Abstract
An observer, fixating at one location, selectively processes stimuli in some surrounding region when searching for targets in visual search tasks. This region is called the functional visual field (FVF). Recent FVF research has sought to explain why observers sometimes miss a target even though they fixated nearby, placing the target clearly within the FVF. Such miss errors become especially important when the targets are, for example, tumors on x-rays or threats in baggage at airport security checkpoints. Understanding of such "look but fail to see" errors would benefit from a direct measure of the FVF during search. We use the duration of the “pre-target fixation” (PTF) as such a measure. The PTF is the fixation that immediately precedes a saccade to the target. In previous experiments where participants search for an O among Cs, PTF duration is shorter than other fixation durations when search is easy and when the target is near to the current fixation. It is hypothesized that the presence of a strong target signal inside the FVF speeds saccade planning and the resulting release of a saccade. In the present study, we tested whether similar effects can also be found in search through natural scene images where observers do not have precise target templates but only know the target category. To that end, we analyzed the large open source COCO-Search18 gaze dataset (Chen, Yang, Ahn, Samaras, Hoai, & Zelinsky, 2021). As in search for artificial stimuli, results showed shorter PTF durations if the target is close. As would be expected, the FVF as estimated from PTF duration is smaller when search is more difficult. The PTF duration method can be used to estimate the size of the FVF in a manner more natural and more convenient than gaze contingent moving window paradigms.