Human cochlear masking and OAE suppression measured using dynamic stimuli

Background

Previous recordings of mouse basilar-membrane (BM) motion in response to exponential frequency chirps have shown that upward sweeps (i.e., from low to high frequency) are more effective suppressors of tonal probe signals than downward sweeps (high to low) (Charaziak, ARO 2022). That study found that the differential suppressive effects of sweep direction depend strongly on sweep rate, being stronger for faster sweeps. These results in mice are reminiscent of findings from human perceptual studies using so-called “Schroeder-phase complexes,” which are stimuli that contain multiple harmonic frequency components of equal amplitude but variable starting phase, summing to produce a series of periodic linear frequency sweeps. Behavioral studies find that negative Schroeder-phase maskers (e.g., upward sweeps) are more effective maskers of tonal signals than positive Schroeder-phase maskers (e.g., downward sweeps). The psychoacoustic literature on Schroeder-phase masking interprets the dependence on sweep direction as arising not primarily from suppression, but from cochlear dispersion (phase curvature), an interpretation supported by animal measurements and physiological models.

Methods

The current study investigates the effects of sweep rate and direction in normal hearing humans, both behaviorally and physiologically. We use exponential frequency sweeps, as in Charaziak (2022), rather than the linear sweeps produced by Schroeder-phase complexes, hypothesizing that the use of exponential sweeps will provide new opportunities to relate masking and suppression to cochlear processing, which is roughly exponential with frequency. For the psychoacoustic experiments, detection thresholds for tonal signals masked by upward and downward sweeps are measured as a function of sweep rate and direction. In the same subjects, stimulus-frequency otoacoustic emissions (SFOAEs) at the tone frequency are recorded using the sweep combinations from the psychoacoustic paradigm as suppressors. In addition to providing a measure of the suppression produced by the various sweep waveforms, the SFOAE measurements provide estimates of cochlear dispersion and delay that can be used to interpret sweep rates (e.g., as “fast” or “slow”).

Results 

Differences in psychoacoustic masking and OAE suppression for various sweep combinations will be explored and compared. To the extent that the two phenomena share common generation mechanisms, we expect behavioral masking and OAE suppression to depend similarly on sweep characteristics.

Conclusions

The long-term goal of our study is to better understand dynamic aspects of nonlinear signal processing in the cochlea. Here, we expand on previous work in mice (Charaziak, 2022) to explore the effects of chirp sweep rate and direction on signal suppression and masking in normal hearing human listeners.