The influence of tDCS on the speed-accuracy tradeoff and metacognitive decision making.

A fundamental tradeoff exists between speed and accuracy when performing a decision (speed-accuracy tradeoff, SAT). Metacognition allows for the adjustment, monitoring, and evaluation of one's own decisions and strategies. While these aspects of cognition are central to human behavioural performance, their respective causal neural underpinnings are not well understood. Here, we used transcranial direct current stimulation (tDCS) to investigate the causal roles of the prefrontal cortex (PFC), superior medial frontal cortex (SMFC), and posterior parietal cortex (PPC) in the SAT and metacognition. Subjects received active or sham tDCS before completing a perceptual task with explicit SAT cues and reported confidence in their decisions. We fit the linear ballistic accumulator model to behavioural data to extract latent decision variables and used confidence judgments to compute two common indices of metacognition: meta-d' and m-ratio. Stimulation influenced performance on the perceptual task but there was no meaningful evidence for an effect on metacognition. Specifically, PFC stimulation reduced subjects' response caution, especially when accuracy was emphasised; SMFC stimulation decreased response caution and increased the discriminability between choices; and PPC stimulation increased both response caution and discriminability. These results show that the impact of tDCS on the SAT critically depends on the frontoparietal region stimulated. In addition, there was little to no evidence of any effect of tDCS on metacognition, hinting at potential differences in the neural circuitry supporting aspects of object-level computation and meta-level processing. In sum, our findings provide further evidence that tDCS can alter decision making and strategic processes in the human brain.