Speech unfolds at different time scales. Therefore, neuronal mechanisms involved in speech processing should likewise operate at different (corresponding) time scales. The present study aimed to identify speech units relevant for selecting speech streams in a multi-talker situation. Functional connectivity was extracted from the continuous EEG while young adults detected targets within one stream in the presence of a different, task-irrelevant stream. In two separate groups, either the attended or the ignored stream was manipulated so that it contained intact, word-wise scrambled, syllable-wise scrambled, or spectrally scrambled speech. We found functional brain networks that were sensitive to the difference between the situations when speech was meaningful at sentence vs. at word level, but not between when speech was meaningful at word vs. only valid at syllable level, irrespective of whether the speech units were manipulated in the attended or the ignored stream. These functional brain networks operated in the delta and theta bands corresponding to integrating information from longer time windows. Further, the networks, which could be linked with suppressing information from the to-be-ignored stream included brain areas associated with high-level processing of speech. These results are compatible with late filtering models of auditory attention, as they suggest that the length of intact speech units in the to-be-ignored stream affects processes of attentional selection. However, we found no evidence for speech-to-brain coupling differences as a function of the intact unit of speech in either stream. Thus, although the current results do not rule out that early processes of speech processing affect stream selection in a cocktail party situation, neither do they provide supporting for it.
Keywords: EEG; Functional connectivity; Neural oscillations; Selective attention; Speaker segregation; Speech processing.
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