Left temporal alpha-band activity reflects single word intelligibility

Abstract

The electroencephalographic (EEG) correlates of degraded speech perception have been explored in a number of recent studies. However, such investigations have often been inconclusive as to whether observed differences in brain responses between conditions result from different acoustic properties of more or less intelligible stimuli or whether they relate to cognitive processes implicated in comprehending challenging stimuli. In this study we used noise vocoding to spectrally degrade monosyllabic words in order to manipulate their intelligibility. We used spectral rotation to generate incomprehensible control conditions matched in terms of spectral detail. We recorded EEG from 14 volunteers who listened to a series of noise vocoded (NV) and noise-vocoded spectrally-rotated (rNV) words, while they carried out a detection task. We specifically sought components of the EEG response that showed an interaction between spectral rotation and spectral degradation. This reflects those aspects of the brain electrical response that are related to the intelligibility of acoustically degraded monosyllabic words, while controlling for spectral detail. An interaction between spectral complexity and rotation was apparent in both evoked and induced activity. Analyses of event-related potentials showed an interaction effect for a P300-like component at several centro-parietal electrodes. Time-frequency analysis of the EEG signal in the alpha-band revealed a monotonic increase in event-related desynchronization (ERD) for the NV but not the rNV stimuli in the alpha band at a left temporo-central electrode cluster from 420–560 ms reflecting a direct relationship between the strength of alpha-band ERD and intelligibility. By matching NV words with their incomprehensible rNV homologues, we reveal the spatiotemporal pattern of evoked and induced processes involved in degraded speech perception, largely uncontaminated by purely acoustic effects.

Effects of acoustic periodicity, intelligibility, and pre-stimulus alpha power on the event-related potentials in response to speech

“…Becker, Pefkou, Michel, & Hervais-Adelman, 2013;Ding, Chatterjee, & Simon, 2014;Obleser & Weisz, 2012;Peelle, Gross, & Davis, 2013) and imaging work (e.g. Davis & Johnsrude, 2003;Evans et al, 2014;Scott, Blank, Rosen, & Wise, 2000).…”

“…This second analysis additionally included spectrally rotated speech, a completely unintelligible non-speech analogue that has been used in a number of the previously mentioned studies (Becker et al, 2013;Peelle et al, 2013;Scott et al, 2000), as a baseline condition (henceforth the rotated condition). In contrast to several recent M/EEG studies that have investigated the perception of noise-vocoded (Becker et al, 2013;Obleser & Weisz, 2012;Obleser, Wöstmann, Hellbernd, Wilsch, & Maess, 2012) and unprocessed speech (e.g. Kerlin, Shahin, & Miller, 2010;Müller & Weisz, 2012;Wilsch, Henry, Herrmann, Maess, & Obleser, 2015) by analysing neural activity in the frequency domain, the current study focusses on timedomain responses.…”

Neural and Cognitive Effects of Hearing Loss on Speech Processing

“…Importantly, reducing the spectro-temporal information result in an alpha-power change during processing of intelligible speech, but not when processing unintelligible (spectrally rotated) speech (Becker et al, 2013). Thus, changes in alpha power relate to speech processing and are not induced by alterations in the acoustic detail of the stimuli.…”

“…Studies investigating the effect of temporal-fine structure on the alpha power report reduced inhibitory activity when listening to less degraded speech in younger (Obleser and Weisz, 2012;Becker et al, 2013) and older listeners (Wöstmann et al, 2015). Importantly, reducing the spectro-temporal information result in an alpha-power change during processing of intelligible speech, but not when processing unintelligible (spectrally rotated) speech (Becker et al, 2013).…”

“…That is, during a difficult listening task, the functional inhibition theory predicts an increase in alpha power because of the increased WM involvement, or cognitive load. Indeed, increased alpha power during auditory processing has been observed when (1) increasing the memory load (Leiberg et al, 2006;Kaiser et al, 2007;van Dijk et al, 2010;, (2) reducing the spectro-temporal detail (temporal-fine structure) of the stimuli (Obleser and Weisz, 2012;Becker et al, 2013;Wöstmann et al, 2015), (3) increasing the complexity of the speech material (Meyer et al, 2013), and when reducing the (4) temporal (Wilsch et al, 2014) and (5) contextual (Wöstmann et al, 2015) predictability of the stimuli. In all of the studies mentioned above (with the exception of Wilsch (2014)), alpha power increased over the parietal-occipital regions usually associated with processing of visual information.…”