Age-Related Changes in the Shape and Frequency Limits of the Amplitude-modulated cVEMP Temporal Modulation Transfer...

Amplitude-modulated cervical vestibular evoked myogenic potentials (AMcVEMPs) are hypothesized to reflect nonlinear rectification by saccular hair cells. Their temporal modulation transfer function (TMTF) offers a noninvasive measure of vestibular temporal processing. While age-related changes in TMTFs have been reported, the bandwidth and shape of the TMTF across age groups remain undefined. Defining these limits may clarify age-related vestibular processing changes, and examining age-related changes in shape may reveal insights into saccular-collic pathway integrity. This study aimed to (1) examine age-related differences in AMcVEMP TMTF shape and (2) define TMTF limit and optimal modulation frequencies across age groups.

Forty-nine participants grouped as young (N = 16), middle-aged (N = 17), and older adults (N = 16) were included. AMcVEMPs were recorded using a 500 Hz carrier frequency, 100% amplitude-modulated at 10 modulation frequencies (11 to 397 Hz). Stimuli were delivered to the mastoid via B81 transducers, and participants were instructed to maintain ipsilateral sternocleidomastoid muscle activation (electromyographic ≈ 50 µV). Responses were characterized using established AMcVEMP analyses for amplitude, signal to noise ratio (SNR), and phase coherence (PC). The TMTF shapes were assessed using individually normalized metrics. The TMTF limits were defined as modulation frequencies with >75% response rates (SNR, PC) and modulation gain >-10 dB.

AMcVEMP responses (amplitude, SNR, and PC) declined with age. AMcVEMP tuning showed age-related trends, with amplitude TMTF peaks tending to shift upward (37 to 53 Hz) and SNR and PC peaks tending to shift downward (79 to 53 Hz); however, none of these peak shifts were statistically significant. TMTF bandwidth reduced with age, decreasing from 17 to 173 Hz in young to 23 to 113 Hz in middle-age adults. In older adults, 53 Hz yielded the highest response rate (approximately %). All of the young and middle-aged adults had a present response at least at one modulation frequency, with 79 Hz yielding the highest response rate (100% in young and >80% among middle-aged adults). Across groups, frequencies between 23 and 113 Hz elicited the strongest responses.

Age-related degradations are seen in AMcVEMP. The trend toward an upward shift in normalized amplitude and downward shifts in normalized SNR and PC may reflect age-related changes in sternocleidomastoid properties; however, these interpretations should be made with caution. Reduced TMTF limit suggests diminished vestibular temporal processing with aging. The best response between 23 and 113 Hz across age groups informs optimization of AMcVEMP protocols and their clinical utility; the high inter-subject variability across groups, and lower response rates among older adults, remain a challenge. Exploring alternative carrier frequencies (e.g., 100 to 250 Hz) that are closer to the resonance frequency of the otolith organs would be an interesting avenue for future investigation.