Visual Influences on Vestibular Adaptation

Impairment of vestibular pathways can lead to deficits in balance, gait, and gaze stability. Gaze-stability exercises are a central component of vestibular rehabilitation and have been shown to improve vision during head movement as well as functional mobility in individuals with peripheral or central vestibular dysfunction. Improvements in gaze stability may occur through vestibulo-ocular reflex (VOR) adaptation or through compensatory saccadic eye movements. However, many adults with vestibular hypofunction also present with uncorrected visual acuity deficits or binocular vision abnormalities, such as low vision, convergence insufficiency, or ocular misalignment. These visual conditions are common but understudied in the context of vestibular rehabilitation, and it is not known whether they limit the capacity for VOR adaptation.

Incremental vestibulo-ocular reflex adaptation (IVA) is a non-invasive, 15-minute training method that strengthens the VOR by exposing users to a controlled visual error signal. IVA uses a moving laser target whose velocity is programmed as a function of the participant's head movement, producing immediate increases in VOR gain. The method can be customized to provide unilateral, bilateral, or asymmetric adaptation, allowing targeted training for individuals with unilateral or bilateral vestibular deficits. IVA has been studied extensively in adults with vestibular hypofunction, but its effectiveness in individuals with impaired visual acuity or binocular vision abnormalities has not been evaluated.

This study will examine whether reduced static visual acuity or binocular vision abnormalities affect the magnitude of VOR adaptation in adults with and without vestibular hypofunction. Two experiments will be conducted using a cross-over design. Experiment 1 will enroll adults with abnormal uncorrected static visual acuity, with and withoutvestibular hypofunction, to compare VOR adaptation with and without vision correction. Experiment 2 will enroll adults with binocular vision abnormalities, with and without vestibular hypofunction, to evaluate VOR adaptation in their best corrected visual state. All participants will complete IVA training during two study visits separated by a washout period.

Inclusion criteria

For All Participants (All Groups)

• Age 18 to 60 years
• Able to provide informed consent

Group-Specific Inclusion Criteria:

• Group 1: Abnormal Uncorrected Static Visual Acuity (No Vestibular Hypofunction) Normal peripheral vestibular function
• Group 2: Abnormal Uncorrected Static Visual Acuity + peripheral Vestibular Hypofunction
• Group 3: Binocular Vision Abnormalities (No Vestibular Hypofunction) Normal peripheral vestibular function
• Group 4: Binocular Vision Abnormalities + peripheral Vestibular Hypofunction

Individuals who have abnormal static visual acuity, a binocular vision abnormality (ocular misalignment, convergence insufficiency), and vestibular loss will be assigned to Group 4.

The following definitions will be used when determining group placement:

• Abnormal Static Visual Acuity: Uncorrected visual acuity (head is still) ≥0.30 logMAR in both eyes.
• Unilateral Vestibular Hypofunction: 60 ms VOR gain <0.80 unilaterally for the lateral semicircular canal.
• Bilateral Vestibular Hypofunction: 60 ms VOR gain <0.80 bilaterally for the lateral semicircular canals.
• Normal Vestibular Function: 60 ms VOR gain of 0.80 to 1.20 bilaterally for the lateral semicircular canals.
• Convergence Insufficiency: ≥6 cm near point of convergence.
• Ocular Misalignment: ≥4 prism diopters of manifest deviation of the eyes (tropia) on cover/uncover testing.

Exclusion criteria

• Diagnosis of fluctuating vestibular disorders (e.g., benign paroxysmal positional vertigo)
• Neurologic conditions (e.g., multiple sclerosis), dementia,
• Alcohol or drug abuse,
• A major psychiatric disorder (e.g., schizophrenia),
• Pain that limits cervical spine range of motion by >50% or that results in an -altered gait pattern.