Language processing remains one of the most striking examples of hemispheric asymmetry. While much research has focused on elucidating the functional differences, the exact underlying mechanisms of the laterality remain unknown. Language function is severely disrupted in left but not right hemisphere lesions while damage to the right hemisphere impairs the processing of prosody. Many studies in both humans and animal models have exhibited similar functional asymmetries, nevertheless, the exact neuronal mechanisms underlying hemispheric lateralization in language remain unknown. Several hypotheses have been proposed explaining the functional asymmetries, including a differing hemispheric capacity for temporal and spectral resolution, a difference in the temporal scale of information integration as well as global vs. local processing of stimuli.
The aim of this study is to propose and test a unifying hypothesis whereby the left hemisphere processes a wide range of temporal modulations and a limited range of spectral modulations and the right hemisphere processes a wide range of spectral modulations and a limited range of temporal modulations. Using novel stimuli filtering techniques, we propose altering speech stimuli on a spectral modulation and temporal modulation axis and directly test our hypothesis using Magnetoencephalography (MEG) and Electrocorticography (ECoG). Specifically, we will address: 1) Left and right hemisphere sensitivity to spectral and temporal modulations;2) Task-related modulations of auditory cortex;3) Spectrotemporal auditory receptive fields in higher order auditory cortical fields. !
Clinical observations dating back to the 19th century provide clear evidence that damage to the left but not right hemisphere impairs language processing. While there exist many hypotheses regarding the roles of the left and right hemispheres in speech, language, and other aspects of cognition, the exact neural mechanisms underlying hemispheric asymmetries remain unknown. The aim of this proposal is to elucidate the mechanisms underlying left and right processing of speech, in a hope to further our understanding of basic mechanisms of speech analysis and enrich the diagnostic and treatment tools for language disorders.