E-field modeling of transcranial magnetic stimulation differentiates parietal contributions for grasping after stroke

After a motor stroke, brain networks mediating reaching and grasping undergo functional reorganization, particularly in the parietal cortex. Online repetitive transcranial magnetic stimulation (rTMS) can probe the behavioral relevance of stimulated cortical territories. However, anatomical interpretation is limited because the induced electric field is spatially distributed and may not correspond precisely to the nominal stimulation target.

To investigate how individual rTMS-induced E-field distributions relate to interference effects of online rTMS over the anterior intraparietal sulcus (IPS) on grasping in stroke patients and healthy controls.

Eighteen chronic stroke patients and eighteen matched controls performed a reach-grasp-lift task during online rTMS of the IPS. Individual E-fields were modeled and correlated with rTMS-induced changes in 3D kinematic measures. The overlap between E-field maxima and a meta-analytically defined grasping network, as well as cytoarchitectonic parietal regions, was quantified.

In both groups, spatial overlap between the induced E-field and the grasping-related network was linked to rTMS effects on movement smoothness. Changes in grip shaping and smoothness were associated with E-field overlap in the IPS in healthy controls. In patients, the analogous association was observed for the inferior parietal lobule (IPL) rather than the IPS, a pattern compatible with reorganization of parietal specialization after stroke.

E-field-based analyses may improve the interpretation of rTMS effects on grasping in healthy and reorganized neural circuits. This can provide a framework for neuromodulation strategies that focus on individually defined functional targets rather than standard landmarks.