Australasian Mathematical Psychology Conference 2019

The effect of stimulus presentation time on response and stimulus bias: A diffusion-model based analysis

Jeremy Ngo
University of New South Wales
Chris Donkin
University of New South Wales

There are two main types of bias in simple decision tasks, response bias and stimulus bias. In the context of a diffusion model, response bias is a shift in the start point, reflecting that each decision starts with some evidence in favour of one of the responses. Stimulus bias is bias in the drift rate, where evidence accumulation is faster for one response compared to the alternative. Differences in the relative frequency of presented stimuli are typically thought to induce response bias, though previous research has sometimes reported stimulus bias as well. We used a two-alternative forced-choice brightness discrimination task in which we manipulated the presentation length of the stimuli. We analysed the data using a hierarchical diffusion model. The results show an overall response bias, but also find stimulus bias increases as the presentation time of the stimulus decreases. We argue that the results are difficult to explain using the traditional diffusion model, which assumes stationarity of the drift rate. It is plausible the drift rate of individuals may depend on factors other than the stimulus early in a trial, but is updated as information becomes readily available.