Accounting for the last-sampling bias in perceptual decision-making

Human decisions are replete with biases that often reflect the underlying mechanisms of the decision-making process. The current study focused on a bias observed in different modalities and decision domains; the last-sampling bias, whereby people tend to choose the option they sampled last. One common explanation for this bias is that attention causally affects choices: The option now being attended to accumulates evidence faster than its unattended alternative. An alternative explanation is that the state of the accumulated evidence influences sampling patterns, but this mechanism remains controversial. To test these explanations, we designed a sequential sampling paradigm based on a simple two-alternative perceptual decision task. In one condition subjects could freely sample the options and in another condition the last-sampled option was predetermined. Significant last-sampling biases were observed in both conditions. We then examined possible causes of the last-sampling bias under the attentional drift-diffusion model (aDDM) framework. By comparing empirical results and model behaviors, we inferred that the influence between attention and evidence accumulation is bi-directional during perceptual decision-making.