Cortical distribution of speech and language errors investigated by visual object naming and navigated transcranial magnetic stimulation

Abstract

Navigated transcranial magnetic stimulation (nTMS) gains increasing importance in presurgical language mapping. Although bipolar direct cortical stimulation (DCS) is regarded as the gold standard for intraoperative mapping of language-related areas, it cannot be used to map the healthy human brain due to its invasive character. Therefore, the present study employed a non-invasive virtual-lesion modality to provide a causality-confirmed cortical language map of the healthy human brain by repetitive nTMS (rTMS) with functional specifications beyond language-positive/language-negative distinction. Fifty right-handed healthy volunteers underwent rTMS language mapping of the left hemisphere combined with an object-naming task. The induced errors were categorized and frequency maps were calculated. Moreover, a principal component analysis (PCA) was performed on the basis of language-positive cortical regions for each error category. The left hemisphere was stimulated at 258-789 sites (median: 361.5 sites), and 12-241 naming errors (median: 72.5 errors) were observed. In male subjects, a total number of 2091 language errors were elicited by 9579 stimulation trains, which is equal to an error rate of 21.8 %. Within females, 10,238 stimulation trains elicited 2032 language errors (19.8 %). PCA revealed that the inferior parietal lobe (IPL) and middle frontal gyrus (MFG) were causally involved in object naming as a semantic center and an executive control center. For the first time, this study provides causality-based data and a model that approximates the distribution of language-related cortical areas grouped for different functional aspects of single-word production processes by PCA.

Results on the spatial resolution of repetitive transcranial magnetic stimulation for cortical language mapping during object naming in healthy subjects

“…Hence, considerable error rates were achieved during mapping of the opIFG and trIFG, but belonging to other types than no responses. As a possible explanation for the low rate of no responses in these regions when compared to previous investigations [6–8, 20], the stimulation frequency of 7 Hz (instead of 5 Hz) has to be considered, but the set of objects, which was different when compared to these previous publications, might have also played a role. However, the distinct cause for the specific pattern of no responses observed in the present study cannot be fully assessed within the scope of our approach.…”

“…In the recent years, the differentiation of naming errors into various predefined error categories has been demonstrated repeatedly within the scope of rTMS and DCS trials [4, 6, 8, 16, 20, 34]. Although other error categories except no responses are based on stimulation-induced disruption of important language subfunctions as well, we decided to not take these error types into account since no responses have proven to be among the most frequent error types [6, 20].…”

“…Although other error categories except no responses are based on stimulation-induced disruption of important language subfunctions as well, we decided to not take these error types into account since no responses have proven to be among the most frequent error types [6, 20]. Incorporation of other error types could have led to different hotspots, but potentially on the basis of more unclear language disruption.…”

“…Other error types, like hesitations or performance errors, were not taken into account in the present investigation. Although other error types represent stimulation-induced disruption of important language subfunctions as well, we decided to not take these categories into account since no responses have proven to be among the most frequent error types whilst being easy to detect during video analysis [6, 20]. …”

Interhemispheric connectivity revealed by diffusion tensor imaging fiber tracking derived from navigated transcranial magnetic stimulation maps as a sign of language function at risk in patients with brain tumors

“…24,44 In this context, the current approach for nTMS language-mapping trials has been repeatedly described. 15,22,23,30,41,42,54,56 Briefly, the resting motor threshold (RMT) was first determined to be able to individually adjust the stimulation intensity during language mapping. Muscle electrodes (Neuroline 720, Ambu) were placed over the abductor pollicis brevis muscle.…”