Abstract
Several physiological mechanisms act on the response of the auditory nerve (AN) during acoustic stimulation, resulting in an adjustment in auditory gain. These mechanisms include—but are not limited to—firing rate adaptation, dynamic range adaptation, the middle ear muscle reflex, and the medial olivocochlear reflex. A potential role of these mechanisms is to improve the neural signal-to-noise ratio (SNR) at the output of the AN in real time. This study tested the hypothesis that neural SNRs, inferred from non-invasive assessment of the human AN, improve over the duration of acoustic stimulation. Cochlear potentials were measured in response to a series of six high-level clicks embedded in a series of six lower-level broadband noise bursts. This paradigm elicited a compound action potential (CAP) in response to each click and to the onset of each noise burst. The ratio of CAP amplitudes elicited by each click and noise burst pair (i.e., neural SNR) was tracked over the six click/noise bursts. The main finding was a rapid (< 24 ms) increase in neural SNR from the first to the second click/noise burst, consistent with a real-time adjustment in the response of the auditory periphery toward improving the SNR of the signal transmitted to the brainstem. Analysis of cochlear microphonic and ear canal sound pressure recordings, as well as the time course for this improvement in neural SNR, supports the conclusion that firing rate adaptation is likely the primary mechanism responsible for improving neural SNR, while dynamic range adaptation, the middle ear muscle reflex, and the medial olivocochlear reflex played a secondary role on the effects observed in this study. Real-time improvements in neural SNR are significant because they may be essential for robust encoding of speech and other relevant stimuli in the presence of background noise.
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Notes
An artifact created by the probe microphone/tube assembly occurred for the first burst, resulting in a spectral difference between the first burst compared to the remaining bursts. This spectral difference was identified as an artifact, rather than of biological origin, by conducting probe microphone measurements in a syringe with the probe microphone tubing position, foam ear tip depth, and syringe volume set to approximate those associated with measurement in the human ear canal. Sound pressure measurements in the syringe resulted in a constant spectral difference between the first burst and the remaining bursts, similar to that observed in ear canal measurements of human participants. The spectral configuration of the artifact varied among participants, consistent with individual differences in outer and middle ear anatomy and probe insertion depth. Thus, the use of the second burst as the reference provides a custom correction factor for each participant. Note that this approach should have little to no impact on assessing the effects of the MEM reflex on neural SNR given that MEM activity is not expected for the 24 ms between the first and second burst, as the time constant for the MEM reflex is roughly 50 ms (Hung and Dallos 1972).
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This work was supported by grant K23 DC014752 from NIH/NIDCD (PI: Jennings).
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Jennings, S.G., Dominguez, J. Firing Rate Adaptation of the Human Auditory Nerve Optimizes Neural Signal-to-Noise Ratios. JARO 23, 365–378 (2022). https://doi.org/10.1007/s10162-022-00841-7
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DOI: https://doi.org/10.1007/s10162-022-00841-7