A behavioural perspective of visual search and the low prevalence effect

Abstract

In visual search, rare items are missed disproportionately often. This Low Prevalence Effect (LPE) is a robust phenomenon with important societal consequences. We asked the question, “Is the schedule of signal presentation controlling eye movements and if so, how?” and hypothesised that visual search is an operant behaviour. To answer this question, we examined the LPE using eye-tracking and incorporated a rich schedule of signal presentation (medium-prevalence condition: 0.50 target probability across 200 trials) and a lean schedule (low-prevalence condition: 0.02 or 0.00 target probability across 1,000 trials) over three experiments. Experiment One was the control, in Experiments Two and Three we increased task difficulty by incorporating a staircase titration procedure, and in Experiment Three we also removed all target-present trials in the low-prevalence condition. We replicated the LPE in all experiments and observed increased levels of searching eye movements throughout the medium-prevalence condition due to an increase in behaviour for the first correct rejection immediately following a correct target-present response which we attribute to the local effects of reinforcement and the strengthening of search behaviour when we find what we are looking for. We also observed that visual attention was being guided towards the densest population of stimulus items (centroid) at the beginning of their search as a search strategy and conclude that this centroid zone is acting as a discriminative stimulus signalling the availability of reinforcement. We contend that eye movements are an operant behaviour controlled by environmental contingencies and reinforcement mechanisms are behind the LPE phenomenon.