Effects of prism adaptation combined with electrical stimulation on cortical excitability of upper and lower limb muscles.

Right-shifting prism adaptation (PA) is a well-characterized sensorimotor behavioral phenomenon that induces leftward visuospatial-motor behavioral aftereffects. Somatosensory electrical stimulation (Stim) has been shown to upregulate corticomotor excitability during motor training, and can accentuate the effects of PA. In this study, we investigated the effects of combining PA+Stim on corticomotor excitability and upper limb visuospatial-motor behavior, and whether these effects transfer to the lower limb. We used a single-blind, 2-arm randomized crossover repeated-measures design to compare the effects of 1 session of PA+Stim versus PA+Sham in 15 young able-bodied individuals (18-35 years). During PA, either sensory-level Stim or Sham was delivered to the left upper limb. In addition to upper limb pointing performance, corticomotor excitability, and intra-cortical inhibition were measured before and after PA in the bilateral first dorsal interossei (FDI) and left tibialis anterior and soleus muscles using motor-evoked potentials (MEPs) elicited in response to single and paired-pulse transcranial magnetic stimulation (TMS). Our results showed that behaviorally, both PA+Stim and PA+Sham showed significant sensorimotor aftereffects, inducing the expected leftward shift during upper limb pointing. Neurophysiologically, only PA+Stim, but not PA+Sham, significantly increased MEP amplitudes in the left FDI. Additionally, PA+Sham versus PA+Stim showed significant differences in PA-induced modulation of intra-cortical inhibition in the left FDI and tibialis anterior. In summary, while both PA+Stim and PA+Sham had similar behavioral aftereffects, only PA+Stim increased cortical excitability and modulated intra-cortical inhibition of the left upper and lower limb. These findings suggest that combining PA with Stim may elicit greater neurophysiological effects and inter-effector transfer to the lower limb compared to PA alone.