Continuous theta-burst stimulation to primary motor cortex reduces the overproduction of forces following removal of visual feedback

Abstract

Forward models, generated from the efference copies of motor commands, are thought to monitor the accuracy of ongoing movement. By comparing predicted with actual afferent information, forward models also aid in the differentiation of self-produced movements from externally generated ones. Many have proposed that a consequence of this comparison is attenuation of the predicted component of incoming sensory signals. Previous work from our laboratory has shown that following the removal of an external visual reference, discrete sequential forces exceed target values. Forces produced at the fingertip were perceived as weaker, which lead to a systematic, compensatory over-production of the magnitudes required. The relatively new repetitive TMS protocol of continuous theta-burst stimulation (cTBS) has been shown to reliably depress cortical excitability for a period following stimulation. If sensory attenuation mechanisms were responsible for the overproduction of forces found in our previous results, we hypothesized that reducing cortical excitability of M1 through application of cTBS would induce discrepancy between the efference copy generated and motor output produced. As a result, we expected the overproduction of forces following visual feedback removal would be reduced after receiving cTBS. Participants produced series of pinch grip forces in time to a metronome and to visually specified force magnitudes. Visual feedback of force output was extinguished 10 s into experimental trials and participants performed continued responses for the remaining 10 s. Results confirmed our hypothesis. Mean peak force and constant error were greater and more positive in the absence of visual feedback regardless of stimulation condition; however, the magnitude of increase was significantly reduced following cTBS compared with baseline and sham conditions. Variability was not differentially affected by stimulation condition, increasing only with removal of visual feedback contingent upon the larger forces produced in these trials. Our findings provide further evidence to support the idea that TBS may differentially affect motor output and efference copy generation.