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 affects humans and whether it contributes to suprathreshold speech-in-noise intelligibility deficits. For 94 human listeners with normal audiometry (aged 12-68 years; 64 women), we measured click-evoked auditory brainstem responses (ABRs), self-reported lifetime noise exposure, and speech reception thresholds 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 function, 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. Speech reception thresholds in noise were not correlated with wave-I growth rates and mean speech reception thresholds 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.