Anodal high-definition transcranial direct current stimulation reveals a specific role for posterior parietal cortex in interlimb generalization.

Motor adaptation involves recalibration of well-learned movements in response to sensory or mechanical perturbations. This is believed to rely on internal models-neural representations that predict motor command outcomes-enabling computation of sensory prediction errors. These models support learning generalization, ie application of newly learned movements to new situations, effectors, or state space regions not directly encountered during practice. Prior studies show that focal left posterior parietal cortex lesions impair visuomotor adaptation in both arms, implicating posterior parietal cortex in sensorimotor recalibration. Here, we investigated the posterior parietal cortex's role in visuomotor adaptation and interlimb generalization using anodal high-definition transcranial direct current stimulation (HD-tDCS). Sixty adults performed dominant (right) arm reaches while adapting to a 2D 30° cursor rotation, concurrent with HD-tDCS targeting the left posterior parietal cortex, right posterior parietal cortex, or under sham conditions. Stimulation was then discontinued, and participants repeated the task with their non-dominant (left) arm to assess interlimb generalization. HD-tDCS over left or right posterior parietal cortex had no effect on initial adaptation (P = 0.77). However, left posterior parietal cortex stimulation was associated with enhanced interlimb generalization (P = 0.003) whereas right posterior parietal cortex stimulation was not (P = 0.77). This suggests that anodal transcranial direct current stimulation to contralateral posterior parietal cortex may contribute to interlimb generalization of visuomotor adaptation, rather than adaptation itself.