator (converting the signal into a tonotopically arranged set of electrical pulses) and an electrode array inserted in the cochlea that will deliver the electrical pulses to different portions of the auditory nerve. Trains of electrical stimulation in the form of charge-balanced, biphasic pulses can be delivered through each of the electrodes in the cochlear array and modulated via computer-controlled signals or audio signals (captured with a microphone).

Diagnosis established on the basis of the consensus document of the Classification Committee of the Bárány Society (Strupp et al., Journal of Vestibular Research, vol. 27, no. 4, pp. 177-189, 2017).

Patients with documented diagnosis of unrecovered unilateral vestibulopathy, consistent with the current classification of vestibular disorders of the Bárány Society (www.jvr-web.org/ICVD.html).

Different combinaisons of auditory, visual and vestibular stimuli are used to determine the respective TBW. The participants are sited in a chair and presented with two different stimuli. They are then asked which stimulus comes first. In patients implanted with a VI, the investigators specifically stimulate the vestibular system and pair this stimulation to a visual or auditory one. The time interval between the two stimuli is progressively decreased until they are percieved as simultaneous. It is thus possible to calculate the TBW.

The participant is sited with eyes open. Electrophysiological responses are recorded during the presentation of brief and controlled stimulus. In subjects with a vestibular implant the evoked potentials are recorded following an electrical vestibular stimulation. An electrical cochlear stimulation is used for subjects with a cochlear implant. In healthy controls a brief sound or bone vibration is used.

Electroencephalography (EEG) is a brain exploratory method the investigators will use to estimate the latency of evoked cortical and subcortical potentials following a visual, auditory and vestibular stimulation. The electrical signals are recorded using electrodes placed on the participant scalp.

For cervical-VEMP recording, a very brief electrical stimulation of the vestibular system is applied. The participant lies on an examination table and is asked to lift his head and turn it toward the stimulation opposite side in order to contract the sternocleidomastoid muscle (SCM) whose myogenic activity is recorded using surface electrodes. The stimulus is the same for ocular-VEMP recording. The sited participant is asked to keep his gaze up and the ocular muscles myogenic activity is recorded.

The participant is sited in a chair anchored to a six degrees of freedom motion platform. Stimuli are different types of movements. The participant is asked to mention if he perceives a movement and if so to eventually give its direction. The statistical analyses of these answers allow to determine a perception threshold for each type of movement.

Participant is asked to walk six meters straightforward at: normal, slow, and fast auto-selected speeds as well as the fastest speed possible. Changes in three-dimensional kinematics during the different tasks will be assessed using a 12-camera optoelectronic motion capture system. Participants will be equipped with 35 reflective markers placed on specific anatomical landmarks according to the full-body. The test is performed twice per participant within a 1-week interval, to allow reliability (test-retest) analyses.

Partipants are sited on a chair. At the "Go" of the examinor they are asked to stand up, walk for three meters, make a u-turn, come back and sit back on the chair. Changes in three-dimensional kinematics during the different tasks will be assessed using a 12-camera optoelectronic motion capture system. Participants will be equipped with 35 reflective markers placed on specific anatomical landmarks according to the full-body. The test is performed twice per participant within a 1-week interval, to allow reliability (test-retest) analyses.

The FGA is used to assess postural stability during walking and assesses an individual's ability to perform multiple motor tasks while walking. The FGA comprises ten tasks during which changes in three-dimensional kinematics will be assessed using a 12-camera optoelectronic motion capture system. Participants will be equipped with 35 reflective markers placed on specific anatomical landmarks according to the full-body.The test is performed twice per subject within a 1-week interval, to allow reliability (test-retest) analyses.

The investgators will use a modified SMART EquiTest to implement a custom protocol developed to identify potential biomarkers of vestibular deficiency using pseudorandom stimulus waveforms to perturb balance. The device delivers continuous surface or visual surround rotations that evoke antero-posterior body sway in participants. The test starts with a 4 min warm-up in order to familiarize the participant with the environment. Then participants undergo 4-min test in 3 conditions: (1) surface-tilt stimuli with eyes closed, (2) surface-tilt with eyes open and visual surround fixed, and (3) visual surround tilt with fixed surface with all using 2° peak-to-peak stimulus amplitudes. Participants are also be equipped with a 3-DOF Head Tracker (part of the EquiTest system) continuously recording head movements in the Yaw, Pitch, and Roll planes. Postural assessments areperformed twice per participants within a 1-week interval, to allow reliability (test-retest) analyses.

During the experiments, participants have to read aloud sequences of Sloan optotypes of decreasing size displayed in a random order one at a time on a computer screen . The sequence starts with a five letters presentation at 1 logMAR (logarithm of the Minimum Angle of Resolution). If the letter recognition rate is above chance (>10%), the letter size is decreased by a step of 0.1 logMAR and five new letters are presented one at a time. The experiments is carried out on a treadmill either in statoc or in dynamic (fastest walking speed as possible) condition.

Participants will perform a path integration or 'complete the triangle' virtual reality task in which the subject moves in a virtual environment toward two visual targets following each other and then has to come-back to his starting point. The angular error can then be calculated.

The subject lies on an examination table and is at rest since at least 10mn. Blood pressure and cardiac frequency are measured in lying position. At time zero the subject stands up and his blood pressure and cardiac frequency are then measured at different time points (1mn, 3mn, 5mn and 10mn after time zero). The test will be systematically performed by trained medical staff. All relevant clinical signs of the participant which could appear during the test will be documented.

Using an eye tracker (EyeLink) to provide a reliable and objective measurement of pupillary size, symmetry, and reactivity through measurement of the pupil light reflex, the investigators will measure changes in pupil size before and right after the activation of the vestibular implant.