Auditory brainstem responses to level-specific chirps in normal-hearing adults

Upward chirps are often designed to compensate for the cochlear traveling wave delay which is regarded as independent of stimulation level. A chirp based on a traveling wave model is therefore referred to as a level-independent chirp. Another compensation strategy, for instance based on frequency-specific auditory brainstem response (ABR) latencies, results in a chirp that changes with stimulation level and is therefore referred to as a level-dependent chirp. One such strategy, the direct approach, results in a chirp family that is called the level-specific chirp. The level dependence is in agreement with the findings that the chirp, which generates the largest ABR in normal-hearing adults, has a duration (sweeping rate) that changes with stimulus level. A direct comparison of ABRs to a fixed chirp and to a level-specific chirp has not been performed at higher levels of stimulation where the differences are thought to have the greatest effect on the ABR characteristics from normal-hearing adults.

To make a direct comparison of the ABRs to two different chirp stimuli-a level-specific chirp (LS-Chirp) and a level-independent chirp (CE-Chirp)-and to evaluate the hypothesis that at higher levels of stimulation the LS-Chirp generates significantly higher response amplitudes, and produces higher resolution of the different peaks in the ABR than the CE-Chirp.

ABRs are recorded in 10 normal-hearing adults (20 ears) in response to three stimuli at four presentation levels using ER-3A insert earphones. The three stimuli are (1) a level-specific chirp (LS-Chirp), (2) a level-independent chirp (CE-Chirp), and (3) a standard 100-μs click as a reference. The recorded ABRs are evaluated by the peak to trough amplitude (wave V), the peak latency (wave V), the frequency of appearance of wave I, III, and V, and the Grand Average waveforms. Amplitude and latency differences are evaluated statistically by the Wilcoxon matched-pair signed rank test.

At higher levels (80 dB nHL), the amplitude and waveform resolution of the ABR to the LS-Chirp are significantly higher than to the CE-Chirp. At lower levels (20, 40, and 60 dB nHL), no significant differences are found between the amplitudes of the ABR to the two stimuli, but at 60 dB nHL the waveform resolution is better for the LS-Chirp than for the CE-Chirp. For all levels, the amplitude of the ABRs to the two chirps are significantly larger than to the Click, except at 80 dB nHL where the ABR to the CE-Chirp gets distorted and low in amplitude. The differences between the ABR latencies to the three stimuli are large at higher levels, but small at lower levels. At higher levels, the LS-Chirp and the Click generate similar resolutions of the main ABR peaks, but the ABRs to the LS-Chirp are significantly larger than to the Click.

The study confirms the experimental hypothesis that at higher levels of stimulation the LS-Chirp generates significantly higher response amplitudes than both the CE-Chirp and the Click. It also generates a much better response resolution than the CE-Chirp, but the same response resolution as the Click.