Abstract
Humans excel at understanding speech even in adverse conditions such as background noise. Speech processing may be aided by cortical activity in the delta and theta frequency bands, which have been found to track the speech envelope. However, the rhythm of non-speech sounds is tracked by cortical activity as well. It therefore remains unclear which aspects of neural speech tracking represent the processing of acoustic features, related to the clarity of speech, and which aspects reflect higher-level linguistic processing related to speech comprehension. Here we disambiguate the roles of cortical tracking for speech clarity and comprehension through recording EEG responses to native and foreign language in different levels of background noise, for which clarity and comprehension vary independently. We then use a both a decoding and an encoding approach to relate clarity and comprehension to the neural responses. We find that cortical tracking in the theta frequency band is mainly correlated to clarity, whereas the delta band contributes most to speech comprehension. Moreover, we uncover an early neural component in the delta band that informs on comprehension and that may reflect a predictive mechanism for language processing. Our results disentangle the functional contributions of cortical speech tracking in the delta and theta bands to speech processing. They also show that both speech clarity and comprehension can be accurately decoded from relatively short segments of EEG recordings, which may have applications in future mind-controlled auditory prosthesis.SIGNIFICANCE STATEMENT Speech is a highly complex signal whose processing requires analysis from lower-level acoustic features to higher-level linguistic information. Recent work has shown that neural activity in the delta and theta frequency bands track the rhythm of speech, but the role of this tracking for speech processing remains unclear. Here we disentangle the roles of cortical entrainment in different frequency bands and at different temporal lags for speech clarity, reflecting the acoustics of the signal, and speech comprehension, related to linguistic processing. We show that cortical speech tracking in the theta frequency band encodes mostly speech clarity, and thus acoustic aspects of the signal, whereas speech tracking in the delta band encodes the higher-level speech comprehension.
Highlights
Speech comprehension requires real-time extraction of acoustic features from sound signals and their transformation into lin-Received July 18, 2018; revised May 1, 2019; accepted May 11, 2019
We show that cortical speech tracking in the theta frequency band encodes mostly speech clarity, and acoustic aspects of the signal, whereas speech tracking in the delta band encodes the higher-level speech comprehension
Using natural speech in a subject’s native as well as in a foreign language, and in different levels of babble noise, allowed us to separate neural correlates of speech clarity from those of speech comprehension. We used this experimental paradigm to disambiguate the role of the cortical entrainment to the speech envelope for the processing of lower-level acoustic features as well as of higher-level linguistic ones
Summary
Speech comprehension requires real-time extraction of acoustic features from sound signals and their transformation into lin-Received July 18, 2018; revised May 1, 2019; accepted May 11, 2019. The cortical activity in the delta and theta frequency band has been found to track the speech rhythm, evident in its envelope, and this entrainment has been suggested as a neural mechanism for parsing speech into linguistic constituents (Ding and Simon, 2012, 2014; Giraud and Poeppel, 2012; Horton et al, 2013; Thwaites et al, 2015; Keitel et al, 2018) Evidence for this hypothesis comes from the modulation of cortical entrainment to the speech envelope through attention to one of several competing speakers, and cortical tracking may be important for understanding speech in adverse conditions such as
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