Analysis of click-evoked auditory brainstem responses using time domain cross-correlations between interleaved responses.

Objectives: The rapidly evolving field of early diagnostics after the introduction of newborn hearing screening requires rapid, valid, and objective methods, which have to be thoroughly evaluated in adults before use in infants. The aim was to study cross-correlation analysis of interleaved auditory brainstem responses (ABRs) in a wide dynamic range in normal-hearing adults. Off-line analysis allowed for comparison with psychoacoustical click threshold (PCT), pure-tone threshold, and determination of ABR input/output function. Specifically, nonfiltered and band-pass filtered ABRs were studied in various time segments along with time elapsed for ensemble of sweeps reaching a specific detection criterion.

Design: Fourteen healthy normal-hearing subjects (18 to 35 years of age, 50% females) without any history of noise exposure participated. They all had pure-tone thresholds better than 20 dB HL (125 to 8000 Hz). ABRs were recorded in both ears using 100 μsec clicks, from 71.5 dB nHL down to -18.5 dB nHL, in 10 dB steps (repetition rate, 39 Hz; time window, 15 msec; filter, 30 to 8000 Hz). The number of sweeps increased from 2000 at 71.5 dB nHL, up to 30000 at -18.5 dB nHL. Each sweep was stored in a data base for off-line analysis. Cross-correlation analysis between two subaverages of interleaved responses was performed in the time domain for nonfiltered and digitally band-pass filtered (300 to 1500 Hz) entire and time-windowed (1 to 11 and 5 to 11 msec) responses. PCTs were measured using a Bekesy technique with the same insert phone and stimulus as used for the ABR (repetition rate, 20 Hz). Time elapsed (≈ number of accepted sweeps/repetition rate) for the ensemble of sweeps needed to reach a cross-correlation coefficient (ρ) of 0.70 (=3.7 dB signal-to-noise ratio [SNR]) was analyzed.

Results: Mean cross-correlation coefficients exceeded 0.90 in both ears at stimulus levels ≥11.5 dB nHL for the entire nonfiltered ABR. At 1.5 dB nHL, mean(SD) ρ was 0.53(0.32) and 0.44(0.40) for left and right ears, respectively (n = 14) (=0 dB SNR). In comparison, mean(SD) PCT was -1.9(2.9) and -2.5(3.2) dB nHL for left and right ears, respectively (n = 14), while mean pure-tone average (500 to 2000 Hz) was 2.5 dB HL (n = 28). Almost no effect of band-pass filtering or reduced analysis time window existed. Average time elapsed needed to reach ρ = 0.70 was approximately 20 seconds or less at stimulus levels ≥41.5 dB nHL, and ≈30 seconds at 31.5 dB nHL. The average (interpolated) stimulus level corresponding to ρ=0.70 for the entire nonfiltered ABR was 6.5 dB nHL (n = 28), which coincided with the estimated psychoacoustical threshold for single clicks.

Conclusions: ABR could be identified in a short period of time using cross-correlation analysis between interleaved responses. The average stimulus level corresponding to 0 dB SNR in the entire nonfiltered ABR occurred at 1.5 dB nHL, 4 dB above the average PCT. The mean input/output function for the ensemble of sweeps required to reach ρ = 0.70 increased monotonically with increasing stimulus level, in parallel with the ABR based on all sweeps (≥1.5 dB nHL). Time domain cross-correlation analysis of ABR might form the basis for automatic response identification and future threshold-seeking procedures.