Right-shifting prism adaptation (PA) is a well-characterized sensorimotor intervention that induces a leftward visuomotor aftereffect on upper-limb pointing. Separately, somatosensory electrical stimulation (Stim) has been shown to upregulate corticomotor excitability during motor training and can accentuate sensorimotor learning. The present study tested whether combining PA with sensory-level electrical stimulation to the left upper limb (PA+Stim) produces greater neurophysiological effects than PA with sham stimulation (PA+Sham), and whether any neuromodulatory effects transfer from the upper limb to lower-limb muscles.
This investigation used a single-blind, two-arm randomized crossover repeated-measures design. Fifteen young able-bodied adults (ages 18–35 years) completed the study. Participants received one session of PA+Stim and one session of PA+Sham in randomized order, with at least a 3-week washout between sessions. Behavioral and neurophysiological assessments were obtained immediately before (Pre) and immediately after (Post) each PA session.
During the PA protocol, participants wore rightward-shifting prism glasses to induce a recalibration of visuo-proprioceptive mapping that typically produces a leftward aftereffect once prisms are removed. Concurrently, sensory-level electrical stimulation (Stim) or a Sham condition was delivered to the left upper limb. The Stim was somatosensory in nature (sensory-level), intended to modulate sensorimotor cortical excitability during adaptation.
Behavioral outcomes included visuo-proprioceptive pointing (eyes open) and proprioceptive pointing (eyes closed) to quantify the magnitude and direction of PA-induced aftereffects. Neurophysiological outcomes were measured using transcranial magnetic stimulation (TMS): single- and paired-pulse TMS-evoked motor-evoked potentials (MEPs) were recorded to assess corticomotor excitability and intra-cortical inhibition. Muscles tested were bilateral first dorsal interossei (FDI) for the upper limbs and the left tibialis anterior and soleus for the lower limb. Measures were compared Pre versus Post within each condition and contrasted between PA+Stim and PA+Sham.
Both PA+Stim and PA+Sham produced the expected leftward sensorimotor aftereffect on upper-limb pointing. Specifically, visuo-proprioceptive pointing with eyes open showed a significant leftward change from pre to post for PA+Stim (p < 0.001) and for PA+Sham (p < 0.001). Proprioceptive pointing (eyes closed) also shifted leftward for PA+Stim (p < 0.001) and for PA+Sham (p < 0.002). Direct comparison of change scores between conditions revealed no significant difference in the magnitude of the PA-induced behavioral aftereffect between PA+Stim and PA+Sham.
Despite similar behavioral aftereffects, neurophysiological outcomes differed between conditions. Only the PA+Stim condition produced a significant increase in MEP amplitude in the left FDI, indicating enhanced corticomotor excitability for that upper-limb representation after PA when paired with sensory electrical stimulation. In contrast, PA+Sham did not produce a comparable increase in left FDI MEP amplitude.
Intra-cortical inhibition, probed via paired-pulse TMS measures, was modulated differently between conditions. The study reports significant differences between PA+Sham and PA+Stim in PA-induced modulation of intra-cortical inhibition in the left FDI and in the left tibialis anterior. These differences indicate that adding sensory-level electrical stimulation to PA altered intracortical inhibitory processes not only in the stimulated upper-limb representation but also in a lower-limb muscle representation (tibialis anterior), suggesting some degree of inter-effector cortical modulation.
Figure 1 illustrated the randomized crossover design, representative behavioral pointing traces (pre, during with prisms, and post), and representative TMS-evoked MEP traces from the left FDI for PA+Sham and PA+Stim. Figure 2 presented group behavioral data showing significant leftward aftereffects for both conditions and indicated no difference in behavioral change magnitude between PA+Stim and PA+Sham. Figure panels summarised visuo-proprioceptive (eyes open) and proprioceptive (eyes closed) pointing shifts and compared post-minus-pre change scores across conditions. Subsequent figures summarized neurophysiological changes, highlighting the selective increase in left FDI MEP amplitude after PA+Stim and differential intracortical inhibition changes across conditions.
In this cohort of young, able-bodied adults, combining prism adaptation with sensory-level electrical stimulation to the left upper limb produced greater neurophysiological modulation than PA alone, despite similar behavioral aftereffects. Specifically, PA+Stim increased corticomotor excitability in the left FDI and produced distinct modulation of intra-cortical inhibition in both the left FDI and left tibialis anterior relative to PA+Sham. These findings suggest that pairing PA with somatosensory stimulation can amplify cortical excitability changes and may promote inter-effector transfer from upper to lower limb muscle representations.
Clinically and experimentally, the results support the concept that concurrent peripheral somatosensory input can augment the neurophysiological impact of sensorimotor adaptation protocols. The study reports neurophysiological markers that differ by condition even when behavioral outcomes are similar, highlighting the value of integrating TMS-based measures when evaluating neuromodulatory interventions.
All details reported here are taken from the article abstract and figure legends available via the source. Specific methodological parameters beyond those summarized (for example, exact stimulation parameters, TMS intensities, or full statistical tables) were not reported in the abstract text provided and would require consultation of the full-text article for additional granularity.