Abstract
Speech perception is mediated by both left and right auditory cortices but with differential sensitivity to specific acoustic information contained in the speech signal. A detailed description of this functional asymmetry is missing, and the underlying models are widely debated. We analyzed cortical responses from 96 epilepsy patients with electrode implantation in left or right primary, secondary, and/or association auditory cortex (AAC). We presented short acoustic transients to noninvasively estimate the dynamical properties of multiple functional regions along the auditory cortical hierarchy. We show remarkably similar bimodal spectral response profiles in left and right primary and secondary regions, with evoked activity composed of dynamics in the theta (around 4–8 Hz) and beta–gamma (around 15–40 Hz) ranges. Beyond these first cortical levels of auditory processing, a hemispheric asymmetry emerged, with delta and beta band (3/15 Hz) responsivity prevailing in the right hemisphere and theta and gamma band (6/40 Hz) activity prevailing in the left. This asymmetry is also present during syllables presentation, but the evoked responses in AAC are more heterogeneous, with the co-occurrence of alpha (around 10 Hz) and gamma (>25 Hz) activity bilaterally. These intracranial data provide a more fine-grained and nuanced characterization of cortical auditory processing in the 2 hemispheres, shedding light on the neural dynamics that potentially shape auditory and speech processing at different levels of the cortical hierarchy.
Highlights
Contrary to the classic neuropsychological perspective, speech processing is known to be distributed across the 2 hemispheres, with some models positing a leftward dominance for verbal comprehension and a rightward dominance for processing suprasegmental features, including aspects of prosody or voice processing [1]
A time-frequency representation of the evoked responses, as computed through intertrial phase coherence (ITPC; Fig 2A), demonstrates the presence of a dynamical response composed of multiple spectral modes, which, could differ between regions of interest (ROIs)
Asymmetric processing in human auditory cortex out of the 24 (67%) patients implanted in association auditory cortex (AAC), their ITPC spectrum was more similar to the model of their hemisphere of implantation than to the one of the opposite hemisphere
Summary
Contrary to the classic neuropsychological perspective, speech processing is known to be distributed across the 2 hemispheres, with some models positing a leftward dominance for verbal comprehension and a rightward dominance for processing suprasegmental features, including aspects of prosody or voice processing [1]. The origin and function of lateralization continues to be vigorously debated, for example, with regard to its domain-general or domainspecific nature [2,3] The domain-specific view postulates that speech is processed in a dedicated system lateralized to the left hemisphere On this view, processing critically depends on the specific linguistic properties of a stimulus. Crucial to this debate is proper understanding of the distinctive sensitivity of the left and right auditory cortical regions to acoustic features, which should be grounded in characteristic anatomic-functional signatures
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