Cortical network mechanisms of anodal and cathodal transcranial direct current stimulation in awake primates

Transcranial direct current stimulation (tDCS) is a non-invasive neuromodulation technique that is widely used in clinical research to modulate cortical excitability and plasticity. At the same time, the mechanism of action and its actual impact on neuronal firing is unknown. To characterize potential effects of tDCS on sensorimotor neurons of behaviorally active cortical circuits, we implanted 96-channel microelectrode arrays in the sensorimotor cortex of two monkeys trained to perform a simple visuo-motor task. We recorded cortical spiking before, during, and after the most common forms of tDCS used in humans (unilateral sensorimotor stimulation) and found that neural responses were sensitive to both dose and polarity, and outlasted stimulation. Roughly 15% of cells were modulated by tDCS, and the ratio of excited to inhibited neurons was different for anodal and cathodal currents. At high currents, the cell-type specificity of these effects was indicative of direct effects on the neurons. Dynamics of neural ensembles during forearm contractions reflected dose-dependent changes in ensemble size and activation patterns. We conclude that tDCS induces reproducible and noticeable changes in cortical neuron activity at high doses, likely through a combination of single neuron polarization and network interactions.