Determinants of dwell time in visual search: similarity or perceptual difficulty?

Abstract

The present study examined the factors that determine the dwell times in a visual search task, that is, the duration the gaze remains fixated on an object. It has been suggested that an item's similarity to the search target should be an important determiner of dwell times, because dwell times are taken to reflect the time needed to reject the item as a distractor, and such discriminations are supposed to be harder the more similar an item is to the search target. In line with this similarity view, a previous study shows that, in search for a target ring of thin line-width, dwell times on thin linewidth Landolt C's distractors were longer than dwell times on Landolt C's with thick or medium linewidth. However, dwell times may have been longer on thin Landolt C's because the thin line-width made it harder to detect whether the stimuli had a gap or not. Thus, it is an open question whether dwell times on thin line-width distractors were longer because they were similar to the target or because the perceptual decision was more difficult. The present study de-coupled similarity from perceptual difficulty, by measuring dwell times on thin, medium and thick line-width distractors when the target had thin, medium or thick line-width. The results showed that dwell times were longer on target-similar than target-dissimilar stimuli across all target conditions and regardless of the line-width. It is concluded that prior findings of longer dwell times on thin linewidth-distractors can clearly be attributed to target similarity. As will be discussed towards the end, the finding of similarity effects on dwell times has important implications for current theories of visual search and eye movement control.

Figure 1. Example of the search display and the conditions.

An example of a search display, depicting a target present trial in search for a thin target ring, among thin, medium and thick Landolt Cs. The Figure is not drawn to scale; the stimuli were much smaller and had a larger distance than depicted. The bottom row depicts examples of the possible target-distractor combinations used in each block.

Figure 2. Results: Mean Number of Distractor Fixations.

The mean number of fixations on each distractor type during visual search for a thin, medium or thick target, respectively. Error bars depict +1 SEM.

Figure 3. Results: Mean Dwell Times of Fixations on Each Distractor.

The mean dwell times on each distractor type, depicted separately for the thin, medium and thick search target. Error bars depict +1 SEM.