Effects of contralateral acoustic stimulation on spontaneous otoacoustic emissions and hearing threshold fine structure

Abstract

Medial olivocochlear (MOC) influence on cochlear mechanics can be noninvasively, albeit indirectly, explored via the effects of contralateral acoustic stimulation (CAS) on otoacoustic emissions. CAS-mediated effects are particularly pronounced for spontaneous otoacoustic emissions (SOAEs), which are typically reduced in amplitude and shifted upward in frequency by CAS. We investigated whether similar frequency shifts and magnitude reductions were observed behaviorally in the fine structure of pure-tone hearing thresholds, a phenomenon thought to share a common underlying mechanism with SOAEs. In normal-hearing listeners, fine-resolution thresholds were obtained over a narrow frequency range centered on the frequency of an SOAE, both in the absence and presence of 60-dB SPL broadband CAS. While CAS shifted threshold fine structure patterns and SOAEs upward in frequency by a comparable amount, little reduction in the presence or depth of fine structure was observed at frequencies near those of SOAEs. In fact, CAS typically improved thresholds, particularly at threshold minima, and increased fine structure depth when reductions in the amplitude of the associated SOAE were less than 10 dB. Additional measurements made at frequencies distant from SOAEs, or near SOAEs that were more dramatically reduced in amplitude by the CAS, revealed that CAS tended to elevate thresholds and reduce threshold fine structure depth. The results suggest that threshold fine structure is sensitive to MOC-mediated changes in cochlear gain, but that SOAEs complicate the interpretation of threshold measurements at nearby frequencies, perhaps due to masking or other interference effects. Both threshold fine structure and SOAEs may be significant sources of intersubject and intrasubject variability in psychoacoustic investigations of MOC function.

FIG. 1

A Ear canal spectra showing the selected SOAE for one subject in quiet and with CAS. B Frequency and level of the twelve SOAEs chosen for study in the SOAE-specific measurements in quiet (blue circles) and with CAS (red squares). Solid lines connect the frequencies/levels of the same SOAE in the two conditions. C CAS-mediated changes in the frequency (as a percentage of the frequency in quiet) and level of each SOAE.

FIG. 2

A–C Thresholds measured in quiet (blue) and with CAS (red) for three subjects with no measurable SOAEs within the test range. Representative ear canal spectra are shown for each condition (light blue and red traces). Thresholds measured in each of two sessions (“S1” and “S2”, thin solid and dashed traces, respectively) are shown along with their average (thick solid trace). D–F CAS-mediated changes in threshold (CAS-quiet) for each of the individual sessions (thin and solid dashed traces, as above) and the average change (thick trace) for each subject.

FIG. 3

SOAE-specific measurements in quiet and with CAS for three representative subjects. Each column contains data from a single subject plotted versus frequency distance (in octaves) from the SOAE frequency measured in quiet. The nominal SOAE frequency, f _SOAE, is indicated at the top of the column. Threshold curves and representative ear canal spectra from the two individual test sessions are shown separately in panels A–C and D–F, respectively. For clarity, blue and red arrows indicate the SOAE amplitude in quiet and with CAS for subject 26FR (panels C, F). Changes in threshold (CAS-quiet) for each of the two sessions (thin solid and dashed gray lines) and their average (thick black line) are shown in panels G–I.

FIG. 4

Threshold curves from the three subjects in Figure 3 replotted to minimize the effect of the frequency shift in the fine structure pattern. A–C Thresholds for each session (thin and dashed lines) and the two-session average (thick lines) are plotted as a function of the frequency distance from the SOAE frequency measured in the same condition (quiet or CAS), such that all curves are aligned relative to their minima. D–F CAS-mediated changes (CAS-quiet) in the aligned threshold curves are shown for each session (thin and dashed lines) along with their average (thick lines).

FIG. 5

Individual (thin gray lines) and average (thick black line) CAS-mediated changes in thresholds for all twelve SOAE-specific measurements. In A, threshold changes were calculated at frequencies relative to the SOAE frequency in quiet, thus preserving the effects of the frequency shift in the fine structure patterns (as in Fig. 3). In B, changes were calculated after first aligning the threshold curves with respect to their minima, i.e., by plotting thresholds as a function of frequency distance from the SOAE in the same condition (as in Fig. 4).

FIG. 6

A–B The frequency of each threshold minimum (f _Min) corresponded closely with the associated SOAE frequency (f _SOAE), both in quiet and CAS. The frequency difference (f _Min re f _SOAE) is shown in Hz (A) and as a percentage (B) for each subject (open symbols) along with the average (solid symbols, +/− one standard deviation). Threshold minima tended to occur slightly lower in frequency than the associated SOAE. Asterisks indicate that this frequency difference was significant at the P < 0.05 level according to two-tailed, paired t tests for the CAS condition. C–D CAS-mediated changes in SOAE frequency (Δ f _SOAE) were significantly linearly correlated with changes in the threshold minimum frequency (Δ f _Min), whether expressed in Hz (C) or as a percentage (D). Adjusted r ² values are shown in each panel with asterisks indicating significance at the P < 0.05 level. Solid black lines indicate the linear regression fits, and gray dashed lines are unity.

FIG. 7

A–D Relationships between CAS-mediated changes in SOAE level and changes in average threshold level (T _Ave), threshold minimum level (T _Min), threshold maxima level (T _Max), and threshold fine structure depth. At least for SOAE level reductions less than 10 dB (the ten points to the right of the dashed vertical lines), larger SOAE level reductions were associated with greater threshold improvements and increases in fine structure depth. Spearman’s rank correlation coefficients calculated for these ten data points are shown in each panel, with asterisks indicating significance at the P < 0.05 level. For reference, solid black lines are the linear regression fits computed for these data, and dashed gray lines are y = x in A–C or y = −x in D. Data from the two subjects with SOAE reductions larger than 10 dB indicate that threshold improvements may saturate or reverse with increasing CAS effects.

FIG. 8

Thresholds and representative ear canal spectra for three subjects with distinct SOAE profiles and CAS-mediated threshold changes. Each subject completed threshold measurements from 1 to 4 kHz in 1/100th-octave steps over the course of approximately 20 test sessions. Threshold measurements were repeated twice, with frequency lists between sessions overlapped by five points, so that all thresholds represent the average of two to four measurements. For subjects 08FR (B) and 17FL (C), black triangles and brackets indicate frequency regions where thresholds were improved by CAS. Blue and red arrows in subject 17FL’s plot indicate the peak spectral level of an SOAE measured in quiet and with CAS, respectively. See text for further discussion.

FIG. 9

CAS-mediated changes in thresholds across frequency for the three subjects in Figure 8.