

Poster E78, Friday, November 10, 10:00 – 11:15 am, Harborview and Loch Raven Ballrooms

Phase entrainment of neural oscillations with tACS causally modulates fMRI responses to intelligible speech

Benedikt Zoefel1, Alan Archer-Boyd1, Matthew H Davis1;1MRC Cognition and Brain Sciences Unit, Cambridge, UK

Alignment between neural oscillations and speech rhythm, “entrainment”, is often observed to be enhanced during speech comprehension (e.g., Peelle et al., 2013). Nevertheless, the relation between entrainment and comprehension might merely be correlational, introduced by stimulus manipulations that simultaneously reduce speech intelligibility and remove entraining cues for neural oscillations (e.g., Doelling et al., 2014). Only if we manipulate entrainment as a dependent variable and observe consequences for speech comprehension, can we can conclude that there is a causal relation between the two. This is possible using transcranial alternating current stimulation (tACS): tACS has been shown to impose a rhythm on neural oscillations (e.g., Herrmann et al., 2013) and can thus be used to manipulate entrainment in an experimental setting. However, based on behavioural measures alone, it is difficult to distinguish a specific modulation of speech processing and changes to low-level auditory processes that would affect processing of non-speech or unintelligible stimuli. We therefore combined tACS at 3.125 Hz over lateral temporal regions with concurrent fMRI to measure BOLD responses to intelligible (16-channel vocoded) and unintelligible (1-channel vocoded) rhythmic speech stimuli. Stimuli consisted of 5-syllable sentences with syllables recorded by someone speaking in time with a 3.125 Hz metronome (metronome pulses were inaudible to listeners). We manipulated entrainment by systematically varying the phase relation between tACS and speech rhythm, and measured the consequences for neural activity by comparing BOLD responses during true and sham tACS stimulation in speech-specific and auditory brain regions. We found that, for intelligible speech, the phase relation between tACS and speech rhythm significantly modulates the magnitude of the BOLD response in the Superior Temporal Gyrus (compared to a surrogate distribution). Importantly, this modulation was specific to tACS stimulation during intelligible speech; a significant interaction showed that the effect was reduced and absent for unintelligible speech and during sham stimulation. Furthermore, comparison of stimulation and sham conditions confirmed that tACS had a suppressive effect on the magnitude of the BOLD response. Our results therefore suggest that entrainment has a specific, causal influence on neural responses to intelligible speech. We anticipate that tACS may have specific neural effects on perturbing or enhancing speech perception and comprehension which we will explore in follow-up behavioural studies. These studies will further establish the causal role of neural entrainment in speech processing, and (given the BOLD suppression observed) also help to establish causal links between the magnitude of fMRI responses and speech comprehension.

Topic Area: Perception: Speech Perception and Audiovisual Integration

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