Functional MRI reveals the cerebral substrates of linguistic and affective prosody
The cerebral organization of speech is still a subject of debate. A vast body of evidence supports the view that left perisylvian areas subserve lexical and grammatical aspects of speech comprehension, while earlier lesion studies and recent data obtained from functional neuroimaging point to an essential role of right perisylvian areas during discrimination of prosodic information in tonal sequences. Here, data obtained from three recent fMRI studies on speech processing at the sentence-level will be introduced. The results demonstrate a strong involvement of distinct areas in the left hemisphere whenever lexical and syntactic information is emphasized but challenge the widely held notion of Broca's area accomodating syntactic functions. Further, the data provide strong evidence that right cortical areas particularly subserve slow prosodic modulations (sentence melody). A bifrontal network including the fronto-opercular cortex might serve as an interface synchronizing prosodic and syntactic information during speech processing. A third experiment investigated how the brain processes affective intonation. Volunteers heard sentences which either had a neutral or an emotional intonation. Results suggest that neutral intonation involves core peri-sylvian regions. In contrast, positive and negative intonations produce selective brain activation in additional frontal, temporal and subcortical areas. Thus, the current data call into question an exclusive right hemisphere lateralization of affective prosody. Taken together, the results demonstrate how fMRI helps to gather new insight about the neurocognition of speech by identifying distinct cerebral regions subserving in distinct functions of speech.