Mapping of Broca's region using repetitive transcranial magnetic stimulation in correlation to functional and structural imaging

Objective: Broca's area in the left hemisphere is macro-anatomically defined by the pars opercularis and the pars triangularis of the inferior frontal gyrus, represented in cytoarchitectonic Area 44 and Area 45 respectively. Beyond speech production in general, Broca's area is supposed to be linked to differentiated functional roles in language processing. The objective of our study is to combine complementary data from functional and structural-anatomical imaging with the electrophysiological method of transcranial magnetic stimulation (TMS) in order to individually map the Broca's region within the speech network. The background idea is to identify the functionally mandatory proportions of Broca's region which is a prerequisite in neurosurgical tumor resection. In analogy to electrical brain stimulation in awake surgery, the so called “virtual lesion technique” TMS allows for inducing an acute and exhaustive reversible dysfunction without structural damage and thus directly identifies cortical areas involved in language processing. Therefore, it is an elegant method to enable for causative evidence, which vice versa complement imaging data.

Methods: In the left inferior frontal gyrus of healthy subjects, non-motor speech phenomena was induced by neuro-navigated repetitive TMS in a picture naming paradigm at cortical sites designated as TMS positive sites, at individually adapted stimulation intensity levels of up to 160% of resting motor threshold. While functional and connectivity were explored by both task-related (analogous picture naming task) and task-negative (resting state) functional magnetic resonance imaging, structural fiber connectivity data were obtained by diffusion tensor imaging (DTI) at 3 T. TMS positive sites were compared to TMS negative sites as seeds regions in resting state correlation analyses as well as probabilistic DTI tractography.

Results: Preliminary analyses in a subset of subjects indicate language-related functional connectivity in a fronto-parieto-temporal network correlated to the TMS language positive seed areas, corresponding to fronto-parieto-temporal fiber connections as revealed by the DTI data starting from the same TMS positive sites.

Conclusions: First results of causative TMS language mapping in the inferior frontal gyrus are consistent with correlation data obtained by functional and structural imaging in healthy subjects and thus suggests data complementation in order to specify functional mandatory proportions of Broca's Region. Beyond that, convergent functional-structural findings from different methods will be of increasing relevance for preoperative planning in brain surgery.