Modulating what is and what could have been: The effect of transcranial direct current stimulation on the evaluation of attained and unattained decision outcomes.

The affective evaluation of decision outcomes, whether attained (e.g., disappointment) or based on the conscious realization that a decision made differently would have led to a better or worse outcome (e.g., regret), greatly influence future decisions. Prior research has demonstrated a role of the medial and orbitofrontal cortex (M/OFC) in decision valuation and the experience of regret and relief. Here we examined whether inhibitory transcranial direct current stimulation (tDCS) could dampen the experience of decision-induced affect, with a focus on regret and relief. Thirty-eight participants completed a previously used gambling task and were asked to rate their happiness with attained outcomes of a chosen gamble before and after being shown unattained, counterfactual outcomes (i.e., what would have happened had they selected the other gamble). The difference in happiness rating before and after revealing these unattained counterfactual outcomes was taken as a measure of regret (negative shift) or relief (positive shift). During this task, 20 participants received 2 mA cathodal tDCS over EEG coordinate Fp1 for 20 minutes, and 18 participants received sham stimulation over the same location. Linear mixed-model results showed that, compared to sham, participants who received cathodal tDCS reported less intense emotions in response to attained as well as counterfactual outcomes. These findings were not due to the groups differing in the gambles they selected or attained monetary outcomes, demonstrating that tDCS can modulate decision-induced (counterfactual) affect. This may have implications for the ability to modulate value-based decision-making using brain stimulation techniques more broadly.