A Pilot Study of Phase-Evoked Acoustic Responses From the Ears of Human Subjects

Temporal properties of otoacoustic emissions (OAEs) are of interest as they help understand the dynamic behavior and spatial distribution of the generating mechanisms. In particular, the ringing behavior of responses to clicks and tone bursts have been investigated, and times of arrival and roundtrip delays have been related to properties of the dispersive cochlea and internal reflections. Temporal suppression experiments (e.g. Kemp and Chum, 1980; Verhulst et al., 2008), where a suppressor click is presented just before the stimulus click, have shown how a click response depends on preceding cochlear excitation revealing properties of the nonlinearity responsible for OAE generation.

To explore the temporal properties of OAEs further, we studied acoustic responses from the ears of nine young, normal-hearing human subjects to abrupt changes in the phase of pure tones. The measurement paradigm was similar to a standard scale-subtract technique, but instead of scaling the stimulus level linearly, to average out linear parts of the response, the phase was scaled linearly. Specifically, the phase of 500-Hz, 1000-Hz and 2000-Hz pure tones at 50 and 60 dB SPL was cosine-faded 90, 120 and 180 degrees within one, three and five periods of the stimulus-frequency every 64 ms (54 conditions). Using a combination of level and phase variation, emissions linked to any time-invariant nonlinearity could be extracted. Phase-evoked residual responses (PERRs) look like tone bursts with a phase-shift in the middle.

According to a 6-dB criterion on the signal-to-noise ratio, five/nine subjects had PERRs in more than 15/36 conditions, disregarding the 18 180-degree conditions which evoked only five responses in total. Across subjects and conditions, stimulus-frequency OAEs were present in 358/468 (76%) measurements. 125 of those and six of those without SFOAEs had PERRs. The 120-degree conditions evoked more PERRs than the 90- and 180-degree conditions. One-kHz conditions evoked slightly more than 2-kHz conditions and many more than 500-Hz conditions. Furthermore, the prevalence decreases with lower stimulus-level and longer phase-transitions.

It is possible that PERRs originate in the same dynamic OAE mechanisms as those which can be probed in click- and click-suppression experiments. Contrary to the click stimulus, however, the phase-changing stimulus is narrower in the frequency domain and the responses may therefore reflect the dynamic behavior of more localized regions of OAE generators.