Ambiguous tDCS: transcranial direct current stimulation effects on performance of monetary incentive delay task reinforcement learning

Transcranial electrical stimulation (tES) serves as a powerful technique for assessing the causal role of specific brain regions in behavior, including decision-making. While tES studies investigating sensorimotor function yield unequivocal results, cognitive research reveals considerable ambiguity and diversity in stimulation-related effects. The consequences of transcranial direct current stimulation (tDCS) on cognitive functioning are not consistently predictable based on the current direction (cathodal or anodal), which limits its applicability in cognitive research. In the present study, we further explored tES effect ambiguity in cognitive tasks by modulating medial frontal cortex (MFC) activity in an auditory monetary incentive delay (MID) task, where participants responded to acoustic cues encoding expected monetary losses using cathodal tDCS. We analyzed feedback-related negativity (FRN), reflecting prediction error processing when participants encountered losses compared to no losses during two subsequent MID task sessions, and exogenous P2 response to stimulus onset unrelated to anticipated monetary incentives. We anticipated an inhibitory cathodal tDCS effect on both P2 response and FRN. Contrary to our expectations, we observed a facilitatory effect of cathodal tDCS on FRN, replicating our earlier results (Gorin et al., 2022). No effect of stimulation was observed on P2; however, tDCS influenced the learning effect of P2. The difference in P2 amplitude between the first and second sessions, observed in the sham group, was absent in the group that received cathodal tDCS. We offered the interpretation of the complex picture of tES effects during auditory MID performance in light of brain plasticity theory for P2 and reward-learning mechanisms for FRN. Importantly, our findings regarding the multidirectionality of tDCS effects on cognitive function challenge the utility of tES as a readily employable method for testing brain causality in highly complex neurocognitive events, such as decision-making.