Cochlear synaptopathy and speech-in-noise deficits in normal hearing listeners
Animal studies have well established that hair cell loss following noise exposure or aging is often preceded by loss of synapses between the sensory cells and the auditory nerve fibers. A recent temporal bone study provides histopathological evidences that primary neural degeneration greatly exceeds inner hair cell loss in aging human ears as well (Wu et al., 2018). The silencing of these neurons, especially those with high thresholds and low spontaneous rates, degrades auditory processing and may contribute to difficulties understanding speech in noise. In mice, cochlear synaptopathy has been diagnosed by supra-threshold amplitude of ABR wave 1 (Kujawa and Liberman, 2009), the summed activity of cochlear neurons. The fractional reduction in responses to moderate level tone-pips is correlated with the fractional reduction in synaptic counts (Sergeyenko et al., 2013). Cochlear synaptopathy is also correlated with measures of middle-ear muscle reflex (MEMR) strength, possibly because the missing high-threshold neurons are important drivers of this reflex (Valero et al., 2015, 2018).
We recruited 165 normal hearing subjects (≤ 25 dB HL from 0.25 – 8 kHz) between the ages of 18 and 63, with no history of ear or hearing problems, no history of neurologic disorders and unremarkable otoscopic examinations. All subjects were native speakers of English and passed the Montreal Cognitive Assessment. Word recognition scores were assessed at 55 dB HL in quiet and in difficult listening situations using the NU-6 corpus (with competing white noise at 0 SNR or with a 45% or 65% compression with 0.3 s reverberation). Performance on a modified version of the QuickSIN was measured as well. Outer hair cell function was assessed using DPOAEs from 0.5 kHz to 16 kHz while cochlear function was assessed by electrocochleography (ECochG). EcochG waveforms were obtained in response to 100-µs clicks delivered in alternate polarity at 125 dB pSPL with a repetition rate of 9.1 Hz in absence or in presence of a forward masker consisting of an 8-16 kHz noiseband of 90-ms duration presented at 15 dB SL or 35 dB SL. Finally, one EcochG waveform was obtained from all subjects at a rate of 40.1 kHz. The total noise dose for all ECochG measurements was well within OSHA and NIOSH standards.
MEMR effects were assessed in a second session using a custom method similar to that of Keefe and colleagues (Keefe et al. 2010). This approach measures changes in ear-canal sound pressure to a click probe evoked by an ipsilateral noise elicitor. Specifically, we use a pair of 100-µs clicks at 95 dB SPL separated by a 500-msec elicitor (noise burst with a 2.5 ms ramp) presented 30 ms after the first click and preceding the second by 5 ms. This click-noise-click complex was repeated every 1535 ms, leaving 1 s of silence between noise bursts to allow relaxation of the MEMs. Four complexes were presented at each elicitor level, and elicitor level was raised in 5 dB steps from 40 to 95 dB SPL. For each click-noise-click complex, the spectral difference between the two click waveforms was computed.
Our results show that middle-ear reflex thresholds and electrocochleographic measures of neural health are correlated with speech-recognition performance whereas measures of hair cell function are not, consistent with selective neural loss. Furthermore, forward masking has a differential suppressive effect: both SP- and AP- amplitudes decrease to a greater extent with masking levels in subjects who obtained the poorest word recognition scores when compared to those who did best. These results further support the idea that cochlear synaptopathy may lead to deficits in hearing-in-noise, despite the presence of normal thresholds at standard audiometric frequencies.
Keefe DH, Fitzpatrick D, Liu YW, et al. (2010). Wideband acoustic-reflex test in a test battery to predict middle-ear dysfunction. Hear Res 263:52-65.
Kujawa SG, Liberman MC (2009) Adding insult to injury: cochlear nerve degeneration after "temporary" noise-induced hearing loss. J Neurosci 29: 14077-85.
Sergeyenko Y, Lall K, Liberman MC, et al. (2013) Age-related cochlear synaptopathy: an early-onset contributor to auditory functional decline. J Neurosci 33:13686-94.
Valero MD, Hancock KE, Liberman MC (2015) The middle ear muscle reflex in the diagnosis of cochlear neuropathy. Hear Res 332:29-38.
Valero MD, Hancock KE, Liberman MC (2016) The middle ear muscle reflex in the diagnosis of cochlear neuropathy. Hear Res 332:29-38.
Wu PZ, Liberman LD, Bennett K, et al. (2018) Primary neural degeneration in the human cochlea: evidence for hidden hearing loss in the aging ear. Neuroscience, in press.
Research supported by a grant from the NIDCD (P50 DC015857)