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Chia-Chien Wu, Eileen Kowler; Timing of saccadic eye movements during an accumulative visual search task. Journal of Vision 2012;12(9):735. doi: 10.1167/12.9.735.
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Efficient visual search requires rapid sequences of decisions about where to look, and when to initiate each saccade. To study temporal aspects of saccadic decisions, we used an accumulative visual search task in which several targets (thin outline circles, line width 4’) were located in arrays of distractors (line width 5’-7’). Each target contained a randomly-oriented line (discernible only when fixated) and Ss reported mean line orientation at the end of trials. Durations of fixations varied according to the type of fixated location, with fixations on targets lasting ~40% longer than those on distractors. The same was found in control ("look-only") trials when orientation was not reported. The longer fixations may have aided search in that saccades launched from targets were more likely to land on targets than saccades launched from distractors. When fixation duration was prolonged by increasing the difficulty of the perceptual task, the probability of landing on targets also improved to some degree. Fixation durations also varied according to where the saccade landed. Saccades made from distractors to targets had shorter latencies, and smaller sizes, than saccades made from distractor to distractor. This suggests that saccades were delayed when a target was not immediately visible, but not delayed past the point of diminishing returns. Fixation durations increased with trial length and over time within trials, as the number of locations previously visited increased, producing a greater demand on memory in order to avoid revisiting targets. These results suggest that saccadic timing is modulated to produce efficient search. Searchers neither make saccades at highest possible rates, in the hope of reaching many targets through exploration, nor delay saccades past the point where useful information is likely to be obtained. Instead they adopt a strategy that takes into account the requirements of foveal processing, the difficulty of peripheral selection, and the momentary memory load.
Meeting abstract presented at VSS 2012
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