Effects of Transcranial Direct Current Stimulation on Neural Activity in the Primate Brain

Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique that has shown promise in improving cognitive functioning and as a therapy for neurological disorders. Recent studies have shown that it can modulate widespread neural activity; yet these effects are highly variable and depend on the areas being stimulated (montage) and the type of stimulation (polarity and intensity). We tested two tDCS stimulation montages (unilateral and bilateral) targeting the nonhuman primate prefrontal cortex (PFC) and examined their influence on spectral power in scalp-recorded electroencephalogram (EEG) and intracortical recorded local field potentials (LFPs). The effects of polarity (anodal or cathodal) and intensity (0.5-1.5 mA) were also examined. Our results suggest that tDCS with a bilateral montage may be more effective in changing spectral power than unilateral, causing a robust increase in power post-stimulation in frontal scalp electrodes, focused in the lower frequency bands. No effects of polarity were seen, but intensity of stimulation showed a roughly linear effect on power. Minimal EEG power changes were observed in parietal recordings post-stimulation, which tended to show decreases in power. These results highlight the importance of electrode placement and targeting specific brain areas when considering the potential efficacy of tDCS. Furthermore, these data allow a better understand the underlying mechanisms of tDCS and can help translate findings across experimental studies