Motor adaptation and internal model formation in a robot-mediated forcefield

Background: Motor adaptation relies on error-based learning for accurate movements in changing environments. However, the neurophysiological mechanisms driving individual differences in performance are unclear. TMS-evoked potential can provide a direct measure of cortical excitability. Objective: To investigate cortical excitability as a predictor of motor learning and motor adaptation in a robot-mediated forcefield. Methods: 15 right-handed healthy participants (mean age 23 years) performed a robot-mediated forcefield perturbation task. There were 2 conditions: unperturbed non-adaptation and perturbed adaptation. Transcranial magnetic stimulation (TMS) was applied in the resting state at baseline and following motor adaptation over the contralateral primary motor cortex (left M1). EEG was continuously recorded, and cortical excitability was measured by TMS-evoked potential (TEP). Motor learning was quantified by the motor learning index. Results: Larger error-related negativity (ERN) in fronto-central regions was associated with improved motor performance as measured by a reduction in trajectory errors. Baseline TEP N100 peak amplitude predicted motor learning (p = 0.005), which was significantly attenuated relative to baseline (p = 0.0018) following motor adaptation. Conclusions: ERN reflected the formation of a predictive internal model adapted to the forcefield perturbation. Attenuation in TEP N100 amplitude reflected an increase in cortical excitability with motor adaptation reflecting neuroplastic changes in the sensorimotor cortex. TEP N100 is a potential biomarker for predicting the outcome in robot-mediated therapy and a mechanism to investigate psychomotor abnormalities in depression.