Speech auditory brainstem response (speech ABR) characteristics depending on recording conditions, and hearing status: an experimental parametric study.

Speech elicited auditory brainstem responses (Speech ABR) have been shown to be an objective measurement of speech processing in the brainstem. Given the simultaneous stimulation and recording, and the similarities between the recording and the speech stimulus envelope, there is a great risk of artefactual recordings. This study sought to systematically investigate the source of artefactual contamination in Speech ABR response. In a first part, we measured the sound level thresholds over which artefactual responses were obtained, for different types of transducers and experimental setup parameters. A watermelon model was used to model the human head susceptibility to electromagnetic artefact. It was found that impedances between the electrodes had a great effect on electromagnetic susceptibility and that the most prominent artefact is due to the transducer's electromagnetic leakage. The only artefact-free condition was obtained with insert-earphones shielded in a Faraday cage linked to common ground. In a second part of the study, using the previously defined artefact-free condition, we recorded speech ABR in unilateral deaf subjects and bilateral normal hearing subjects. In an additional control condition, Speech ABR was recorded with the insert-earphones used to deliver the stimulation, unplugged from the ears, so that the subjects did not perceive the stimulus. No responses were obtained from the deaf ear of unilaterally hearing impaired subjects, nor in the insert-out-of-the-ear condition in all the subjects, showing that Speech ABR reflects the functioning of the auditory pathways.

Relationship between loudness growth function and auditory steady-state response in normal-hearing subjects.

The present study investigates the relationship between auditory steady-state responses (ASSRs) and loudness growth function. ASSR amplitudes were compared to the perceived loudness level at frequencies of 500 and 2000Hz in 11 normal-hearing subjects. As a first step, loudness growth function was estimated for the two test frequencies. Then ASSR amplitude was recorded for each of the two frequencies at different stimulus intensities, each corresponding to a loudness level as given by the first part of the study. Normalized results show that the ASSR amplitude correlates well with the loudness function (R(2)=0.81). A stepwise multiple linear regression confirmed these results with loudness explaining almost all the ASSR amplitude (loudness R(2)=0.81, p<0.001, f=562 and for intensity f=1.1, p=0.29). The non-linearity of the ASSR amplitude for low loudness levels can be explained by both the active amplification in the cochlea and the noise in the recording. The results suggest that ASSRs can be used for "objective" loudness measurement.

Auditory steady-state response evaluation of auditory thresholds in cochlear implant patients.

The aims of this work were to characterize the electrophysiologic response obtained by measurement of the auditory steady-state response (ASSR) in patients with a cochlear implant (MXM Digisonic) and to study the relationship between the subjective thresholds of the implantees and those estimated using electrical auditory steady-state response (ASSR)-based objective audiometry. Five subjects were examined with the use of four carrier frequencies--600, 1000, 2000 and 3500 Hz--modulated at frequencies between 70 and 85Hz, a particular frequency of modulation being represented at a specific electrode (for each carrier frequency) as a particular pulse-width modulation frequency. The protocol consisted of testing output and thresholds for different overall pulse durations for several stimulus (pulse) intensities, rendering multiple threshold measures (in duration) for each subject tested. The non-linearity of response growth, as a function of duration, provided the basis for teasing apart physiologic response and electrical artefact in the suprathreshold recorded responses. Thresholds estimated with use of the electrical ASSR demonstrated reasonably good agreement with the subjective thresholds. The results obtained thus demonstrated the efficacy of the approach and are encouraging for further advances in cochlear implant applications.