Can tDCS affect acquisition of a simple motor skill in neurotypical adults?

Abstract

Abstract Excitatory tDCS (positive current into the brain) enhances skill acquisition and task dependent adaptation (TDA). Little is known about tDCS impact on spinal cord. Spinal reflexes participate in movements, change through life, and can be modified by experience or injury (Ann Rev Neurosci 24: 807-843). People can change spinal reflex size through operant conditioning (J Neurosci 29: 5784-5792). Acquiring this simple motor skill (larger or smaller reflex size) requires supraspinal drive (J Neurophysiol 87: 645-652). This study asks if tDCS affects the magnitude or consistency of H-reflex conditioning. The MUSC IRB approved the study. Neurotypical participants receive Active (excitatory, 2mA, 0.06mA/cm2, 30m) or Sham tDCS over contralateral leg region of primary motor cortex during soleus H-reflex down-conditioning. Tibial nerve stimulation elicits H-reflexes. Participants stand comfortably and maintain soleus background activity in a defined range. In 4 Baseline (BL) sessions, H-reflexes are simply recorded (control trials, three 75-trial blocks). In 12 subsequent Conditioning sessions (CS), participants practice reducing the H-reflex (conditioning trials, 3 blocks of 75). Visual feedback about reflex size follows each conditioning trial. Every session collects H-reflex recruitment curves (RCs) and control reflexes. TMS MEP RCs are collected periodically. A 1-month follow-up examines retention. To date, 8 participants (4 women, 5 Active tDCS, Age 22-36) have completed the study. Final conditioned H-reflex sizes (mean of CSs 10-12 as % BL) were 58, 67, 90, 99, 215% for Active and 71,115,125% for Sham participants. MEPmax change was -0.66, -0.28, -0.07, +0.04, +0.30mv and +0.09, +0.11, +1.11mV, Active and Sham respectively. The study is ongoing. The full results may illuminate tDCS impact on skill acquisition and guide its therapeutic applications as a neuromodulator. Keywords: tDCS, Hoffman reflex, operant conditioning, motor learning