Eye movements during a realistic search task

Purpose. If visual search is a serial process behaving like a spotlight, then a sequence of saccades directed to individual objects in a display would be expected. We tested this hypothesis by monitoring the eye movements from 6 subjects as they searched for target objects in realistic scenes. Methods. Stimuli consisted of high-resolution color images of objects (e.g., tools) arranged on an appropriate background surface (e.g., a workbench). Scenes depicted either 1, 3, or 5 objects, with each object appearing initially at 7* eccentricity from a fixation cross. The subject's task was to indicate the presence or absence of a target object in each of these scenes, during which time eye position data was collected. Results. We failed to observe the sequential fixation of individual objects suggested by serial search. Surprisingly, most initial saccades were directed towards the center of the scenes, even though no objects ever appeared there. The eyes then typically moved to another intermediate location on one side of the display before finally fixating an individual object by the third saccade. Conclusions. Contrary to our intuitions about search (that we look directly to individual objects) and the spotlight metaphor (that objects in a display are inspected serially), finding a gradual convergence of oculomotor search towards the target instead suggests that search in complex scenes may be better described as a process which collapses over time and space. Specifically, these center-of-gravity averaging patterns observed in the primary and secondary saccades argue for an initially broad search window which constricts during the course of search to surround a single object.