Aging of the medial olivocochlear reflex and associations with speech perception

Abstract

The medial olivocochlear reflex (MOCR) modulates cochlear amplifier gain and is thought to facilitate the detection of signals in noise. High-resolution distortion product otoacoustic emissions (DPOAEs) were recorded in teens, young, middle-aged, and elderly adults at moderate levels using primary tones swept from 0.5 to 4 kHz with and without a contralateral acoustic stimulus (CAS) to elicit medial efferent activation. Aging effects on magnitude and phase of the 2f1-f2 DPOAE and on its components were examined, as was the link between speech-in-noise performance and MOCR strength. Results revealed a mild aging effect on the MOCR through middle age for frequencies below 1.5 kHz. Additionally, positive correlations were observed between strength of the MOCR and performance on select measures of speech perception parsed into features. The elderly group showed unexpected results including relatively large effects of CAS on DPOAE, and CAS-induced increases in DPOAE fine structure as well as increases in the amplitude and phase accumulation of DPOAE reflection components. Contamination of MOCR estimates by middle ear muscle contractions cannot be ruled out in the oldest subjects. The findings reiterate that DPOAE components should be unmixed when measuring medial efferent effects to better consider and understand these potential confounds.

Figure 1

Audiograms for 35 individuals in the elderly group. The typical and atypical subgroups are designated by line, and the grand mean is superimposed. Atypical elderly had slightly elevated mean thresholds compared to typical elderly; relative threshold elevation ranged from 0.7 dB (at 6 kHz) to 5.3 dB (at 1 kHz).

Figure 2

Individual examples of DPOAE fine structure in no-noise (NoN) and during contralateral acoustic stimulation (CAS) at 60 dB SPL for each of the four age groups. The inset in each group shows two superimposed NoN averages and illustrates the typical level variability.

Figure 3

Mean DPOAE medial olivocochlear reflex in dB (MOCR_dB) and standard error of the mean (SEM) for the four age groups plotted at third-octave intervals. Although data are shown at individual center frequencies, frequency was collapsed into low and high categories (1414 Hz boundary) for statistical testing. Negative values indicate reductions in DPOAE level with CAS; the dashed line represents no level change.

Figure 4

Mean DPOAE medial olivocochlear reflex (A) normalized by baseline magnitude (MOCR_n) and (B) as magnitude of the difference vector (MOCR_Vn) for the four age groups plotted at third-octave intervals. Frequency was collapsed into low and high categories (1414 Hz boundary) for statistical testing. Error bars represent the SEM. For MOCRn, significant age (F = 10.4; p < 0.0001) and frequency effects (F = 40.4; p < 0.0001) were observed and an interaction (F = 4.9; p = 0.003); for MOCR_Vn, significant age (F = 7.9; p < 0.0001) and frequency effects (F = 35.5: p < 0.0001) were observed but no interaction.

Figure 5

Mean (A) distortion- (MOCR_Dn) and (B) reflection-component (MOCR_Rn) MOC reflex normalized by baseline magnitude for each of the four age groups and plotted at third-octave intervals. Error bars represent the SEM. There were significant effects of age for both indices (MOCR_Dn: F = 7.6, MOCR_Rn: F = 8.2; p < 0.001 both) and frequency (MOCR_Dn: F = 107.4, MOCR_Rn: F = 23.3; p < 0.0001 both). MOCR_Dn also showed an interaction: F = 3.3; p < 0.024.

Figure 6

DPOAE fine structure in NoN and CAS at 60 dB SPL recorded from one atypical elderly adult. In this group of elderly (not used for calculation of MOC reflexes), the presentation of CAS produced more pronounced fine structure, even at the lowest CAS levels (60 dB SPL).

Figure 7

Total reflection-component energy measured from time domain filters centered to estimate energy (A) at the DP place (2f₁−f₂) which is considered the main reflection site and (B) associated with longer latencies (LL), presumably related to multiple internal reflections. Both DP place and LL reflection is shown in NoN and at three levels of contralateral noise (CAS) for both the typical and atypical elderly groups. Neither group shows systematic changes in DP-place reflection once CAS is introduced; the atypical subset shows statistically significant increases in LL reflection with increases in CAS. Error bars represent SEM.

Figure 8

The additional phase periods accumulated (re: main reflection at 2f₁−f₂) for longer-latency reflection. All age groups show additional phase accumulation for the later component (as expected because of the time-domain filters utilized), but only the elderly groups, and most notably the atypical elderly, show increased phase accumulation when CAS was presented. This result is consistent with multiple internal reflections. Error bars represent SEM.

Figure 9

There were ten significant positive correlations between indices of the MOCR reflex and performance on vowels and consonants (overall percent-correct), as well as the effectiveness in transmission of speech feature information at 0 and −3 dB SNR. These correlations were conducted without the elderly subjects, and included data from individuals ranging from 13 to 58 years of age.