Contribution of Resolved and Unresolved Harmonic Regions to Brainstem Speech-Evoked Responses in Quiet and in Background Noise

Speech auditory brainstem responses (speech ABR) reflect activity that is phase-locked to the harmonics of the fundamental frequency (F0) up to at least the first formant (F1). Recent evidence suggests that responses at F0 in the presence of noise are more robust than responses at F1, and are also dissociated in some learning-impaired children. Peripheral auditory processing can be broadly divided into resolved and unresolved harmonic regions. This study investigates the contribution of these two regions to the speech ABR, and their susceptibility to noise. We recorded, in quiet and in background white noise, evoked responses in twelve normal hearing adults in response to three variants of a synthetic vowel: i) Allformants, which contains all first three formants, ii) F1Only, which is dominated by resolved harmonics, and iii) F2&F3Only, which is dominated by unresolved harmonics. There were no statistically significant differences in the response at F0 due to the three variants of the stimulus in quiet, nor did the noise affect this response with the Allformants and F1Only variants. On the other hand, the response at F0 with the F2&F3Only variant was significantly weaker in noise than with the two other variants (p<0.001). With the response at F1, there was no difference with the Allformants and F1Only variants in quiet, but was expectedly weaker with the F2&F3Only variant (p<0.01). The addition of noise significantly weakened the response at F1 with the F1Only variant (p<0.05), but this weakening only tended towards significance with the Allformants variant (p=0.07). The results of this study indicate that resolved and unresolved harmonics are processed in different but interacting pathways that converge in the upper brainstem. The results also support earlier work on the differential susceptibility of responses at F0 and F1 to added noise.