Prism Adaptation-Induced Modulation of Cortical Excitability of Upper and Lower Limb Muscles is Enhanced with Electrical Stimulation

BackgroundPrism adaptation (PA) is a sensorimotor behavioral phenomenon. Right shifting PA induces a shift in global visuospatial motor behavior toward the left hemi-space (aftereffect) leading to immediate and transient changes in visuomotor behavior. Non-invasive sensorimotor electrical stimulation (Stim) may upregulate corticomotor excitability, is commonly used as a therapeutic adjunct during motor training, and may accentuate the effects of PA. However, the cortical plasticity mechanisms related to the behavioral effects of PA, its generalization to the lower limb, and the combinatorial effects of PA and Stim are poorly understood. ObjectiveTo evaluate the effects of combining PA with Stim on corticomotor excitability and visuo-spatial-motor behavior, and its generalization to the lower limb. MethodsWe used a repeated-measures design to evaluate the effects of 1 session of PA with and without Stim in 15 young able-bodied individuals (18-35 years). Before and after PA, visuomotor pointing task performance, corticomotor excitability, short-interval intra-cortical inhibition (SICI), long-interval intra-cortical inhibition (LICI), and intra-cortical facilitation (ICF) were evaluated in bilateral upper and left ankle muscles with motor-evoked potentials (MEPs) elicited from single and paired-pulse transcranial magnetic stimulation (TMS). ResultsBehaviorally, both PA+Stim and PA+Sham showed significant sensorimotor aftereffects, inducing a leftward shift in visuo-spatial and proprioceptive pointing. Neurophysiologically, suprathreshold MEP amplitude increased in the left first dorsal interossei (FDI) and left soleus following the PA+Stim condition but not the PA+Sham condition. PA+Stim showed statistical trends for inducing larger changes in ICF of the left FDI and left tibialis anterior. Additionally, compared to PA+Stim, PA+Sham induced larger changes in LICI of the left FDI and left tibialis anterior, and in SICI of the left tibialis anterior. ConclusionAlthough both PA+Stim and PA+Sham had similar behavioral aftereffects, only PA+Stim increased cortical excitability in M1 representations of the left upper and lower limb (toward the direction of the PA aftereffect), suggesting that PA+Stim may elicit greater neurophysiological changes and generalization to lower limb than PA alone.