Hemispheric laterality in neural processing of speech and non-speech temporal information on multiple timescales

Numerous studies have investigated hemispheric laterality of multi-band neural oscillatory activities in response to speech temporal information, but their speech- and language-specificity remain elusive. In the present study, using magnetoencephalography, we examined laterality patterns of theta (4-8 Hz) and high gamma-band (more than 80 Hz) activities phase-locked to temporal envelope and fine structure, respectively, of speech and non-speech stimuli. Monotone speech (MS) with a fundamental frequency of 80 Hz and its backward sound (bMS) were used as speech stimuli. We also used noise-vocoded speech (NVS), which has the same temporal envelope as MS, but no fine structure, and its backward sound (bNVS) to investigate the effects of temporal fine structure on multi-band oscillatory activities. A 5 Hz amplitude-modulated 80 Hz click-train (AMC) and noise (AMN) were used as non-speech stimuli. We confirmed that speech and non-speech stimuli with fine structure (MS, bMS, and AMC) caused stronger 5 Hz and 80 Hz activities in the right than in the left hemisphere. Regarding the language specificity, we found that the 5 Hz power in the right hemisphere decreased in backward compared to forward speech conditions (MS > bMS and NVS > bNVS). These results suggest that the right hemisphere plays an important role in auditory processing of temporal fine structure through high gamma-band activity in common with speech and non-speech sounds. In addition, theta-band activity phase-locked to the temporal envelope in the right hemisphere contributes to recognition of speech signals.