7/19/2006

Symmetry in Visual Search

Can you identify which item doesn't belong in picture A, to the right? How about in the picture b? As you may have noticed, the second picture is merely a rotated version of the first; nonetheless, people are on average much faster at visually searching the first picture than the second. As it turns out, simple rotations often profoundly alter the efficiency of visual search, but up until an article in the June issue of Psych Science, the reasons for this were unclear.

Authors van Zoest, Giesbrecht, Enns, and Kingstone first ask whether the difference in search efficiency could be due to the fact that it's easier to search through tall objects than wide objects. Based on the reaction times of 45 participants each searching 360 displays showed that the differences in reaction time could not be attributed to target & distractor width. However, the authors did find that black-top targets were easier to find than white-top targets; this is consistent with the idea that viewers expect objects to show coloration consistent with "lighting from above," thus making the objects with black-tops appear odd (incidentally, it is this same expectation that forms the basis for this visual illusion).

Extending this rationale, the same authors next asked whether viewers' lighting expectations could account for the search efficiency differences between pictures a and b above. To test this hypothesis, the authors designed stimuli that did not have a simple "lighting" interpretation, similar to the stimuli in picture C, above. Although these new stimuli removed the difference seen between different distractor/target pairs, they didn't remove the difference between upright and rotated displays.

In a third experiment, the authors considered that perhaps the upright/rotated difference could be caused by "internal symmetry," in that in picture A the items are internally symmetric about the vertical axis, while in picture B the items are internally symmetric about the horizontal axis. To test this hypothesis, the authors used the stimuli in picture C. Unfortunately, this also did not remove the advantage for "upright" vs. rotated displays.

If none of these things can explain the difference in reaction time, then what on earth could? As it turns out, some research has shown that items that are symmetric across the vertical axis (such as the letters "b" and "d") are perceived as more similar to one another than items that are symmetric across a horizontal axis (such as the letters "b" and "p"). What if search is easier when the items are seen as more different from one another, and thus when the targets and distractors are vertically symmetric rather than horizontally symmetric?

To answer this question, the authors used the stimuli in picture D, above. Unlike the stimuli in C, D's targets & distractors are different in that they are not reflections of one another, but instead 180 degree rotations of one another. So, if the direction of interitem symmetry is what causes the differences in search efficiency between upright and rotated displays, then this set of stimuli should show no RT difference between upright and rotated displays.

Indeed, this is exactly the result these authors found. This discovery can be viewed as a refinement of the predictions motivated by Feature Integration Theory (FIT), in that search efficiency is a function of target/distractor difference. But in contrast to the typical interpretation of FIT, these authors have shown that the ways in which targets and distractors differ is not necessarily straightforward - not only does it have to do with lighting expectations, but also interitem symmetry, and this target/distractor need not be "spatially local" in order to affect search efficiency.

But why should interitem symmetry make a difference? As yet, that question remains unanswered.

Related Posts:
Selection Efficiency and Inhibition
The Attentional Zoom Effect
The Attentional Spotlight

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