Multiple indices of the 'bounce' phenomenon obtained from the same human ears

Abstract

Loud low-frequency sounds can induce temporary oscillatory changes in cochlear sensitivity, which have been termed the 'bounce' phenomenon. The origin of these sensitivity changes has been attributed to slow fluctuations in cochlear homeostasis, causing changes in the operating points of the outer hair cell mechano-electrical and electro-mechanical transducers. Here, we acquired three objective and subjective measures resulting in a comprehensive dataset of the bounce phenomenon in each of 22 normal-hearing human subjects. We analysed the level and phase of cubic and quadratic distortion product otoacoustic emissions and the auditory thresholds before and after presentation of a low-frequency stimulus (30 Hz sine wave, 120 dB SPL, 90 s) as a function of time. In addition, the perceived loudness of temporary, tinnitus-like sensations occurring in all subjects after cessation of the low-frequency stimulus was tracked over time. The majority of the subjects (70 %) showed a significant, biphasic change of quadratic, but not cubic, distortion product otoacoustic emissions of about 3-4 dB. Eighty-six percent of the tested subjects showed significant alterations of hearing thresholds after low-frequency stimulation. Four different types of threshold changes were observed, namely monophasic desensitisations (the majority of cases), monophasic sensitisations, biphasic alterations with initial sensitisation and biphasic alterations with initial desensitisation. The similar duration of the three bounce phenomenon measures indicates a common origin. The current findings are consistent with the hypothesis that slow oscillations of homeostatic control mechanisms and associated operating point shifts within the cochlea are the source of the bounce phenomenon.

FIG. 1

The relation between OP shifts and the magnitude of cubic and quadratic distortions, estimated by the absolute value of the second (B, simulated QDP level) and third derivative (B, simulated CDP level), respectively, of a two-exponential Boltzmann function (A) representing the transfer function of the outer hair cell MET. Note that the second derivative shows a notch when the OP is close to the inflection point (in a more symmetric position), whereas the third derivative has a maximum at this position.

FIG. 2

Level and phase of QDPs (f ₂ − f ₁; A, B, respectively) and CDPs (2f ₁ − f ₂; C, D, respectively) as a function of time after LF sound exposure (30 Hz sine wave, 120 dB SPL, 90 s) relative to the corresponding mean of the pre-exposure period. Bold lines represent median DPOAE measures, and grey lines are individual DPOAE measures. For QDPs, only data from subjects showing a significant level change (see ‘Recording of DPOAE Level and Phase After LF Sound Stimulation’ section, two-sided t test, t(35) = 2.8–17.6, p = 10⁻¹⁶–0.01) after LF sound exposure are shown (N = 28 from 14 subjects). For CDPs, only recordings from the same 14 subjects with sufficiently large CDP levels (signal-to-noise ratio ≥ 6 dB) are shown (N = 22 from 11 subjects). CDP levels were extracted from the same data as QDP levels, for which f ₂/f ₁ ratios were optimised. Please note that while CDPs typically do not show significant changes of level (two-sided t test, t(35) = 0.005–2.6, p = 0.011–0.99) and phase (two-sided t test, t(35) = 0.004–2.6, p = 0.011–0.99) after LF sound exposure, we observed ‘abnormal’ behaviour with significant changes (two-sided t test, t(35) = 2.8–9.4, p = 10⁻¹¹–0.008) in two subjects (LS and SL, thin black lines). Controls (E–H) as above, but without LF sound exposure.

FIG. 3

Simulation of the level change (relative to the mean of the pre-exposure period) of CDP and QDP as a function of sinusoidal OP changes. QDP levels change markedly while CDP remain almost unchanged (A, top panel) when the OP is moved sinusoidally (A, bottom panel) around an initial OP position (B, black symbol) near the inflection point of the Boltzmann function. QDP and CDP change with opposite sign (C, top panel) when the OP is sinusoidally changed (C, bottom panel) around an initial OP position further away from the inflection point (D). QDP and CDP change with the same sign (E, top panel), when the OP is changed sinusoidally (E, bottom panel) around an initial OP position far away from the inflection point (F). Dashes lines in B, D and F indicate the range of the OP shift. See also Fig. 1 for the behaviour of QDP and CDP levels as a function of OP shifts.

FIG. 4

Perceived loudness of tinnitus-like sensations after LF sound exposure (30 Hz sine wave, 120 dB SPL, 90 s) relative to control recordings without LF sound exposure as a function of time after LF sound exposure. Individual recordings from 22 subjects are shown in grey, the median in black.

FIG. 5

Pooled monophasic changes of hearing thresholds probed with 1, 2 and 4 kHz after LF sound exposure relative to the mean of the pre-exposure period. Both desensitisation (A) and sensitisation (E) occurred, with desensitisations dominating. A simulated initial OP below (black symbol, B) or above (F) the inflection point on the Boltzmann function can produce a monophasic desensitisation (D) or sensitisation (H), if the first derivative (i.e. the slope) of the Boltzmann function (C, G) is used to simulate outer hair cell efficiency. D, H: outer hair cell efficiency as a function of an OP change following a damped oscillation, in hyperpolarising direction on the MET transducer function and back (i.e. corresponding to a movement of the organ of Corti towards scala tympani and back). Monophasic sensitisation occurred in subjects AK, JU, ML, PP and RM, and monophasic desensitisations occurred in subjects AI, AK, CF, CB, KF, LS, ML, NH, PW, RS, SW, SL, TH and TS. In A and E, solid lines = 1 kHz probe tone, dashed-dotted line = 2 kHz, dotted line = 4 kHz. Dashed lines in B, C, F and G indicate the range of the OP shift.

FIG. 6

Pooled biphasic changes of hearing thresholds probed with 1, 2 and 4 kHz after LF sound exposure relative to the mean of the pre-exposure period. Both initial sensitisation (A) and desensitisation (E) occur. A simulated initial OP above (black symbol, B) or below (F) the inflection point on the Boltzmann function can produce a biphasic change with initial sensitisation (D) or desensitisation (H). D, H: as in Fig. 5, with a sinusoidal OP change. Biphasic changes with initial sensitisation occurred in subjects FK and RS, with initial desensitisation in the subjects AI, CF and KB. Solid lines = 1 kHz probe tone, dashed-dotted line = 2 kHz, dotted line = 4 kHz. Dashed lines in B, C, F and G indicate the range of the OP shift.