The origin of short-latency transient-evoked otoacoustic emissions

Bandpass filtered transient-evoked otoacoustic emission (TEOAE) waveforms are composed of short-latency (SL) and long-latency (LL) portions. The LL portion has latency consistent with generation through linear coherent reflection at the tonotopic place on the basilar membrane. The short-latency (SL) portion occurs earlier in time and exhibits less compressive growth. Several mechanisms have been hypothesized to explain generation of the SL portion, including 2f1-f2 intermodulation distortion and coherent reflection basal to the tonotopic place. Two experiments were designed to examine the generation mechanism and generation location of the SL portion. Experiment 1 tests the hypothesis that the SL portion results from low-side, cubic intermodulation distortion. Experiment 2 determines the region along the basilar membrane at which the SL portion of the TEOAE is generated.

The null hypothesis that the SL portion of the TEOAE is generated through low-side, cubic intermodulation distortion requires stimuli with broad frequency content. Stimulus energy at different frequencies (f1 and f2) is presumed to interact simultaneously across the cochlear partition, generating a distortion-source OAE. To test this hypothesis, OAEs were evoked using 2 kHz tone-bursts with durations spanning the time-frequency continuum between a click and a pure tone. As tone-burst duration increases, stimulus energy at the primary frequencies (f1 and f2) decreases and the input to any nonlinear distortion source is reduced. Accordingly, if generated through 2f1-f2 distortion, the magnitude of the SL portion of the TEOAE was expected to decrease as tone-burst duration increased. Results were inconsistent with generation of the SL portion through intermodulation distortion. As tone-burst duration increased, the SL portion remained present in the TEOAE. The presence of the SL portion influenced the level-dependency of TEOAE latency and magnitude to the same extent across all tone-burst durations.

The region of generation along the cochlear partition of the SL portion has implications for the mechanism through which it is generated. Generation through low-side, cubic intermodulation distortion (2f1-f2) would occur near the f2 tonotopic place. If generation is through coherent reflection, a generation region basal to that of the tonotopic place is hypothesized. To determine the cochlear region where the SL portion is generated, TEOAEs were evoked by 2 kHz tone-bursts in the presence of different suppressor stimuli. The amount of suppression induced by each suppressor on the OAE was measured, and the suppressor frequency causing greatest suppression of a given portion of the TEOAE was interpreted as corresponding to that portion's generation place along the basilar membrane. For analysis purposes, the SL portion was divided into two SL time-windows (SL1 and SL2). The LL portion of the TEOAE was maximally suppressed by a 2.07 kHz tone, consistent with generation at the tonotopic place. Both SL components were generated basal to the tonotopic place. The later-occurring SL portion of the TEOAE (SL1) was generated between 1/4-1/3-octave basal to the tonotopic place while the earlier-occurring SL portion (SL2) was generated 3/5-octave basal to the tonotopic place. The generation region of the SL1 portion of the TEOAE was too apical to be consistent with generation through 2f1-f2 distortion. Although the generation region of the SL2 portion was what would be expected for a 2f1-f2 distortion-source OAE, the latency was too early. Generation of both SL portions may be explained through basal linear coherent reflection. Per this mode of generation, the SL1 and SL2 portions of the TEOAE each likely mirror the underlying mechanics at different regions along the cochlear partition.