Signatures proposed to index perceptual effects emerge in a purely cognitive task

A central question in many studies on perception and consciousness is whether the effects of a given manipulation are perceptual or cognitive. Typically, studies seek to find evidence that the raw sensory experience has been altered-that is, a manipulation produces perceptual rather than cognitive effects. Here, we test two specific proposals for behavioral signatures that could allow this type of inference. First, a long-standing proposition is that different parameters of the drift-diffusion model (DDM) map onto different effects, such that a drift-rate bias indicates a perceptual effect, whereas a starting point shift indicates a cognitive effect. Second, a newer proposal holds that, when plotted as a function of a sensory feature, changes in the peak of the distributions for confidence and reaction time (RT) imply that a given manipulation's effects are perceptual. Here, we test both proposals. We designed a purely cognitive task where no effects can be perceptual and examined whether the proposed signatures of perceptual effects occur in this task. Participants viewed numbers generated by two distributions and indicated which distribution the numbers likely came from. In separate conditions we manipulated prior beliefs and rewards related to each distribution. We found that both manipulations elicited the same signatures previously proposed as indicative of purely perceptual effects. Our results demonstrate that individual DDM parameters and the peaks of confidence and RT distributions in isolation are insufficient to distinguish between perceptual and cognitive effects.