A comparison of auditory brainstem responses elicited by tone pips with the same rise/fall times but different acoustic spectra in subjects with high-frequency hearing loss

It has been suggested that the use of nonlinear envelopes (e.g., Blackman or Gaussian) may improve frequency specificity of auditory brainstem responses (ABRs) elicited by tone pips [1, 5]. However, nonlinear envelopes have been often used for stimuli with a plateau in the mid-part [11]. Spectra of these stimuli are more complex and broader than those without plateau. The spectral splatter in tone pips with plateau results in broadening the region of the basilar membrane eliciting the response. The aim of the study was to compare ABRs elicited by tone pips with and without plateau in subjects with a steeply sloping (60-70 dB/oct) high-frequency hearing loss. Two types of 4 kHz tone-pips were digitally synthesized using a Gaussian envelope with the rise/fall time of 1 ms: (1) with no plateau and (2) with plateau of one cycle (0.25 ms). The ABR thresholds of wave V were determined in 10 and 5 dB steps. The ABR data recorded with the two types of tone pips were divergent. The differences between ABR thresholds for the two stimuli were 15-25 dB and the thresholds of wave V of ABRs elicited by the tone pips without plateau were better correlated with the audiometric thresholds at 4 kHz than those obtained for the tone pips with plateau. The results suggest that embodying the plateau part in tone pips with a nonlinear envelope may weaken frequency specificity of ABRs due to energy spread to frequency bands adjacent to the signal frequency and therefore eliciting a broader region of the cochlea. Thus, the spectrum of tone pips has a significant effect on the differences between audiometric and ABR thresholds in ears with a steeply sloping high-frequency hearing loss.