Identifying the Origin of Effects of Contralateral Noise on Transient Evoked Otoacoustic Emissions in Unanesthetized Mice

Abstract

Descending neural pathways in the mammalian auditory system are known to modulate the function of the peripheral auditory system. These pathways include the medial olivocochlear (MOC) efferent innervation to outer hair cells (OHCs) and the acoustic reflex pathways mediating middle ear muscle (MEM) contractions. Based on measurements in humans (Marks and Siegel, companion paper), we applied a sensitive method to attempt to differentiate MEM and MOC reflexes using contralateral acoustic stimulation in mice under different levels of anesthesia. Separation of these effects is based on the knowledge that OHC-generated transient evoked otoacoustic emissions (TEOAE) are delayed relative to the stimulus, and that the MOC reflex affects the emission through its innervation of OHC. In contrast, the MEM-mediated changes in middle ear reflectance alter both the stimulus (with a short delay) and the emission. Using this approach, time averages to transient stimuli were evaluated to determine if thresholds for a contralateral effect on the delayed emission, indicating potential MOC activation, could be observed in the absence of a change in the stimulus pressure. This outcome was not observed in the majority of cases. There were also no statistically significant differences between MEM and putative MOC thresholds, and variability was high for both thresholds regardless of anesthesia level. Since the two reflex pathways could not be differentiated on the basis of activation thresholds, it was concluded that the MEM reflex dominates changes in TEOAEs induced by contralateral noise. This result complicates the identification of purely MOC-induced changes on OAEs in mice unless the MEM reflex is inactivated surgically or pharmacologically.

Keywords: Contralateral noise; Olivocochlear; Otoacoustic emissions.

FIG. 1

A cartoon of the animal with head plate along with the time-domain measurement procedures. Titanium head plates were affixed to the mouse skull to prevent head movements during recordings. The time-domain measurement contains two parts: (1) TEOAE screening and (2) CAS recording. During the CAS measurements, the 1429 ms broadband noise was presented to the contralateral ear as a sequence interleaved with silent periods of equal length. CAS levels ranged from 55 to 110 dB SPL in 5 or 10 dB steps. Tone pips were presented to the ipsilateral ear 593 ms after the onset and offset of the CAS. Blocks of stimulus presentations were repeated to reach the final number of presentations in the time average used to compute ΔP _stim or ΔP _TEOAE.

FIG. 2

An example of CAS-induced ΔP _stim and/or ΔP _TEOAE in an animal (43 days of age) from group C. The probe and reference were 60 and 80 dB peSPL tone pips at 21.57 kHz. a Time waveforms of probe, TEOAE, and changes in the ear canal pressure induced by different levels of CAS. Shaded regions designate time intervals for the stimulus (red) and the response/TEOAE (Franklin et al. 2007). a On the right side of the ordinate, we also indicate the change in pressure as ΔP in micropascal. b RMS values of ΔP _stim (red), ΔP _TEOAE (purple) and the average (±1 standard deviation) rms value of the NF (gray) for each tested CAS level. NF is the average noise floor in 50 non-overlapping windows (0.5 ms in duration) randomly selected in the non-stimulus portion of each recording. The gray shaded area represents rms values below the average NF. Notice that ΔP _stim at CAS levels of 60, 65, and 70 dB SPL is within this region. c CC, the Pearson product-moment correlation coefficient, of the probe and ΔP _stim during CAS at 75 dB SPL (red), as well as the TEOAE and ΔP _TEOAE for CAS at 60, 65, 70, and 75 dB SPL (purple). CCs below 0.3 were marked as no correlation. When rms values are below the NF, the corresponding CC is not plotted. In other words, no CC is plotted for ΔP _stim when the CAS was presented at 60, 65, and 70 dB SPL.

FIG. 3

An example of CAS-induced ΔP _stim and/or ΔP _TEOAE in an animal (43 days of age) from group C. The figure format is identical to that of Fig. 2. The probe and reference were 60 and 80 dB peSPL, respectively, and the stimulus frequency was 16.88 kHz. a Time waveforms of probe, TEOAE, and CAS-induced changes in ear canal pressure. b RMS values of ΔP _stim (red), ΔP _TEOAE (purple) and the average (±1 standard deviation) rms NF (gray) for each tested CAS level. When the CAS was 50 dB SPL, both ΔP _stim and ΔP _TEOAE were in this range. c CC of the probe and the ΔP _stim (red), as well as the TEOAE and the ΔP _TEOAE (purple) during CAS at 60, 70, and 80 dB SPL. Because rms values for both ΔP _stim and ΔP _TEOAE were below the NF when the CAS was presented at 50 dB SPL, a CC was not plotted.

FIG. 4

a The average ΔP _stim (red, n = 50) and ΔP _TEOAE (purple, n = 15) thresholds were 94.4 ± 13.5 and 90.0 ± 7.1 dB SPL in group A, 78.8 ± 13.1 and 70.6 ± 13.1 dB SPL in group B, and 75.3 ± 14.7 and 69.4 ± 14.0 dB SPL in group C. Error bars represent standard deviations. b The proportion of animals showing ΔP _TEOAE effects without ΔP _stim in groups A (25 %), B (27.8 %), and C (33.3 %). c The average differences of ΔP _stim (red) and ΔP _TEOAE thresholds in animals who showed exclusive ΔP _TEOAE were 7.5 ± 3.5 (n = 2), 9.2 ± 4.7 (n = 5), and 9.8 ± 5.7 (n = 8) dB in groups A, B, and C respectively.

FIG. 5

An example of TEOAEs from an animal receiving 1 % isoflurane. a Probe time waveform used to evoke TEOAEs. Probe and reference tones were 50 and 70 dB peSPL at 24 kHz. TEOAE time waveforms recorded before (b) and 60 min after (c) receiving 1 % isoflurane.

FIG. 6

TEOAEs recorded from group A when the animal was anesthetized and during recovery. The probe and reference levels were 50 and 70 dB peSPL tone pips at 23 kHz. a Probe time waveform. bTEOAE time waveform. c, e Changes induced by CAS at 110 dB SPL. d, f Changes induced by CAS at 100 dB SPL. There was no acoustic cross talk at either 100 or 110 dB SPL.