Abstract
Auditory nerve fibers (ANFs) transmit acoustic information from the sensory hair cells to the cochlear nuclei. In experimental and clinical audiology, probing the whole ANF population remains a difficult task, as the ANFs differ greatly in their threshold and onset response to sound. Thus, low spontaneous rate (SR) fibers, which have rather higher thresholds, delay and larger jitter in their first spike latency are not detectable in the far-field compound action potential of the auditory nerve. Here, we developed a new protocol of acoustic stimulation together with electrophysiological signal processing to track the steady state activity of ANFs. Mass potentials at the round window were recorded in response to repetitive 300-ms bursts of 1/3 octave band noise centered on a frequency probe. Analysis was assessed during the last 200-ms of the response to capture the steady-state response of ANFs. To eliminate the microphonic component reflecting the sensory cells activity, repetitive pairs of sounds of opposite polarities were used. The spectral analysis was calculated on the average of two consecutive responses, and the neural gain was calculated by dividing point-by-point the spectrum to sound over unstimulated condition. In response to low-sound-level stimulation, neural gain predominated in the low-frequency cochlear regions, while a second component of responses centered on higher cochlear frequency regions appeared beyond 30 dB SPL. At 60 dB SPL, neural gain showed a bimodal shape, with a notch near 5.6 kHz. In addition to correlate with the functional mapping of ANFs along the tonotopic axis, the deletion of low-SR fibers leads to a reduction in the high-frequency response, where the low-SR fibers are preferentially located. Thus, mass potentials at the round window may provide a useful tool to probe the SR-based distribution of ANFs in humans and in other species in which direct single-unit recordings are difficult to achieve or not feasible.
Highlights
Within the cochlea, the mechano-transduction is achieved by sensory inner hair cells (IHCs) that convert acoustic stimulus into trains of action potentials along afferent auditory nervePLOS ONE | DOI:10.1371/journal.pone.0169890 January 13, 2017Detection of Low Spontaneous Rate Fibers at the Round Window fibers (ANFs)
To probe the efficiency of power spectral density (PSD) amplitude for the detection of the subtle loss of low-spontaneous rate (SR) fibers, we compared sound-evoked responses recorded 6 days after a 30 min infusion of 33 μM ouabain into the round window niche (n = 9) to those of controls obtained with control artificial perilymph (n = 10)
Mass potentials at the round window were recorded in response to 300-ms bursts of 1/3 octave band noise centered on a frequency probe (Fig 1A)
Summary
The mechano-transduction is achieved by sensory inner hair cells (IHCs) that convert acoustic stimulus into trains of action potentials along afferent auditory nervePLOS ONE | DOI:10.1371/journal.pone.0169890 January 13, 2017Detection of Low Spontaneous Rate Fibers at the Round Window fibers (ANFs). The ANFs show a great diversity in term of threshold, characteristic frequency (CF), and spontaneous rate (SR) of discharge, to encode sounds over a large intensity and frequency range [1,2]. ANFs have the striking capability to “phase-lock” to low-frequency tones up to several kHz [8]. This frequency limit of neural phase-locking, inner hair cell (IHC) membrane potential cannot follow the sound stimulation waveform [9] and frequency coding relies on the place where each frequency produces vibrations along the basilar membrane (i.e., cochlear tonotopy). Phase-locking occurs to stimulus envelope ([10] for a review). The range of modulation frequencies encoded by a single ANF is well characterized by a low-pass modulation transfer function with a 3-dB cut-off frequency, which increases with CF of the fibers [11]
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