Causal Evidence for Dissociable Roles of the Prefrontal and Superior Medial Frontal Cortices in Decision Strategies

The speed-accuracy trade-off (SAT) is arguably the most robust finding in cognitive psychology. This simple and intuitive effect (the faster subjects respond, the more likely they are to make an error) has been the subject of extensive empirical and modeling work to ascertain the underlying latent process(es). One such process is response caution-the amount of evidence to be acquired before a decision is reached-with debate regarding the involvement of another latent variable, the rate of evidence accumulation. Neuroimaging has implicated two frontal regions as neural substrates of the SAT: the posterior lateral prefrontal cortex and the pre-supplementary motor area (preSMA; part of the superior medial frontal cortex; SMFC). However, there is no causal evidence for these regions' involvement in the SAT, nor is it clear what role each plays in the underlying processes. In a double-blind, preregistered study, we applied cathodal transcranial direct current stimulation (offline) to the prefrontal and SMFC. The SAT was measured using a dot-motion task, with differing response instructions (focus on accuracy, speed, or both). The linear ballistic accumulator model indicated performance modulations were driven by response caution. Moreover, both target regions modulated caution but in opposing directions: Prefrontal stimulation increased, and SMFC stimulation decreased, caution. Discriminability (difference between correct and error evidence accumulation rates) was predominantly affected by stimulation targeting the SMFC and did not vary with response instructions. Overall, the findings indicate that while both the SMFC and the prefrontal cortex are causally involved in the SAT, they play distinct roles in this phenomenon. Public Significance Statement We frequently have to balance between making a decision quickly and accurately. Here, we investigated the role of two key brain regions in setting such decision strategies using noninvasive brain stimulation. We found two regions of the frontal cortex-the prefrontal cortex and the superior medial frontal cortex-play distinct roles in the setting of response caution.