Pop-out in visual search

from Michael’s Visual Phenomena & Optical Illusions


What to see

Every 5 seconds the array of “Us” is jumbled. Then search for the one U that has an orientation than all the others. Easy, no?

What to do

Now click the 90° button. That’s already more difficult, would you agree? And clicking the 180° button makes it so hard that often I can’t do it in 5 s. Well, one can always use the stop button, or deselect autorun and use the renew button at your own pace.


Visual search (Wolfe 2014, Wikipedia) is a very frequent task for us, be it in the supermarket or at home looking for the keys. Luckily, when certain local feature differences are present (luminance, orientation, motion, …{Bach & Meigen 1999}), search occurs simultaneously –in parallel– throughout most of our visual field; thus the phrase “pop out” was coined. The alternative, serial search, is much slower. Many search tasks can be classified by their speed as sequential or parallel. [Usually, the slope of reaction time versus number of distractors is compared: nearly horizontal for parallel, linearly rising for sequential search.]

So, why does the orientation make a difference here? At 45° every line-segments’ orientation of the target differs from the distractors. At 90° we have 2:1 versus 1:2, and for 180° all angles are identical, there is just a slight (phase) shift. And the latter is not processed in parallel.

Incidentally, visual search is the one case I know of a sex difference in vision. It concerns the so-called “inhibition of return”, where women may have more (Brown 2013). Inhibition of return makes sense when searching for non-moving targets (e.g. in the fridge): You don’t need to look twice at the same place.