FUNCTIONAL CONNECTIVITY MODULATION INDUCED BY TRANSCRANIAL DIRECT CURRENT STIMULATION OF THE MOTOR NETWORK

Abstract

Background: Brain functions arise from the orchestrated activation and cooperation of networks of regions whose specific relationship varies dynamically across functional states. Resting state functional Magnetic Resonance Imaging (RS-fMRI) analyses focuses on spontaneous low frequency fluctuations (< 0.1 Hz) in the BOLD signal and investigates synchronous activations between regions that are spatially distinct (functional connectivity, FC), occurring in the absence of a task or stimulus. Transcranial Direct Current Stimulation (tDCS) is a non-invasive brain stimulation technique that is known to modulate cortical activity and FC among brain regions, as measured by functional Magnetic Resonance Imaging. It is well established that motor response (motor evoked potentials, MEP) is enhanced by anodal tDCS stimulation and reduced after catodal tDCS stimulation, for a period of about 5 minutes. Further studies showed different patterns of FC modulation after tDCS, depending on polarity but also on the regions studied, but little is known about the duration of the effect. Aim: this study aimed to measure the variation of functional connectivity between cortical brain regions after tDCS along time. Methods: For this purpose we enrolled 30 healthy right-handed subjects. All subjects underwent 4 sessions RS-fMRI (10’each, TR 2’’, 300 volumes, 1.5T scanner): 2 immediately before and 2 after 20’ tDCS over left M1. 20 of them received real (anodal) tDCS, 10 received sham stimulation. Data from 6 subjects (5 real, 1 sham) have been excluded for movement artifacts or other technical problems. We analyzed FC between left and right M1 with two different statistical analyses: Seed-based Correlation Analysis (SCA) and the Temporal Concatenation Group ICA (TC-GICA). Results: Seed-based Correlation Analysis showed a significant decrease of FC during the first fMRI acquisition immediately after anodal tDCS stimulation (p=0.005), that got back to baseline during the last fMRI session. This behavior was not found in subjects who underwent sham stimulation (p=0.12). The Temporal Concatenation Group ICA (TC-GICA) showed that immediately after anodal stimulation the average value of voxels decreases significantly (p <0.05) whereas there is no significant decrease in the case of sham tDCS stimulation. Conclusions: Our results shows that anodal tDCS is able to induce connectivity changes within motor network, that is reversible in a period lasting between 10’and 20’ after stimulation.