Efferent modulation of pre-neural and neural distortion products

Abstract

Distortion product otoacoustic emissions (DPOAEs) and distortion product frequency following responses (DPFFRs) are respectively pre-neural and neural measurements associated with cochlear nonlinearity. Because cochlear nonlinearity is putatively linked to outer hair cell electromotility, DPOAEs and DPFFRs may provide complementary measurements of the human medial olivocochlear (MOC) reflex, which directly modulates outer hair cell function. In this study, we first quantified MOC reflex-induced DPOAE inhibition at spectral fine structure peaks in 22 young human adults with normal hearing. The f1 and f2 tone pairs producing the largest DPOAE fine structure peak for each subject were then used to evoke DPFFRs with and without MOC reflex activation to provide a related neural measure of efferent inhibition. We observed significant positive relationships between DPOAE fine structure peak inhibition and inhibition of DPFFR components representing neural phase locking to f2 and 2f1-f2, but not f1. These findings may support previous observations that the MOC reflex inhibits DPOAE sources differentially. That these effects are maintained and represented in the auditory brainstem suggests that the MOC reflex may exert a potent influence on subsequent subcortical neural representation of sound.

Keywords: Distortion product otoacoustic emissions; Efferent; Frequency following response; Medial olivocochlear reflex.

Figure 1

Pre-neural and neural correlates of the two source model. a) F1 and f2 stimuli (blue and red waveforms, respectively) mix acoustically in the ear canal to form a two-tone complex waveform (purple). The complex waveform is decomposed on the basilar membrane into two cochlear traveling waves representing f1 (blue arrow) and f2 (red arrow) frequencies. Energy from the distortion source, where f1 and f2 traveling waves overlap, propagates backward toward the ear canal and forward to the CDP place. The reflection source arises from coherent scattering of energy at the CDP place, which also propagates toward the ear canal and mixes with distortion source energy. Auditory nerve fibers tuned to f1, f2, and CDP center frequencies feed each component forward into the neural code. b) The DPOAE fine structure represents peaks at which distortion and reflection sources constructively (black arrow) and destructively (subsequent trough) interfere. c) Phase locking to f1, f2, and CDP components initiated by auditory nerve fibers is represented in the ensemble behavior of auditory brainstem nuclei and recorded from the scalp as the FFR. (Note: The f2-f1 (or ASSR) potential corresponding to the amplitude modulated envelope of the two tone stimulus (purple) is not shown.

Figure 2

Effects of CAS on DPOAE fine structure and complementary FFRs in one subject. a) DPOAE fine structure is plotted as a function of f2 on a linear pressure scale. Fine structure is shown in quiet (black) and with CAS (red). The fine structure peak was identified at f2 = 1111 Hz. b) The probe tones corresponding to the DPOAE fine structure peak were used to evoke FFRs. Spectra are plotted for quiet (black line) and with CAS (red line) conditions. CDP= 721 Hz, F1-FFR = 916 Hz, F2-FFR = 1111 Hz. Note also that an additional FFR corresponding to 3f1-2f2 at 526 Hz is apparent.

Figure 3

DPOAE fine structure peak inhibition for each subject plotted as a function of f2 frequency (Error bars = SEM of repeated measurements). The gray triangle corresponds with the subject data shown in Figure 2a.

Figure 4

FFR peak amplitudes in quiet and with CAS (Error Bars = SEM).

Figure 5

Scatter plots showing relationships between (square-root transformed) DPOAE inhibition and FFR inhibition as a function of FFR spectral peak. Lines of best fit for each comparison are shown; note that the outlier in the first panel (gray circle) was not included in the model. Regression equations for each comparison were: F1-FFR (y= −0.9473x + 2.95), F2-FFR (y= 5.1641x − 4.252), and CDP-FFR (y = 8.3733x − 3.4955). See text for more detail.