A diffusion model analysis of target detection in near-threshold visual search

When searching for a target briefly presented among distractors how do people combine information across display locations to make a decision and how does the quality of the evidence entering the decision process vary with the type of items in the display? Research on accuracy in near-threshold visual search has had difficulty in distinguishing between models that make similar predictions about accuracy but make different assumptions about the underlying psychological processes. We used the diffusion model to analyse response times and accuracy data from four near-threshold search tasks which showed striking asymmetries between response-time distributions on target-present and target-absent trials. We found that performance was better explained by a model in which evidence was accumulated in parallel about each stimulus separately than one in which the evidence was pooled into a single decision process. We found that as contrast increased, the quality of the evidence entering the decision process about targets was markedly stronger than the evidence about nontargets. The overall pattern of evidence strength for stimuli on target-present and target-absent trials was consistent with a fixed-capacity memory system in which early visual processes assigned resources preferentially to targets over nontargets. The asymmetry was somewhat reduced in a letter-digit discrimination task that used heterogeneous targets and distractors, likely because heterogeneity reduces the efficiency of the preattentive filtering processes.