Organization of functional interactions within the fronto-temporal language brain system underlying production and perception of regular and irregular Russian verbs
It is largely accepted that the brain system comprising the left inferior frontal gyrus and bilateral areas of the temporal cortex is specifically involved in morphologically complex word processing. Despite a substantial number of experimental neuroimaging studies, the principles of functional organization of this system remain controversial. For instance, a number of studies point to different regimes of involvement of the left IFG and bilateral temporal regions in the processing of regularly or irregularly inflected forms. It is assumed that regular forms are processed using a combinatorial stem-affix rule and that this process is mainly supported by the left IFG and the left superior temporal gyrus (STG). On the other hand, processing of irregular forms is subserved by lexical memory supported by brain areas located in the bilateral STG.
Also there is a controversy in the experimental findings demon- strating different directions of changes in the levels of functional activity in the lIFG and STG regions associated with processing of regularly or irregulary inflected forms. Therefore, it is hard to consistently support the claim of differential involvement of this system in the rule or memory-based processing. Since there is a lack of studies dedicated to the investigation of functional interactions within this fronto-temporal brain system it is also hard to infer the principles of its functional organization.
To fill this gap we recently conducted two fMRI studies in which healthy participants actively and overtly inflected or passively perceived and actively selected regular and irregular Russian verb forms. Irrespective of the task type, processing of irregular forms was associated with the greater levels of local functional activity in a number of brain regions including the left IFG. In contrast, the analysis of psychophysiological interactions of the lIFG revealed greater connectivity with brain areas located in the STG for both generating and selecting egular verb forms, as compared to irregular ones. Revealed differential relations between the changes in local activity and distant functional interactions can be seen as alterna- tions in the regime of fronto-temporal network activity as a function of morphological process involved.