The Spectral Extent of Phasic Suppression of Loudness and Distortion-Product Otoacoustic Emissions by Infrasound and Low-Frequency Tones

Abstract

We investigated the effect of a biasing tone close to 5, 15, or 30 Hz on the response to higher-frequency probe tones, behaviorally, and by measuring distortion-product otoacoustic emissions (DPOAEs). The amplitude of the biasing tone was adjusted for criterion suppression of cubic DPOAE elicited by probe tones presented between 0.7 and 8 kHz, or criterion loudness suppression of a train of tone-pip probes in the range 0.125-8 kHz. For DPOAEs, the biasing-tone level for criterion suppression increased with probe-tone frequency by 8-9 dB/octave, consistent with an apex-to-base gradient of biasing-tone-induced basilar membrane displacement, as we verified by computational simulation. In contrast, the biasing-tone level for criterion loudness suppression increased with probe frequency by only 1-3 dB/octave, reminiscent of previously published data on low-side suppression of auditory nerve responses to characteristic frequency tones. These slopes were independent of biasing-tone frequency, but the biasing-tone sensation level required for criterion suppression was ~ 10 dB lower for the two infrasound biasing tones than for the 30-Hz biasing tone. On average, biasing-tone sensation levels as low as 5 dB were sufficient to modulate the perception of higher frequency sounds. Our results are relevant for recent debates on perceptual effects of environmental noise with very low-frequency content and might offer insight into the mechanism underlying low-side suppression.

Keywords: biasing; cochlear mechanics; human cochlea; infrasound; low-frequency hearing.

Fig. 1

Continuous shift of the tone-pip probes (red dots) relative to the cycle of a 15 Hz BT. The probe repetition rate is 8 Hz. Two “beat” periods are shown

Fig. 2

A Individual DPOAE suppression thresholds (biasing-tone level required to suppress the 2f1-f2 DPOAE by 6 dB) for biasing tones of 5, 15, and 30 Hz are shown for 18 participants. Bold lines are linear fits in the frequency range 0.7 − 3 kHz, where data were available from most subjects. The levels of f2 were set to 50 phon, and the f1 parameters were optimized for each f2 to maximize the 2f1-f2 DPOAE level. B Individual loudness suppression thresholds as function of pip-train frequency for the biasing tones of 7, 15, and 31 Hz. Bold lines are linear fits in the frequency range 0.25 − 2 kHz, where data were available for almost all subjects. Both panels: whisker plots show relevant statistics for the BT detection thresholds for the two subject groups participating in each experiment (minimum, maximum, quartiles, and median). Filled circles show average BT levels at 1000 Hz. For individual slope values, see Table 1

Fig. 3

Finite element model of the human cochlea. a Fluid compartments (gray) and BM (blue) of half of the cochlear box model (i.e., half its width). Because its cross sections were symmetrical, only one half of the model had to be numerically solved. b View of the cross-sectional mesh in the x-direction. c View of the cross-sectional mesh in the opposite direction, showing the surface of the helicotrema compartment. The vertical dash-dotted lines in b and c indicate the model’s symmetry. d Lateral view of the mesh at the apical end of the (half) model shown in a in the y-direction, including the 1.35-mm-long helicotrema compartments. e Magnitude and phase of BM displacement along the cochlea in response to tones of various frequencies. The dotted lines beyond 35 mm represent fluid displacement inside the helicotrema. The legend in the lower panel gives also the slopes of the displacement expressed in dB/octave between the characteristic frequencies (lower abscissa scale) of 0.5 and 16 kHz (only 2–16 kHz for the 1-kHz tone), derived from the characteristic places for tonal stimulations. As can be seen in the lower panel, the BM moves in-phase along its entire length when stimulated at and below 30 Hz

Fig. 4

Modulation detection thresholds obtained in control experiments for pip-train probes (black) and continuous-tone probes (red) using a 15-Hz biasing tone. Data were averaged across the twelve new subjects. Also, data from the main experiment, obtained using a full-suppression criterion with a 15-Hz biasing tone, are re-plotted (blue). The criterion in both control experiments was the detection of a modulation. The error bars indicate ± 1 SD of the individual modulation thresholds (in dB SL) at each probe frequency. Note that the steep drop at 8 kHz in the continuous-tone curve is likely due to missing 8-kHz data from two subjects (for whom the safety limit was reached), as suggested by the local slope between 4 and 8 kHz for the remaining 10 subjects (red dashed line)

Fig. 5

Relative BT threshold levels for DPOAE suppression (red) and loudness-suppression (blue) combined over all subjects (main and control experiments) by cumulative-slope analysis (see Methods). Shaded regions represent standard error of the means for the local slopes. The loudness iso-suppression curve was given an arbitrary level of 0 dB at 1000 Hz. The DPOAE iso-suppression curve was set higher by 7 dB at 1000 Hz, corresponding to the difference in BT levels required for DPOAE and loudness suppression at this frequency. (This difference was averaged across all 15-Hz and 30/31-Hz data from the main experiments only). For comparison, chinchilla auditory-nerve data from Fig. 11 of Temchin et al. (1997) are also shown at an arbitrary level scale as function of fiber CF, while keeping their original relative levels. Solid lines represent thresholds for suppression of CF tones. Their slopes are very similar to our loudness iso-suppression curve. Dashed lines represent thresholds to the suppressor tones alone, which had frequencies between 50 and 400 Hz. Below 2 kHz, their slopes rather match that of our DPOAE iso-suppression curve. For convenience, the grid of dotted lines indicates the slope of 9 dB/octave expected for the longitudinal gradient of BM displacement in response to the suppressor tones