Effects of Early- and Late-Arriving Room Reflections on the Speech-Evoked Auditory Brainstem Response.

Background: Room reverberation alters the acoustical properties of the speech signals reaching our ears, affecting speech understanding. Therefore, it is important to understand the consequences of reverberation on auditory processing. In perceptual studies, the direct sound and early reflections of reverberated speech have been found to constitute useful energy, whereas the late reflections constitute detrimental energy.

Purpose: This study investigated how various components (direct sound versus early reflections versus late reflections) of the reverberated speech are encoded in the auditory system using the speech-evoked auditory brainstem response (ABR).

Research Design: Speech-evoked ABRs were recorded using reverberant stimuli created as a result of the convolution between an ongoing synthetic vowel /a/ and each of the following room impulse response (RIR) components: direct sound, early reflections, late reflections, and full reverberation. Four stimuli were produced: direct component, early component, late component, and full component.

Study Sample: Twelve participants with normal hearing participated in this study.

Data Collection And Analysis: Waves V and A amplitudes and latencies as well as envelope-following response (EFR) and fine structure frequency-following response (FFR) amplitudes of the speech-evoked ABR were evaluated separately with one-way repeated measures analysis of variances to determine the effect of stimulus. Post hoc comparisons using Tukey's honestly significant difference test were performed to assess significant differences between pairs of stimulus conditions.

Results: For waves V and A amplitudes, a significant difference or trend toward significance was found between direct and late components, between direct and full components, and between early and late components. For waves V and A latencies, significant differences were found between direct and late components, between direct and full components, between early and late components, and between early and full components. For the EFR and FFR amplitudes, a significant difference or trend toward significance was found between direct and late components, and between early and late components. Moreover, eight, three, and one participant reported the early, full, and late stimuli, respectively, to be the most perceptually similar to the direct stimulus.

Conclusions: The stimuli that are acoustically most similar (direct and early) result in electrophysiological responses that are not significantly different, whereas the stimuli that are acoustically most different (direct and late, early and late) result in responses that are significantly different across all response measures. These findings provide insights toward the understanding of the effects of the different components of the RIRs on auditory processing of speech.