Auditory brainstem response recording to multiple interleaved broadband chirps.

Objective: The simultaneous application of multiple stimuli that excite different frequency regions of the cochlea is a well-established method for recording frequency-specific auditory steady state responses. Because the stimuli are applied at different repetition rates, they actually do not appear exactly simultaneously. There is always a certain time difference between the multiple frequency-specific stimuli. This is true also for multiple interleaved broadband stimuli. Therefore, because of this time difference, one may expect a successful recording of responses to multiple broadband chirp stimuli even when such stimuli activate the whole cochlear partition. This article describes a technique for recording auditory brainstem responses evoked by trains of broadband chirps presented simultaneously at equal stimulus levels but at different repetition rates. The interactions between the interleaved stimulus trains were studied to lay the foundation for a rapid method of assessing temporal aspects of peripheral auditory processing. The first step in laying this foundation is to determine the characteristics of responses from an intact and normal-hearing system to these interleaved chirp trains. Subsequently, the studied interactions between the interleaved applied stimuli may provide a referential framework for future clinical studies aimed at assessing pathological populations. Design: Two chirp trains were applied concurrently at the same stimulus level but at different repetition rates of 20/sec and 22/sec, respectively. Two overall stimulus levels were investigated: 50 and 30 dB nHL. Because of the 2 Hz difference between the repetition rates, the time difference between the stimuli of the two stimulus trains followed a periodic cycling. The cycling period of 0.5 sec contained ten 20/sec stimuli and eleven 22/sec-stimuli. The response to a single train of chirps with the repetition rate of 20/sec was also recorded. The test group consisted of 11 young adult subjects, all with normal hearing. The subjects were between 18 and 21 years of age. Recordings were made with the MB11 Classic instrument equipped with EAR-3A earphones, and special software. During the recording session the EEG was continuously stored on hard disc. The analyzed epoch length was 0.5 sec. Each epoch contained the responses to 10 chirps presented at 20/sec and the responses to 11 chirps presented at 22/sec. The epochs were averaged and the mean value of the amplitude was calculated for each of the 21 chirp responses in the epoch. Results: There were clear interactions between the two interleaved stimulus trains. The amplitude of the grouped responses in the cycle decreased with decreasing distance to the preceding stimulus of the other stimulus train. The mean amplitude of the responses to all ten 20/sec stimuli of a cycle is 82% (50 dB nHL) and 87.3% (30 dB nHL) of the amplitude of the singly presented stimuli. For the mean of the 11 responses to the 22/sec chirps the amplitude values are 84.5% and 87.2%, respectively. Conclusions: The study proved that it is possible to record auditory responses to multiple interleaved stimulation of the whole cochlear partition by broadband chirps. The response reduction caused by the interaction between the stimulus trains is relatively small such that evaluations of the responses are not compromised. The temporal interactions observed in the normal-hearing individuals by using these interleaved trains of chirps might be useful in assessing functional pathological changes in the auditory periphery.