Poster C13, Friday, August 17, 10:30 am – 12:15 pm, Room 2000AB
Brain oscillations and speech rate variations: cortical tracking of syllable rate and fundamental frequency in children and adults
Véronique Boulenger1, Guiraud Hélène1, Hincapié Ana-Sofia2,3, Jerbi Karim2;1Laboratoire Dynamique Du Langage, CNRS/Université de Lyon UMR5596, Lyon, France, 2Département de Psychologie, Université de Montréal, Canada, 3Pontificia Universidad Catόlica de Chile, Santiago de Chile, Chile
Understanding speech is undoubtedly challenging for the listener’s brain, especially given the large variability of the speech signal. For efficient decoding, the continuous acoustic stream has to be parsed into shorter units that can be mapped onto linguistic representations. In this regard, the (quasi-)rhythmicity of speech is fundamental as it offers temporal structure that allows the listener to make predictions about the incoming signal. Cortical oscillations are thought to play a crucial role in the tracking of speech rhythm : by aligning to speech at multiple timescales, oscillations in the gamma, theta and delta frequency bands would sample the signal into phoneme-, syllable- and word-sized packets. Although increasing evidence, mostly in adults, points to such brain-to-speech synchronization, previous studies have barely examined the case of speech rate variations, despite being ubiquitous, and when they did, they used time-compressed speech (i.e. artificially accelerated speech). We here tackled the issue of natural speech perception by investigating changes in the properties of cortico-acoustic coupling when speech naturally shifts from normal to fast rate using magnetoencephalography (MEG). We examined, both in adults and in typically-developing children, cortical tracking of syllable rate as well as of speech fundamental frequency (F0), as changes in this parameter have been reported with increasing speech rate. Brain activity of 24 adults and 14 children (all French native, right-handed speakers with normal hearing) was recorded using a 275-channel MEG CTF system while they listened to sentences naturally produced at a normal rate (~6 syllables/s, mean F0 = 78.6 Hz) or at a fast rate (~9 syllables/s, mean F0 = 88.1 Hz). We estimated coherence between MEG source time-series and 1) amplitude envelope of the speech signal for brain coupling to syllable rate, and 2) the raw acoustic signal for coupling to speech F0. Our results in the two groups indicated entrainment of neuronal oscillations in a widely distributed cortical network, but peaking over the right auditory cortex. In adults, increased coherence was found between cortical oscillatory activity and speech envelope in frequencies centered on the syllable rate, namely 5-7 Hz for normal speech and 8-10 Hz for natural fast speech. In children, although such brain-to-speech alignment was observed in the normal rate condition, the shift in coupling frequency to fast rate speech did not occur. Interestingly, results in both children and adults also revealed stronger cortico-acoustic coupling at pitch frequency, which shifted up with the changes in F0 associated with faster speech rate (76-80 Hz for normal rate speech and 86-90 Hz for fast rate speech). In other words, cortical oscillations synchronized to the speech signal in a higher frequency band to follow the F0 increase that accompanied natural rate acceleration. Overall, our findings provide new evidence for brain alignment to natural speech in adults but also in children for whom evidence is still scarce. They suggest that cortical oscillations display task-specific coupling with the speech signal at frequencies that reflect syllable rate (at least in adults) and pitch.
Topic Area: Perception: Speech Perception and Audiovisual Integration