Lateralised cerebral processing of abstract linguistic structure in clear and degraded speech

Providing a plausible neural substrate of speech processing and language comprehension, cortical activity has been shown to track different levels of linguistic structure in connected speech (syllables, phrases and sentences), independent of the physical regularities of the acoustic stimulus. In the current study, we investigated the effect of speech intelligibility on this brain activity as well as the underlying neural sources. Using magnetoencephalography (MEG), brain responses to natural speech and noise-vocoded (spectrally-degraded) speech in nineteen normal hearing participants were measured. Results showed that cortical MEG coherence to linguistic structure changed parametrically with the intelligibility of the speech signal. Cortical responses coherent with phrase and sentence structures were left-hemisphere lateralized, whereas responses coherent to syllable/word structure were bilateral. The enhancement of coherence to intelligible compared to unintelligible speech was also left lateralized and localized to the parasylvian cortex. These results demonstrate that cortical responses to higher level linguistics structures (phrase and sentence level) are sensitive to speech intelligibility. Since the noise-vocoded sentences simulate the auditory input provided by a cochlear implant, such objective neurophysiological measures have potential clinical utility for assessment of cochlear implant performance.