gms | German Medical Science

Objective: Language mapping with neuro-navigated transcranial magnetic stimulation (TMS) helps to identify language-related cortical regions in individual subjects. To explore cortical sub-areas in group-level analyses, a high spatial resolution approach was proposed with a multitude of stimulation sites. Functional imaging data suggest segregated semantic, syntactic and phonological processing in an anterior-to-posterior direction along the inferior frontal gyrus. As yet, qualitative aspects of language processing during TMS have been rated subjectively. Here, we employed a reaction time paradigm with phonological picture-word interference for objective TMS mapping of phonological processing within Broca's region.

Methods: Twelve healthy participants underwent active as well as sham TMS mapping sessions. In a picture naming paradigm, target pictures were combined with acoustical distractor nouns which were phonologically unrelated, or related to the target noun. The phonological priming effect found in behavioral studies accelerates naming responses to target pictures. We hypothesized that the inhibitory effects of TMS in language processing reduce this phonological facilitation specifically at stimulation sites within Broca's region which are involved in phonological processing. We explored TMS-specific effects on phonological facilitation in probabilistic cytoarchitectonic areas 44 and 45 in a three-way analysis of variance with the factors phonological relatedness, active/sham TMS, and cytoarchitectonic area (clusters of stimulation sites substantially overlapping with probabilistic cytoarchitectonic areas 44 and 45).

Results: We found a reduced phonological priming effect in active as compared to sham TMS stimulation at sites overlapping with the ventral portion of probabilistic cytoarchitectonic area 44, whereas stimulation at sites overlapping with area 45 did not yield an effect on phonological priming, as revealed by a significant threefold interaction (F(1,11) = 6.629; p = 0.026).

Conclusion: We present a novel, reaction-time based approach for objective TMS mapping of functional cortical sub-areas. Our findings indicate phonological processing mainly in area 44. Further studies will focus on other aspects of language processing for a more comprehensive understanding of the functional organization of Broca's region. This will help to improve the interpretation of TMS language mapping in clinical applications.