Interhemispheric facilitation of gesturing: A combined theta burst stimulation and diffusion tensor imaging study.

BACKGROUND:Imaging studies point to a posture (finger vs. hand) and domain-specific neural basis of gestures. Furthermore, modulation of gestures by theta burst stimulation (TBS) may depend on interhemispheric disinhibition. OBJECTIVE/HYPOTHESIS:In this randomized sham-controlled study, we hypothesized that dual site continuous TBS over left inferior frontal gyrus (IFG-L) and right inferior parietal gyrus (IPL-R) predominantly affects pantomime of finger postures. Furthermore, we predicted that dual cTBS improves imitation of hand gestures if the effect correlates with measures of callosal connectivity. METHODS:Forty-six healthy subjects participated in this study and were targeted with one train of TBS in different experimental sessions: baseline, sham, single site IFG-L, dual IFG-L/IPL-R, single site IPL-R. Gestures were evaluated by blinded raters using the Test for Upper Limb Apraxia (TULIA) and Postural Imitation Test (PIT). Callosal connectivity was analyzed by diffusion tensor imaging (DTI). RESULTS:Dual cTBS significantly improved TULIAtotal (F [3, 28] = 4.118, p = .009), but did not affect TULIApantomime. The beneficial effect was driven by the cTBS over IPL-R, which improved TULIAimitation (p = .038). Furthermore, TULIAimitation significantly correlated with the microstructure (fractional anisotropy) of the splenium (r = 0.420, p = .026), corrected for age and whole brain volume. CONCLUSIONS:The study suggests that inhibition of IPL-R largely accounted for improved gesturing, possibly through transcallosal facilitation of IPL-L. Therefore, the findings may be relevant for the treatment of apraxic stroke patients. Gesture pantomime and postural gestures escaped the modulation by dual cTBS, suggesting a more widespread and/or variable neural representation.