An unexpected target of spinal direct current stimulation: Interhemispheric connectivity in humans

Background: Transcutaneous spinal Direct Current Stimulation (tsDCS) is a noninvasive technique based on the application of weak electrical currents over spinal cord. New method: We studied the effects of tsDCS on interhemispheric motor connectivity and visual processing by evaluating changes in ipsilateral Silent Period (iSP), Transcallosal Conduction Time (TCT) and hemifield Visual Evoked Potentials (hVEPs), before (T₀) and at a different intervals following sham, anodal and cathodal tsDCS (T9-T11 level, 2.0mA, 20'). Motor Evoked Potentials (MEPs) were recorded from abductor pollicis brevis (APB), abductor hallucis (AH) and deltoid muscles. hVEPs were recorded bilaterally by reversal of a horizontal square wave grating with the display positioned in the right hemifield. Results: Anodal tsDCS increased TCT (p<0.001) and the interhemispheric delay for both the main VEP components (N1: p= 0.0003; P1: p<0.0001), dampening at the same time iSP duration (APB: p<0.0001; AH: p= 0.0005; deltoid: p<0.0001), while cathodal stimulation elicited opposite effects (p<0.0001). Discussion: tsDCS modulates interhemispheric processing in a polarity-specific manner, with anodal stimulation leading to a functional disconnection between hemispheres. tsDCS would be a new promising therapeutic tool in managing a number of human diseases characterized by an impaired interhemispheric balance, or an early rehabilitation strategy in patients with acute brain lesions, when other non-invasive brain stimulation techniques (NIBS) are not indicated due to safety concerns.