Choice bias, a hallmark of decision-making, is typically conceptualized as an internal reference, or criterion, against which available evidence is compared. The criterion can flexibly be adjusted to anticipate stimulus-response reward contingencies. Yet, although prestimulus neural excitability is thought to affect the criterion, little is known about how strategic criterion shifts are neurally implemented. Crucially, it is unknown whether humans can intentionally adapt neural excitability during criterion shifts and how this affects stimulus-related activity. Here we show that an experimentally induced liberal criterion is effectuated by suppressing prestimulus EEG alpha-band activity (8-12 Hz) in sensory cortex. Moreover, increased excitability boosts subsequent activity in the gamma (59-100 Hz) range by increasing response gain. Drift diffusion modeling of choice behaviour confirms that a liberal criterion biases sensory evidence accumulation towards yes choices. Together, these findings show that humans implement criterion shifts by flexibly adapting neural excitability and sensory evidence accumulation.