Neural entrainment to acoustic edges in speech

Abstract

In the present thesis, I evaluate the role of acoustic edges in the neural tracking of speech syllables. Previous research has shown that neural oscillations exhibit phase locking to the slow temporal modulations (1-10 Hz) of the speech envelope, which is thought to correspond to the syllabic rhythm (Edwards & Chang, 2013; Ghitza, 2013; Giraud & Poeppel, 2012). It has been suggested that this is achieved through the phase resetting of ongoing neural rhythms to specific speech landmarks, such as the fine-grained spectral information placed at the onsets of syllables (Doelling et al., 2014). While some debate exists about whether entrainment occurs as a result of the phase resetting of endogenous oscillations or whether it is simply evoked activity which is temporally aligned to the rhythmic stimulus, I did not specifically investigate this distinction, but based on the present results, I suggest that further investigation into the role of syllabic landmarks in speech tracking is worthwhile nonetheless.Experiment 1 replicated findings from Luo and Poeppel (2007), who suggested the importance of theta oscillations in tracking continuous speech. Here, we used stimuli such as natural speech sentences containing syllable-initial consonants which belonged to different phonemic categories, but we did not see differences in phase locking depending on the amount of edge provided by those phonemes. In Experiment 2, we used series of nearly-isochronous consonant-vowel syllables starting with separate phonemes and showed that syllables starting with some consonants led to less phase locking than others (lowest for sibilants, highest for stops). We also explored different edge markers based on the acoustic properties of the stimuli and, following from suggestions from other research such as Oganian and Chang (2018), considered that information which is critical for speech tracking may be found at the consonant-vowel transition of syllables. In Experiment 3, I tested this hypothesis by placing two different types of noise at various locations of “da” and “ta”syllables. We found that differences in phase locking due to the insertion of noise were the most striking at CV locations and also, the direction of change in entrainment depended on the syllable-initial consonant.I suggest the different phonemes provide different acoustic edges for syllable tracking and that these are most prominent at the CV transition. This claim needs to be tested in the future for a variety of consonants, syllabic structures as well as for continuous speech, but could have crucial implications for the way we currently understand neural phase locking to the speech envelope.

Date of Award	23 Jun 2020
Original language	English
Awarding Institution	The University of Bristol
Supervisor	Nina Kazanina (Supervisor) & Conor Houghton (Supervisor)