Lateralized Cerebral Processing of Abstract Linguistic Structure in Clear and Degraded Speech

Abstract

Human cortical activity measured with magnetoencephalography (MEG) has been shown to track the temporal regularity of linguistic information in connected speech. In the current study, we investigate the underlying neural sources of these responses and test the hypothesis that they can be directly modulated by changes in speech intelligibility. MEG responses were measured to natural and spectrally degraded (noise-vocoded) speech in 19 normal hearing participants. Results showed that cortical coherence to “abstract” linguistic units with no accompanying acoustic cues (phrases and sentences) were lateralized to the left hemisphere and changed parametrically with intelligibility of speech. In contrast, responses coherent to words/syllables accompanied by acoustic onsets were bilateral and insensitive to intelligibility changes. This dissociation suggests that cerebral responses to linguistic information are directly affected by intelligibility but also powerfully shaped by physical cues in speech. This explains why previous studies have reported widely inconsistent effects of speech intelligibility on cortical entrainment and, within a single experiment, provided clear support for conclusions about language lateralization derived from a large number of separately conducted neuroimaging studies. Since noise-vocoded speech resembles the signals provided by a cochlear implant device, the current methodology has potential clinical utility for assessment of cochlear implant performance.

Prior exposure to speech rapidly modulates cortical processing of high-level linguistic structure

“…Our statistical maps indicate that the experience-dependent enhancement of the tracking of phrase-and sentence-level responses are associated with activity in the left cerebral hemisphere. The lateralised responses are entirely consistent with the findings reported in our recent MEG study demonstrating that the speech intelligibility modulates changes in cortical tracking responses to larger linguistic units (phrases and sentences) (Meng et al, 2021).…”

“…Our recent MEG study has demonstrated the effect of spectral degradation modulated speech intelligibility on these cortical tracking responses and investigated the underlying neural sources of the concurrent tracking responses (Meng, Li Hegner, Giblin, McMahon, & Johnson, 2021). Results of this study showed that cortical entrainment -the coherence between brain activities and "abstract" linguistic units with no accompanying acoustic cues (phrases and sentences) -was reduced parametrically as a function of reduced intelligibility.…”

“…We calculated the magnitude-squared coherence between the MEG recordings and a composite signal following the same analysis procedure described in our recent MEG study (Meng et al, 2021). Magnitude-squared coherence is a frequency-domain measure of phase consistency between two signals across multiple measurements, with a normalized value between 0 and 1 at distinct frequencies.…”

“…In addition to the reported cerebral lateralization of brain tracking responses to lower and more abstract levels of linguistic content in our previous MEG study (Meng et al, 2021), we also wished to characterize and contrast the underlying neural sources of the tracking responses facilitated by the perceptual pop-out effect. We specifically predicted that the hypothesised enhancement in tracking responses at the sentence-and phrase-level, driven by prior knowledge on the acoustic and linguistic information, would be more leftlateralised while the largely unchanged word-level responses remain bilateral.…”

“…Unlike previous studies, the experimental paradigm of Ding et al (2016) provides a capable tool that unambiguously separate linguistic and acoustic cues in the speech stream and enables the assessment of intelligibility effects on neural responses at distinct timescales (syllable, phrase and sentence). Based on this paradigm and the direct modulation from intelligibility changes established in the work by Meng et al (2021), we hypothesised that neural responses to abstract linguistic structures should be directly enhanced by experience-facilitated increases in speech intelligibility; while responses to linguistic regularities that are accompanied by physical cues in the speech waveform should be minimally affected by this intelligibility manipulation. Noise-vocoding was used to render speech unintelligible while maintaining its temporal envelope (Shannon et al, 1995).…”