gms | German Medical Science

Objective: Cytoarchitectonic maps of Broca’s region reveal sub-areas of unclear functional relevance. Navigated transcranial magnetic stimulation (TMS) indicates cortical language functions at a high spatial resolution. Here, we correlate functional mapping of Broca’s region by means of TMS with cortical microstructure.

Method: TMS language mapping was performed in 12 healthy, left-dominant native German speakers, with a protocol adapted for high-resolution mapping purposes. Thirty pre-defined target sites covered the entire pars opercularis and the pars triangularis of the left inferior frontal gyrus as well as the anterior part of the inferior precentral gyrus. The mapping procedure was repeated five times per target site including one sham condition. Audiovisual recordings were analyzed off-line by two speech and language therapists. In addition to categorization of language-specific errors, severity was rated and scaled for comparability across subjects. An error severity index per target site was composed from repeated measurements. A hierarchical cluster analysis for 30 stimulation sites across 12 participants was performed for grouping of TMS effects. Target sites corresponding to found clusters were concatenated in standard space. The overlap of these functional volumes with probabilistic cytoarchitectonical area was quantified.

Results: The median of error rates under active stimulation was 34 %. Error severity evaluation indicated a dorso-posteriorly located focus of error severity. The cluster analysis revealed two main clusters dissociating stimulation sites corresponding to probabilistic cytoarchitectonic areas 44 and 45. The dorso-posterior stimulation sites mostly affected were largely split in a posterior and an anterior sub-cluster, whereas the remaining sites were sub-partitioned into a more ventro-dorsal pattern.

Conclusions: Our mapping results indicate functional sub-areas within Broca’s region with significant structure-function overlap.