State dependent effectiveness of cathodal transcranial direct current stimulation (tDCS)

The coupling of tDCS polarity-dependent opposite effect is well established in the sensorimotor domain but is still controversial when higher cognitive functions are targeted. Previous studies from our group showed that at resting state anodal tDCS (a-tDCS) over the right parietal cortex (rPPC) elicited a widespread increase of cortical excitability, whereas cathodal stimulation (c-TDCS) failed to modulate cortical excitability, being indistinguishable from sham stimulation. Here we assessed whether the active state of the targeted region might change the picture. We applied c-tDCS over the rPPC while our participants were performing a visuo-attentional task, namely Posner Cueing task (PCT). TDCS neurophysiological aftereffects were tracked performing TMS-EEG recordings pre- and post- either sham or real stimulation. In particular, single pulses TMS were delivered over the left PPC and TMS evoked potentials were recorded with a 64ch cap. Analyses were then performed both at the sensors and at the cortical source level. Behavioral results showed that c-tDCS significantly slowed down PCT performance in comparison to sham condition. Crucially, at a neurophysiological level, c-tDCS but not Sham significantly reduced cortical excitability in the brain areas involved in task execution, namely in a fronto-parietal network. Therefore, c-TDCS neurophysiological effects over rPPC resulted depending on the background activity. These results hold relevant implications for tDCS set up both in cognitive neuroscience experiments and rehabilitation protocols.