During speech perception, acoustic correlates of syllable structure and pitch periodicity are directly reflected in electrophysiological brain activity. Magnetoencephalography (MEG) recordings were made while 10 participants listened to natural or formant-synthesized speech at moderately fast or ultrafast rate. Cross-correlation analysis was applied to show brain activity time-locked to the speech envelope, to an acoustic marker of syllable onsets, and to pitch periodicity. The envelope yielded a right-lateralized M100-like response, syllable onsets gave rise to M50/M100-like fields with an additional anterior M50 component, and pitch (ca. 100 Hz) elicited a neural resonance bound to a central auditory source at a latency of 30 ms. The strength of these MEG components showed differential effects of syllable rate and natural versus synthetic speech. Presumingly, such phase-locking mechanisms serve as neuronal triggers for the extraction of information-bearing elements.
Blind individuals may learn to understand ultra-fast synthetic speech at a rate of up to about 25 syllables per second (syl)/s, an accomplishment by far exceeding the maximum performance level of normal-sighted listeners (8-10 syl/s). The present study indicates that this exceptional skill engages distinct regions of the central-visual system. Hemodynamic brain activation during listening to moderately- (8 syl/s) and ultra-fast speech (16 syl/s) was measured in a blind individual and six normal-sighted controls. Moderately-fast speech activated posterior and anterior 'language zones' in all subjects. Regarding ultra-fast tokens, the controls showed exclusive activation of supratemporal regions whereas the blind participant exhibited enhanced left inferior frontal and temporoparietal responses as well as significant hemodynamic activation of left fusiform gyrus (FG) and right primary visual cortex. Since left FG is known to be involved in phonological processing, this structure, presumably, provides the functional link between the central-auditory and -visual systems.
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