Comparison of the efficiency of various criteria for artifact rejection in the recording of auditory brain-stem responses (ABR)

On chirp stimuli and neural synchrony in the suprathreshold auditory brainstem response

The chirp-evoked ABR has been regarded as a more synchronous response than the click-evoked ABR, referring to the belief that the chirp stimulates lower-, mid-, and higher-frequency regions of the cochlea simultaneously. In this study a variety of tools were used to analyze the synchronicity of ABRs evoked by chirp- and click-stimuli at 40 dB HL in 32 normal hearing subjects aged 18 to 55 years (mean=24.8 years, SD=7.1 years). Compared to the click-evoked ABRs, the chirp-evoked ABRs showed larger wave V amplitudes, but an absence of earlier waves in the grand averages, larger wave V latency variance, smaller FFT magnitudes at the higher component frequencies, and larger phase variance at the higher component frequencies. These results strongly suggest that the chirp-evoked ABRs exhibited less synchrony than the click-evoked ABRs in this study. It is proposed that the temporal compensation offered by chirp stimuli is sufficient to increase neural recruitment (as measured by wave V amplitude), but that destructive phase interactions still exist along the cochlea partition, particularly in the low frequency portions of the cochlea where more latency jitter is expected. The clinical implications of these findings are discussed.

Effects of Artifact Rejection and Bayesian Weighting on the Auditory Brainstem Response During Quiet and Active Behavioral Conditions

Purpose

To evaluate the effects of 2 noise reduction techniques on the auditory brainstem response (ABR).

Method

ABRs of 20 normal hearing adults were recorded during quiet and active behavioral conditions using 2 stimulus intensity levels. Wave V amplitudes and residual noise root-mean-square values were measured following the offline application of artifact rejection and Bayesian weighting. Repeated measures analysis of variance and Bonferroni adjusted pairwise t tests were utilized to evaluate significant main effects and interactions between the 2 noise reduction techniques.

Results

ABRs recorded during the quiet behavioral condition resulted in minimal differences in wave V amplitude and noise reduction improvement, suggesting that the 2 techniques were equally effective under ideal recording situations. During the active behavioral condition, however, the techniques differed significantly in the ability to preserve the evoked potential and reduce noise. Consequently, strict artifact rejection levels resulted in an inherent underestimation of wave V amplitudes when compared with the Bayesian approach.

Conclusion

Artifact rejection had a detrimental effect on waveform morphology of the ABR. This could lead to difficulty in ABR interpretation when patients are active and ultimately result in diagnostic errors.

Weighted averaging of steady-state responses

OBJECTIVE

To compare weighted averaging and artifact-rejection to normal averaging in the detection of steady-state responses.

METHODS

Multiple steady-state responses were evoked by auditory stimuli modulated at rates between 78 and 95 Hz. The responses were evaluated after recording periods of 3, 6 and 10 min, using 5 averaging protocols: (1) normal averaging; (2) sample-weighted averaging; (3) noise-weighted averaging; (4) amplitude-based artifact-rejection; and (5) percentage-based artifact rejection. The responses were analyzed in the frequency domain and the signal-to-noise ratio was estimated by comparing the signals at the modulation-frequencies to the noise at adjacent frequencies.

RESULTS

Weighted averaging gave the best signal-to-noise ratios. Artifact-rejection was better than normal averaging but not as good as weighted averaging. Responses that were not significant with normal averaging became significant with weighted averaging much more frequently than vice versa. False alarm rates did not significantly differ among the protocols. The advantage of weighted averaging was especially evident when stimuli were presented at lower intensities or when smaller amounts (e.g. only 3 or 6 min) of data were evaluated. Weighted averaging was most effective when the background noise levels were variable. Weighted averaging underestimated the amplitude of the responses by about 2%.

CONCLUSION

Weighted averaging should be used instead of normal averaging for detecting steady-state responses.

Sorted averaging--principle and application to auditory brainstem responses

A new averaging method for evoked potentials, called sorted averaging, is presented. The method requires an ensemble of sweeps stored in memory and is based on the principle of interchangeability of individual sweeps within this ensemble. Sorted averaging is applied by sorting all sweeps according to their estimated background noise and successive averaging of this sorted ensemble, starting with low-noise sweeps. Signal-to-noise power ratio (SNR2) is estimated by calculating the inverse single point variance (Elberling & Don, 1984). This SNR2 estimate increases linearly with the number of sweeps for standard averaging and shows a clear maximum for sorted averaging. Auditory brainstem responses to click stimuli at 70 dB nHL in 20 normal hearing subjects were recorded and 4000 individual sweeps during each run were stored for analysis. In an off-line analysis, SNR2 for standard averaging with 10 microV artefact rejection, for weighted averaging and for sorted averaging were calculated. Sorted averaging was found to yield a significantly higher SNR2.