Pernille Holtegaard1, Mercedes C. Duvig1, Teresa M. C. Gallo1 , Bastian Epp1
1Hearing Systems Section, Department of Health Technology, Technical University of Denmark, DK-2800, Kgs. Lyngby, Denmark
Cochlear synaptopathy (CS) has been suggested to predominantly target low spontaneous rate (SR) fibers. The majority of research in CS hypothesize deficits at supra-threshold levels in accordance with this assumption. However, recent work based on auditory models suggests that the loss also includes high-SR fibers, and that these have an off-frequency contribution to coding of moderate-to-high level sounds. This study aims to investigate the integrity of low- and high-SR fibers in CS. Two outcome measures were assumed to reflect AN-fiber integrity, gap detection thresholds (GDTs) and loudness functions. Low presentation levels were assumed to reflect the integrity of on-frequency high-SR fibers, while moderate-to-high levels were assumed to reflect the integrity of low-SR fibers and off-frequency high-SR fibers. The measure of middle ear muscle reflex (MEMR) strength was used as a proxy measure for CS, and a relationship between MEMR and outcome measures was hypothesized. Listeners with high-frequency sensorineural hearing loss (SNHL) with and without noise-induced tinnitus were recruited. It was hypothesized that listeners with tinnitus, a suggested perceptual consequence of CS, would exhibit higher GDTs and shallower loudness functions compared to listeners without tinnitus. GDTs and loudness functions were measured across a range of presentation levels at low frequencies where hearing sensitivity was normal (<25 dB HL). No significant differences were observed between groups for GDTs or MEMR strength. The slope of the upper part of the loudness function at 0.25 kHz was significantly shallower in the group with tinnitus, but no difference was found at 0.5 or 1 kHz. Also, no correlations were found between MEMR strength and the two outcome measures. Overall, the results did not support the hypotheses. However, the results were confounded by small variations in hearing sensitivity, and therefore can neither confirm nor dismiss the role of high-SR fibers in CS.
Acknowledgements: William Demant Foundation.