Auditory brain stem responses evoked by different chirps based on different delay models.

BACKGROUND

A cochlear delay model has previously been proposed for the construction of a chirp stimulus in order to compensate for the temporal dispersion in the auditory periphery. The large intersubject variability in the model data suggests that a chirp constructed from the average model data will not be able to compensate equally well for the temporal dispersion in all normal-hearing subjects. For the recording of the auditory brain stem response (ABR), it has been suggested that the most efficient chirp for generating the largest response amplitude changes in duration with level, indicating that the delay model exhibits a latency change with frequency, which becomes larger at lower levels.

PURPOSE

To investigate in normal-hearing subjects how the ABR varies in response to five different chirps and to study how the efficiency of each chirp changes with stimulus level.

RESEARCH DESIGN

A click and five chirps of different durations and constructed from the proposed delay model were designed with identical amplitude spectra. The six stimuli were used to record the ABR from 50 normal-hearing test subjects using a quasi-simultaneous stimulation technique at 50 and 30 dB nHL. The ABR recordings were evaluated by the peak-to-trough amplitude and the peak latency.

RESULTS

For the test group the following level effect was demonstrated: at 50 dB nHL the largest response amplitude was provided by a shorter chirp, whereas at 30 dB nHL the largest response amplitude was provided by a longer chirp. There is, however, large variability as to which of the five chirps generated the largest response in each individual subject, but at the two levels of stimulation, the best chirps were significantly correlated across the test group. All five chirps generated significantly larger ABRs than the click, but at 30 dB nHL the gain in response amplitude by using the chirps instead of the click was larger than at 50 dB nHL.

CONCLUSIONS

A chirp that evokes the largest broadband ABRs in normal-hearing subjects changes in duration with level-that is, being relatively short at higher levels (50 dB nHL) and relatively long at lower levels and near the threshold. However, the changes in amplitude in response to chirps of different durations are not very large, and it is therefore uncertain whether the outcome from using such chirps actually would outweigh the instrumental complexity of implementation. It appears that the largest advantage of using the chirp over the click is found at the lower levels of stimulation.