Abstract
The temporal modulation structure of speech plays a key role in neural encoding of the speech signal. Amplitude modulations (AMs, quasi-rhythmic changes in signal energy or intensity) in speech are encoded by neuronal oscillations (rhythmic variations in neural excitability in large cell networks) that oscillate at matching temporal rates. To date, however, all neural studies have investigated adult-directed speech (ADS) as produced and perceived by highly literate adults. Whether temporal features of ADS vary with the skills of the speaker, for example literacy skills, is currently unknown. Here we analyse the temporal structure of ADS spoken by illiterate, low literate (≤ 4 years of literacy) and highly literate (≥ 12 years of literacy) adults. We find that illiterates produce speech differently. Spontaneous conversational speech produced by illiterate adults showed significantly less synchronised coupling between AM bands (less phase synchronisation) than conversational speech produced by low literate and highly literate adults, and contained significantly fewer syllables per second. There was also a significant relationship between years of literacy and the amount of theta-band energy in conversational speech. When asked to produce rhythmic proverbs learned in childhood, all groups could produce speech with similar AM phase synchronisation, suggesting that the differences in spontaneous conversational speech were not caused by physiological constraints. The data suggest that the temporal modulation structure of spoken language changes with the acquisition of cultural skills like literacy that are usually a product of schooling. There is a cultural effect on the temporal modulation structure of spoken language.
Highlights
Recent advances in our understanding of the neural basis of speech encoding suggest that neuronal oscillations at multiple timescales play a key role in encoding amplitude modulation patterns in speech, ‘multi-time resolution processing’ [1,2]
Our study is the first to show that the temporal modulation structure of the speech of adults is not equal
The same effect of literacy was found, with greater theta-beta/low gamma band phase synchronisation for the two literate groups compared to the illiterate group
Summary
Recent advances in our understanding of the neural basis of speech encoding suggest that neuronal oscillations at multiple timescales play a key role in encoding amplitude modulation patterns in speech, ‘multi-time resolution processing’ [1,2]. Adult MEG (magnetoencephalography) and EEG (electroencephalography) research reveals that the speech stream is sampled continuously by networks of cells that vary in endogenous excitability at different temporal rates. Cell networks in auditory cortex form an oscillatory hierarchy [4], and speech intelligibility studies with ADS reveal a key role for neuronal oscillations in four temporal rate bands in this hierarchy, delta, ~1–3 Hz, theta, ~4–8 Hz, beta, ~15–30 Hz, and low gamma, ~30–50 Hz [5]. There is maximal modulation energy in ADS in the theta band, 4–8 Hz [2], and it has been proposed that theta entrainment is a core feature of speech encoding by adults [3]
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