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.