FMRI reveals brain regions mediating slow prosodic modulations in spoken sentences

By means of fMRI measurements, the present study identifies brain regions in left and right peri-sylvian areas that subserve grammatical or prosodic processing. Normal volunteers heard 1) normal sentences; 2) so-called syntactic sentences comprising syntactic, but no lexical-semantic information; and 3) manipulated speech signals comprising only prosodic information, i.e., speech melody. For all conditions, significant blood oxygenation signals were recorded from the supratemporal plane bilaterally. Left hemisphere areas that surround Heschl gyrus responded more strongly during the two sentence conditions than to speech melody. This finding suggests that the anterior and posterior portions of the superior temporal region (STR) support lexical-semantic and syntactic aspects of sentence processing. In contrast, the right superior temporal region, in especially the planum temporale, responded more strongly to speech melody. Significant brain activation in the fronto-opercular cortices was observed when participants heard pseudo sentences and was strongest during the speech melody condition. In contrast, the fronto-opercular area is not prominently involved in listening to normal sentences. Thus, the functional activation in fronto-opercular regions increases as the grammatical information available in the sentence decreases. Generally, brain responses to speech melody were stronger in right than left hemisphere sites, suggesting a particular role of right cortical areas in the processing of slow prosodic modulations.