Ambiguous tDCS: variability of the transcranial direct current stimulation effects in a reinforcement learning task
Transcranial electrical stimulation (TES) is a popular approach for studying and modulating cortical function. According to somatic doctrine, anodal TES increases, while cathodal reduces cortical excitability. Currently, numerous studies use TES in behavioral experiments with no physiological control, relying on the assumption of fairness and complete predictability of stimulation models. However, control reveals the actual effect induced by the stimulation rarely matches one previously modeled.
We conducted an experiment where the feedback-related negativity (FRN) amplitude in the loss prevention version of the monetary incentive delay (MID) task was enhanced by cathodal tDCS (1 mA), even though the modulation of suppression was expected according to results obtained previously by Reinhart and Woodman (2014). The montage involved cathodal stimulation of the ventromedial prefrontal cortex. The effects were tested on 30 healthy participants with sham control.
Considering auditory stimuli in the MID task, and the fact that one of the auditory evoked potentials (P2) has sources in the prefrontal cortex, we also focused on the changes in the P2 caused by tDCS. Reportedly, P2 is sensitive to neuroplastic remodeling in a variety of auditory training tasks. Contrary to our expectations, cathodal tDCS simultaneously enhanced FRN and suppressed plastic changes in the P2 component linked to the auditory cues. We hypothesize that cathodal tDCS could affect the plastic changes of the P2 component by inhibiting P2 sources, one of which, reportedly, is dorsolateral prefrontal cortex.
The present work underlines the multidirectional physiological effects of tDCS that may lead to opposite modulation of neighboring cortical regions and therefore make the observations difficult to interpret. Our results call for future studies using tDCS and suggest more cautious interpretations of tDCS effects in the absence of proper psychophysiological control.