Arousal levels can be used to predict the ongoing effects of neuromodulation

In this study, we aimed to verify whether the behavioural effects induced by a common prefrontal tDCS montage were dependent on the participants’ arousal levels. Pupillary dynamics were recorded during an auditory oddball task while applying either a sham or real tDCS. The tDCS effects on reaction times and pupil dilation were evaluated as a function of subjective (STAI-Y State scores) and physiological (pre-target pupil size) arousal predictors. We showed that prefrontal tDCS hindered task learning effects on response speed such that performance improvement occurred during sham, but not real stimulation. Moreover, both predictors significantly explained performance during real tDCS, with interaction effects showing performance improvement only with moderate arousal levels; likewise, pupil dilation was affected according to the ongoing levels of arousal. These findings highlight the potential role of arousal in shaping the neuromodulatory outcome, and thus encourage a more careful interpretation of null or negative results.