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

Becker SI - PLoS ONE (2011)

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: School of Psychology, The University of Queensland, Brisbane, Queensland, Australia. s.becker@psy.uq.edu.au

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.

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Related in: MedlinePlus

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.
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pone-0017740-g002: 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.

Mentions: A 3×2×3 ANOVA comprising the variables “target type” (thin vs. medium vs. thick), “target presence” (present vs. absent), and “distractor type” (fixation on thin vs. medium vs. thick distractor) computed over the mean number of fixations per trial showed that all main effects and interactions reached significance (all Fs>7.0; all ps<.005). As shown in Figure 2, fixations were clearly modulated by target-distractor similarity, with most fixations being made on distractors that were most similar to the target. Separate 2×2 ANOVAs comparing the number of fixations on two distractor types (thick vs. medium, medium vs. thin, thick vs. thin) on present versus absent trials confirmed that, in search for a thin target, thin distractors were significantly more frequently selected than medium distractors (F(1,11) = 435.5; p<.001) and thick distractors (F(1,11) = 429.5; p<.001). Moreover, the more similar, medium distractors were also more frequently selected than thick distractors (F(1,11) = 60.0; p<.001). These differences were reliable on both target absent and target present trials, but were all significantly stronger on target absent trials (all Fs>10.1; all ps<.009).


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

Becker SI - PLoS ONE (2011)

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.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3050928&req=5

pone-0017740-g002: 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.
Mentions: A 3×2×3 ANOVA comprising the variables “target type” (thin vs. medium vs. thick), “target presence” (present vs. absent), and “distractor type” (fixation on thin vs. medium vs. thick distractor) computed over the mean number of fixations per trial showed that all main effects and interactions reached significance (all Fs>7.0; all ps<.005). As shown in Figure 2, fixations were clearly modulated by target-distractor similarity, with most fixations being made on distractors that were most similar to the target. Separate 2×2 ANOVAs comparing the number of fixations on two distractor types (thick vs. medium, medium vs. thin, thick vs. thin) on present versus absent trials confirmed that, in search for a thin target, thin distractors were significantly more frequently selected than medium distractors (F(1,11) = 435.5; p<.001) and thick distractors (F(1,11) = 429.5; p<.001). Moreover, the more similar, medium distractors were also more frequently selected than thick distractors (F(1,11) = 60.0; p<.001). These differences were reliable on both target absent and target present trials, but were all significantly stronger on target absent trials (all Fs>10.1; all ps<.009).

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: School of Psychology, The University of Queensland, Brisbane, Queensland, Australia. s.becker@psy.uq.edu.au

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.

Show MeSH
Related in: MedlinePlus