Target Detection in Visual Search: Unravelling the Pupillary Response

Pupillometry, the measurement of pupil size, is a technique used widely in psychological research to infer arousal, cognitive effort and processing load in various observer settings. Specifically, pupil dilation has been linked to increased effort in extended visual search (Porter, Troscianko & Gilchrist, 2007) and with the detection of targets in rapid serial visual presentation (Privetera et al., 2010). Whether pupil dilations elicited by target detection during visual search can be resolved against the backdrop effects of search-induced effort is a question of current interest. We sought to addresses this question with a novel signal-detection-style brief visual search paradigm. Pupil size was recorded as participants searched for a feature-distinct target item in arrays of similar items, responding either ‘target present’ or ‘target absent’ for each trial. Targets were presented briefly at the centre of the screen and participants were asked to maintain central fixation, which minimised eye movements and the pupil-size measurement errors they cause. The factor levels Target (present vs. absent) and Set Size (6 vs. 12) combined in equal parts to make four conditions. Overall, reaction times (RTs) were greater for target-absent trials. Pop-out search effects for target-present trials were evidenced by near-identical RTs for the two set sizes, whereas a marked difference in RTs for target-absent trials suggested that greater effort was required to arrive at the correct response for the larger set sizes. Despite the pop-out search effects on target-present trials and the greater effort required for target-absent searches, pupil dilation was consistently greater when a target was present. These results suggest that pupil dilations resulting from target detection could be reliably separable from the background effects caused effort during visual search. Meeting abstract presented at VSS 2015