Low-frequency versus high-frequency synchronisation in chirp-evoked auditory brainstem responses

This study investigates the frequency specific contribution to the auditory brainstem response (ABR) of chirp stimuli. Frequency rising chirps were designed to compensate for the cochlear traveling wave delay, and lead to larger wave-V amplitudes than for click stimuli as more auditory nerve fibres fire synchronously. Traditional click stimuli were believed to only excite high-frequency fibres synchronously. It is still currently unclear whether the broad-band chirp stimulus leads to increased synchronisation of both low- and high-frequency fibres. It is also unclear if both these groups of fibres contribute significantly to the overall wave-V amplitude. In the present study, ABRs were recorded from 10 normal-hearing listeners using low- and high-frequency band-limited chirps and clicks (0.1 – 1.5 kHz and 1.5 - 10 kHz) presented at a level of 40 dB HL. The results showed significantly larger wave-V amplitudes for both low and high-frequency band-limited chirps than for the filtered clicks. This demonstrates that the synchronisation of nerve fibres occurs across the entire frequency range at this presentation level, and this leads to significant increases in wave-V amplitudes. The increase for the low-frequency chirp was found to be clearly larger than that obtained at the higher frequencies.