Proceedings of the Annual Meeting of the Cognitive Science Society

There are two main types of bias in simple decision tasks,response bias and stimulus bias. Response bias is a startinglevel of evidence in favor of a biased response, whereas stim-ulus bias is the evaluation of stimuli in favor of a biased re-sponse. Previous research typically dissociates between thesetwo types of bias. Some studies suggest that it can be diffi-cult to induce response bias without stimulus bias (Ratcliff &McKoon, 2008; van Ravenzwaaij, Mulder, Tuerlinckx, & Wa-genmakers, 2012). We used a two-alternative forced-choicebrightness discrimination task in which we manipulated thepresentation length of the stimuli. We analyzed the data witha hierarchical diffusion model. The results show an overall re-sponse bias, as well as stimulus bias that increases as stimuluspresentation time decreases. We argue that the results suggesta need to revise how stimulus bias is conceptualized throughthe drift rate parameter of the diffusion model.