Serial bias and response time: Prior stimulus not only biases but also modulates the speed of decision for a new stimulus

A decade of research has demonstrated that the reported perception of a new stimulus can be biased by task-irrelevant prior stimuli. However, existing studies have primarily focused on explaining the direction and magnitude of this bias effect, often neglecting other relevant aspects of perceptual behavior that may also be influenced by prior stimuli. In this study, we examined how decision speed for a new stimulus might be influenced by prior stimuli in motion-direction estimation tasks. We found that direction reports exhibited a repulsive serial bias along with a systematic response time (RT) effect, where reports were faster when the prior motion direction was more dissimilar to the current motion direction. Follow-up experiments replicated this RT effect and showed that it occurred only when repulsive serial bias was evident. Subsequent analyses revealed that the RT effect was positively correlated with repulsive serial bias, indicating that both effects are driven by common underlying mechanisms. Together, these results demonstrate that prior stimuli not only bias but also modulate response speed to new stimuli, suggesting that existing theories should incorporate decisional mechanisms that influence response speed to fully account for the serial bias phenomenon.