Distributed neuronal assemblies for word processing as revealed by fMRI, EEG, MEG, TMS, and lesion evidence
Language has been viewed as an encapsulated modular system subdivided into autonomous processors for speech perception and production, and for speech sounds, words, and their meaning. This talk presents evidence to the contrary: Functional links between speech perception and speech production systems in inferior frontal and superior temporal lobe are necessary to account for basic characteristics of language deficits after stroke, aphasia syndromes 1, and neuroimaging studies demonstrate near-simultaneous activation of superior temporal and inferior frontal areas during spoken word perception 2. The meaning of words does not seem to be linked to one specific cortical "meaning centre", but to multiple cortical sites involved in processing perceptual and action aspects of concepts 3. Looking at action words that refer to different parts of the body, such as lick, pick and kick, we recently found fMRI evidence for a somatotopic line-up of activation along the motor strip which reflects the meaning of words under processing 4. EEG and MEG studies showed that this differential semantic activation occurs rapidly and automatically 4. Evidence from TMS and neuropsychological patient studies indicates that the motor strip plays a category-specific role in the processing of action related words and concepts 5,6. These results indicate that form and meaning of a word are cortically linked by cell assemblies distributed over perisylvian language areas and additional sites. The specific cortical topographies of these assemblies may reflect semantic aspects of words and concepts 7. This new view on the brain basis of language has implications for aphasia therapy that will be discussed briefly.