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Poster Slam Session A, Tuesday, August 20, 2019, 10:00 – 10:15 am, Finlandia Hall, Michal Ben-Shachar

Human speech cortex encodes amplitude envelope as transient, phase-locked responses to discrete temporal landmarks

Katsuaki Kojima1, Yulia Oganian1, Chang Cai1, Anne Findlay1, Edward Chang1, Srikantan Nagarajan1;1University of California San Francisco

The slow (1-10Hz) temporal amplitude envelope of speech reflects acoustically and perceptually relevant information about speech temporal structure and content, including syllabic structure. It is well-established that the phase of neural activity in the delta-theta bands (1 - 10 Hz) is aligned to the phase of the speech amplitude envelope during listening. This has been taken as evidence for continuous entrainment of endogenous low-frequency oscillations to the speech envelope, possibly driven by phase-reset at some landmark event in the envelope, such as local peaks in the envelope (peakEnv) or times of rapid increases in amplitude (peakRate). We recently showed using direct electrocorticography (ECoG) that local neural populations in speech cortical areas selectively encode peakRate events in continuous speech. This finding suggests that peakRate is the primary envelope cue represented in speech cortex. However, it leaves open whether phase-locking of low-frequency oscillatory activity observed with M/EEG reflects these transient responses. Alternatively, it might reflect the phase-reset of endogenous oscillatory activity by either peakRate or peakEnv events. We predicted that if it reflects transient responses, phase-locking would 1) rapidly diminish between consecutive acoustic edge events and 2) cover a frequency range reflective of the temporal structure of the speech stimulus envelope. In contrast, if phase-alignment reflects phase-reset of ongoing oscillatory activity, it should continue for several cycles between consecutive acoustic edges and its frequency range should be independent of stimulus envelope dynamics. To contrast these predictions, we recorded neural activity using MEG while participants (n = 6) listened to regularly-paced and 1/3-slowed continuous speech. We analyzed the phase of neural activity in the delta-theta band (1 - 10Hz) over bilateral temporal regions, aligned to acoustic edges and peaks in the speech envelope. Phase-locking was increased when neural activity was aligned to peakRate events, more than it was aligned to peakEnv events, replicating our intracranial results. Crucially, phase-locking in lower frequency bands increased for slowed speech compared to regular speech. Finally, phase-locking peaked after peakRate events and diminished within a single cycle. This pattern of phase-locking is suggestive of an underlying transient response, rather than continuous oscillatory entrainment. These data confirm and extend our previous intracranial findings to low-frequency activity and provide a link between results from intracranial electrophysiology and non-invasive MEG recordings. Taken together, our results demonstrate that the speech envelope induces a series of evoked responses at times of rapid increases in the speech amplitude envelope, rather than continuous alignment of intrinsic oscillatory activity.

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

Poster A69

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