Evidence for age-related cochlear synaptopathy in humans unconnected to speech-in-noise intelligibility deficits

Cochlear synaptopathy (or the loss of primary auditory synapses) remains a subclinical condition of uncertain prevalence. Here, we investigate whether it occurs in humans, and whether it contributes to suprathreshold speech-in-noise intelligibility deficits. For 94 human listeners with normal audiometry (aged 12-68 years; 64 female), we measured click-evoked auditory brainstem responses (ABRs), self-reported lifetime noise exposure, and speech reception thresholds (SRTs) for sentences (at 65 dB SPL) and words (at 50, 65 and 80 dB SPL) in steady-state and fluctuating maskers. Based on animal research, we assumed that the shallower the rate of growth of ABR wave-I amplitude versus level, the higher the risk of suffering from synaptopathy. We found that wave-I growth rates decreased with increasing age but not with increasing noise exposure. SRTs were not correlated with wave-I growth rates, and mean SRTs were not statistically different for two subgroups of participants (N=14) with matched audiograms (up to 12 kHz) but different wave-I growth rates. Altogether, the data are consistent with the existence of age-related but not noise-related synaptopathy. In addition, the data dispute the notion that synaptopathy contributes to suprathreshold speech-in-noise intelligibility deficits. [We thank Filip Rønne, Niels H. Pontoppidan, and James M. Harte for useful discussions. BCZ was supported by a postdoctoral scholarship from the Conselho Nacional de Desenvolvimento Científico e Tecnológico (Brasil). Work supported by the Oticon Foundation, Junta de Castilla y León (grant SA023P17), European Regional Development Fund and the Spanish Ministry of Economy and Competitiveness (grant BFU2015-65376-P) to EAL-P.]