The dynamics of perceptual decision making

Decisions about noisy stimuli require evidence integration over time. Traditionally, evidence integration and decision making are described as a one-stage process: a decision is made when evidence for the presence of a stimulus crosses a threshold. However, this model is incompatible with psychophysical experiments on feature fusion, where two visual stimuli are presented in rapid succession. Paradoxically, the second stimulus biases decisions more strongly than the first one, contrary to predictions of one-stage models and intuition. This can only be explained using a two-stage model where sensory information is integrated and buffered before it is fed into a drift diffusion process. I will present a series of psychophysical experiments to test both, accuracy and reaction time distributions predicted by the model.