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Poster A64, Tuesday, August 20, 2019, 10:15 am – 12:00 pm, Restaurant Hall

Cortical entrainment in the alpha rather than the theta frequency range predicts speech comprehension in noise

Antje Strauß1,2, Vincent Aubanel2, Anne-Lise Giraud3, Jean-Luc Schwartz2;1Zukunftskolleg, University of Konstanz, Germany, 2CNRS, GIPSA-Lab, Université Grenoble Alpes, 3University of Geneva, Campus Biotech

Cortical entrainment to the syllable rhythm has been repeatedly shown to be correlated with speech comprehension, however, usually by using highly controlled, artificial speech stimuli. This is why the exact nature of the process remains unclear. In the current EEG study (N=16), we therefore used more naturalistic speech material by contrasting natural, isochronous and randomly rhythmic sentences in noise that all had a mean imposed syllable frequency of 5 Hz. First, we show that in noise, natural sentences are better understood than isochronous or randomly rhythmic sentences. This finding suggests that any temporal manipulation of natural speech is detrimental for comprehension even if temporal predictability as such is increased in isochronous sentences. Second, we find that overall Brain-Audio Coherence shows a 5 Hz peak only for isochronous sentences over central electrodes suggesting a strong acoustic byproduct. Third, we find a positive correlation of single-trial Brain-Audio Coherence with single-trial speech intelligibility scores over right fronto-temporal electrodes in the alpha frequency range (10.7 Hz ±0.7 Hz) irrespective of the rhythmic condition. Since previous models and experimental work rather suggested that syllable entrainment (as reflected by Brain-Audio Coherence at 5 Hz in the current data) plays a crucial role for speech comprehension, we conducted post-hoc analyses in the theta frequency range (5 Hz ±0.7 Hz). These analyses reveal, finally, that correlations with speech intelligibility scores are significant for both isochronous and natural but not for randomly rhythmic sentences. However, individual slopes of linear regression lines are steeper for isochronous than natural sentences. These results suggest that speech rhythm-independent alpha oscillations are used for speech comprehension in noise. Additionally, tracking the syllabic rhythm to improve performance is only employed if temporal predictability is a reliable bottom-up acoustic cue as in isochronous sentences. In over-learned natural speech rhythms, syllable entrainment presumably plays a secondary role. In randomly rhythmic sentences, decent speech intelligibility scores are achieved even without syllable entrainment. Altogether, these data contribute to clarify the respective roles and limitations of temporal predictions vs. speech intelligibility in cortical entrainment. This work was supported by the European Research Council under the 7th European Community Program (FP7/2007–2013 Grant Agreement No. 339152 – “Speech Unit(e)s”).

Themes: Speech Perception, Perception: Auditory
Method: Electrophysiology (MEG/EEG/ECOG)

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