Abstract
Speech perception is dynamic and shows changes across development. In parallel, functional differences in brain development over time have been well documented and these differences may interact with changes in speech perception during infancy and childhood. Further, there is evidence that the two hemispheres contribute unequally to speech segmentation at the sentence and phonemic levels. To disentangle those contributions, we studied the cortical tracking of various sized units of speech that are crucial for spoken language processing in children (4.7-9.3 years old, N = 34) and adults (N = 19). We measured participants' magnetoencephalogram (MEG) responses to syllables, words, and sentences, calculated the coherence between the speech signal and MEG responses at the level of words and sentences, and further examined auditory evoked responses to syllables. Age-related differences were found for coherence values at the delta and theta frequency bands. Both frequency bands showed an effect of stimulus type, although this was attributed to the length of the stimulus and not the linguistic unit size. There was no difference between hemispheres at the source level either in coherence values for word or sentence processing or in evoked response to syllables. Results highlight the importance of the lower frequencies for speech tracking in the brain across different lexical units. Further, stimulus length affects the speech-brain associations suggesting methodological approaches should be selected carefully when studying speech envelope processing at the neural level. Speech tracking in the brain seems decoupled from more general maturation of the auditory cortex.
Highlights
Brain structure and function continue to develop into early adulthood, with some evidence for different trajectories for the left and right hemispheres (Gogtay et al, 2004; Pang & Taylor, 2000; Parviainen et al, 2019)
This study investigated whether children and adults differ in overall brain activity as well as in left and right auditory cortex activity while listening to various speech units that are essential for spoken language processing
We examined how auditory processing of words and sentences is reflected in the level of coherence and hemispheric lateralization across development, and how these are related to the processing of syllables, at both the sensor and source levels
Summary
Brain structure and function continue to develop into early adulthood, with some evidence for different trajectories for the left and right hemispheres (Gogtay et al, 2004; Pang & Taylor, 2000; Parviainen et al, 2019). Functional near-infrared spectroscopy (fNIRS) and magnetic resonance imaging (MRI) have provided evidence for a significant leftward asymmetry for speech processing that is already present from birth (Dehaene-Lambertz et al, 2002; Pena et al, 2003). Drawing on these findings, we used magnetoencephalography (MEG) to examine how hemispheric specialization is reflected in brain responses to various speech units (sentence, words, syllables) and to uncover whether this specialization differs between children and adults. We combined two experimental approaches: examining general indices of auditory maturation as reflected in the age-related changes of onsetresponses (event-related fields [ERF]) to simple speech sounds alongside examination of word and sentence tracking in different frequency bands, as measured by coherence
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