Latencies of Pulsed Distortion-Product Otoacoustic Emissions and Their Relation to Auditory Brainstem Responses

To assess system properties of the human auditory system, such as cochlear gain, frequency selectivity, and their dependence on frequency and level, it is essential to examine the interrelation of various readouts. By measuring and analyzing otoacoustic emission (OAE) and auditory brainstem response (ABR) latencies, among others, predictions of cochlear models and applicability of properties such as the minimum-phase principle, level dependence of latencies, or related changes of the gain of a presumed positive-feedback mechanism can be investigated.

Here, we present measurements of the latency of the nonlinear-distortion component of pulsed distortion-product otoacoustic emissions (DPOAE) ( $f_2$ = 1-14 kHz, $L_2$ = 25-85 dB SPL) in 20 ears (12 female, 8 male). This yields a direct estimate of intracochlear traveling-wave build-up by recording the time elapsed between the $f_2$ primary stimulus and the distortion-product pulse response. Thus, this technique does not require deriving latency from phase gradients of the coherent-reflection component of different frequencies, as is done using swept-tone DPOAE or SFOAE.

At low stimulus levels ( $L_2$ = 35 dB), DPOAE latency was 13 ms at $f_2$ = 1 kHz, exponentially to  2 ms at $f_2$ = 12-14 kHz. In periods of the corresponding frequency, this rose from $\approx$ 13 periods at 1 kHz to $\ge$ 25 periods above 6 kHz. Between 3 and 6 kHz, latency showed a steeper rise, departing from a pure exponential relation. Level dependence of latencies varied among subjects, with changes ranging from -2 to -12% per 10 dB level increase. Test-retest reliability of latency determination with pulsed DPOAE was excellent.

For frequencies above 1 kHz and up to 14 kHz, OAE latency data align with a scaling law of $\approx$ 0.3 dB/dB. A transition region between 3 and 6 kHz shows scaling in some ears approaching 1 dB/dB, violating local scaling symmetry. Although comparison with ABR literature reveals some unresolved discrepancies, latencies of pulsed DPOAE allow a way to estimate cochlear tuning properties.