Lack of an effect of transspinal and transcranial direct current stimulation on performance, perception of effort, and physiologic function in humans

Abstract

Introduction: For nearly two decades, direct current stimulation has been used to attempt to alter cognitive function, particularly in individuals who exhibit symptoms of mental illness. More recently, human performance enhancement has been a target of direct current stimulation. Direct current stimulation may be administered through non-invasive cutaneous transcranial or transspinal stimulation. Where altered motor cortex excitability or altered cortical activity are potential targets of transcranial stimulation, transspinal stimulation may affect spinal and supraspinal neuron function. However, the body of research related to the effects of transcranial and transspinal direct current stimulation on human performance is small and the mechanisms that may explain the effects are debated or unclear. Methods: One transcranial and two transspinal direct current studies were completed. The first study investigated the effects of transcranial direct current stimulation of the temporal lobe on high intensity work capacity and heart rate variability. The second project analyzed the potential modulatory effects of transspinal direct current stimulation on motor unit function and perception of effort. The third study quantified the effects of transspinal direct current stimulation on cycling time to exhaustion and perception of effort. Conclusions: In all studies, there was no effect of direct current stimulation condition, anodal or cathodal, on any measure of human performance, perceived exercise intensity, or physiologic function. Therefore, more research is required to determine how to maximize the efficacy of direct current stimulation with respect to acute modulation of human performance.