Criterion Setting is Implemented through Flexible Adjustment of Neural Excitability in Human Visual Cortex

Choice bias, a hallmark of decision-making, is typically conceptualized as an internal reference, or criterion, against which accumulated evidence is compared. Flexible criterion adjustment allows organisms to adapt to the reward structure associated with the choice alternatives, and is assumed to arise from shifts in this reference. Here, in contrast, we show that criterion setting is implemented by modulating evidence accumulation rather than shifting an internal reference. Compared to a conservative criterion, experimentally inducing a liberal criterion during a visual detection task suppressed prestimulus oscillatory 8-12 Hz (alpha) activity in visual cortex, suggesting increased neural excitability. Increased excitability, in turn, boosted stimulus-related 59-100 Hz (gamma) activity by enhancing cortical response gain. Drift diffusion modeling of choice behaviour confirmed that a liberal criterion specifically biases the process of sensory evidence accumulation. Together, these findings provide a unique insight into the neural determinants of decision bias and its flexible adjustment.