Restoring Limb Function by Letting the Brain Control Muscle Stimulators Directly
*Dawn Taylor1,2,3, Frank Willett1,2,3, Tyler Johnson1,2 , Harrison Kalodimos1,2,3
1The Cleveland Clinic, Dept. of Neurosciences, 2Cleveland Functional Electrical Stimulation (FES) Center, Louis Stokes Cleveland VA Medical Center, 3Case Western Reserve University, Dept. of Biomedical Engineering
Introduction: Determining the stimulation patterns that will generate one’s desired limb motion under every use condition is a challenge. Complex stimulation algorithms have to be customized to each user’s unique post-injury limb, and the level of expertise required to engineer these stimulation algorithms for each unique user hinders wide spread clinical deployment. Instead of trying to solve this difficult engineering problem, we’ve taken a fundamentally different approach that makes use of the brain’s ability to learn—we let the brain control the stimulators directly.
Method: Rhesus macaques used the firing rates of ~60 cortical neurons to control the stimulation levels applied to six muscles in a ‘dynamic arm simulator’—a realistic computational model of a paralyzed human arm that takes in stimulation values and calculates how a paralyzed human arm would respond in real time. The animals received real-time visual feedback of the model arm’s fingertip position in the form of a moving cursor and had to move the cursor to different targets for rewards. The linear brain-to-muscle-stimulation weight matrix was generated by initially taking some simple empirical measurements of the model limb’s static force response to stimulation and the neurons’ response to thinking about movements while not actually moving—a process that could be easily implemented clinically.
Results: Subjects were able to move the model arm’s fingertip cursor to the desired targets immediately but with curved paths. With practice, they learned to adjust their cortical output to make straighter trajectories. Subjects also could quickly adjust the stimulation to compensate for external forces applied to the limb.
Discussion and conclusions: By enabling the brain to control one’s FES system directly, we can make use of the brain’s inherent adaptability to refine and expand its repertoire of skilled movements with practice, thus reducing the engineering challenges to clinical deployment.
This work was supported by NIH/NINDS Grant R01NS058871, Merit Review I01 RX001296 from the US. Dept. of Veteran’s Affairs Rehabilitation Research and Development Service, the National Science Foundation, and the US Dept. of Education. The arm model was developed by the R. F. Kirsch lab and graciously made available under contracts from the NIH. The contents do not represent the views of the U.S. Dep U.S. Department of Veterans Affairs or the United States Government.