Reaction times are quite different when the target shares some, but not all, of the features of some of the distractors. In this case, the target does not pop out ("non-pop-out target"), and the reaction times go up according to the number of objects in the visual scene.
Why does this happen? What is the simplest explanation for both types of reaction time patterns? A critical insight came when Treisman and colleagues closely examined the errors made by the observers. When the observers misperceived what the target was (for instance, when they reported that the odd-man-out was a red vertical bar, when it actually was a red horizontal bar, they were also likely to misperceive where the target was. This means that, in order to find a target correctly, one also has to perceive the target correctly. Additional experiments showed that mental ability to focus on a particular location in the visual scene—say, the bar at the top left corner of the image—is needed when the target does not pop out, and not needed when it does.

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