Evaluation of optimal masking levels in place-specific low-frequency chirp-evoked auditory brainstem responses

The aim of the study is the experimental determination of the optimal required masking level for a given stimulus level when using a band limited "low-frequency chirp" in order to improve frequency and place specificity of auditory brainstem responses (ABRs). A low-frequency chirp (100-850 Hz) at stimulation levels between 40 and 80 dB normalized hearing level was presented to 12 normal hearing subjects. During presentation of each stimulus, the level of a high-pass noise with a low cutoff frequency of 1100 Hz was varied between 0 and 25 dB signal-to-noise ratio (SNR) by using 5 dB steps (at 0 dB SNR the same level of both the chirp and the masker in dB sound pressure level was presented). Measurements without masking were used as a reference. In all masking conditions, the latency of wave V was significantly increased compared to unmasked ABRs. The amplitude of wave V decreased when reaching the effective and therefore optimal masking level. Accordingly, in order to ensure place specificity of the ABR, ipsilateral masking is essential. At lower stimulus levels the SNR can be substantially increased (i.e., the masker level decreased) without loss of place specificity.