Effect of spectral resolution on neural entrainment of the speech envelope

Objectives: Research shows that neural entrainment of the speech envelope correlates with speech intelligibility. While the speech envelope is known to be the primary cue of speech intelligibility, other features still have an effect on speech reception. While keeping the envelope fixed we investigated the effect of spectral resolution on neural entrainment, thus varying speech intelligibility. Methods: Nine normal-hearing subjects listened to vocoded running speech in quiet and stationary noise (3 dB SNR) while their EEG was recorded. Using vocoded speech, we were able to preserve temporal envelope cues while degrading the spectral resolution. By measuring behavioural speech understanding during the EEG experiment we known the words understood by the subject. We trained a linear multi-channel decoder to reconstruct the speech envelope from the EEG signal, and by correlating it with the original envelope, we estimated the neural entrainment of the stimulus. Results: We found that vocoded speech in noise elicited different neural entrainment in different oscillation bands compared to vocoded speech in quiet. We also found an increase in neural entrainment when more vocoder bands were used, indicating that spectral resolution can have measurable effects on speech envelope based methods. These results correlate with behaviourally measured speech intelligibility. Conclusion: Since neural entrainment appears to be differently affected by noise regarding which oscillation band is considered, we can hypothesise the existence of a neural mechanism related to the suppression of the added stationary noise. Furthermore, even though the neural entrainment measure only depends on the speech envelope, we can still see an effect of spectral resolution, indicating that it is not merely a measure of peripheral coding of the envelope, but also relates to speech intelligibility.status: publishe