Peripheral and central vestibular contributions to perception of rotation in healthy aging

The vestibular system, located bilaterally in the inner ear, plays an integral role in maintaining standing balance and interpreting our perception of self-motion. Healthy aging causes anatomical deterioration in both peripheral/brainstem vestibular regions and central vestibular integration structures including thalamus, cortex, and cerebellum. While traditional assessments of rotational perception rely on minimal detection thresholds, they fail to examine central integration of vestibular information using suprathreshold stimulation. Moreover, these detection threshold protocols often fail to identify well-known effects of aging on the vestibular system. Therefore, this thesis investigated how healthy aging affects peripheral and central vestibular perception of rotation using a suprathreshold stimulation paradigm. Twenty healthy older adults (range: 60-78 years old) and twenty-five healthy young adults (range: 18-29 years old) performed a rotation perception task on a custom-built rotating platform. Participants tracked platform rotation using a rotary dial while experiencing suprathreshold pseudorandom sinusoidal rotations (0.02-0.11 Hz; ±30°) with eyes closed and white noise masking. Peripheral/brainstem vestibular function was assessed by testing the visual vestibulo-ocular reflex (VVOR) using identical pseudorandom sinusoids. Visual-vestibular interactions were evaluated using the subjective visual vertical test (SVV), where participants determined when a vertical line returned to 0° after 30° clockwise and counterclockwise rotations. Performance metrics included frequency-based measures of coherence, gain, and phase between platform and dial rotation, VVOR gain and phase comparing eye velocity to head velocity, and SVV error from both rotation directions. Older adults demonstrated significantly lower coherence (p=0.002) and significantly higher phase shift (p<0.001) in the rotation perception task, indicating impaired central integration of vestibular information. Older adults also showed significantly lower VVOR gain (p<0.001), suggesting peripheral/brainstem vestibular deficits. However, gain differences between age groups were not observed in the perception of rotation task. SVV performance showed no age-related differences, with all participants performing within the accepted error range (±2°). These findings demonstrate that older adults exhibit deficits in central integration of vestibular information that do not directly correlate with peripheral/brainstem function assessments using suprathreshold stimulation. This understanding may inform future clinical techniques for assessing vestibular deficits in older adults and patients with vestibular disorders.