It’s impossible to pay attention to everything in the visual field at once. If we could, magicians would be out of business: most “magic” tricks work by distracting the viewers’ attention while the real trick is being done in plain sight.
However, if a new object enters our field of view, we quickly direct our attention that way and make note of it. This would have come in handy for our ancestors as they kept watch for predators. Now its primary use is probably avoiding car accidents (or shutting down Web browsers when the boss shows up at your cubicle!). Interestingly, this facility is not perfect. The image below links to an animation (read the instructions before you click!) where two small Xs will appear in the blue boxes. If you focus on the X in the center, then see how long it takes you to notice both additional Xs appearing, you’ll find that the first X serves as a distraction, making it more difficult for you to notice the second X. Go ahead and click now to try it out.
Interestingly, the farther away the second X appears, the less distracting the first X is. Try this second animation.
This effect was first observed by Michael Posner—if an object appears in one part of our field of view, it temporarily delays our ability to detect another object appearing near it. The effect begins about a third of a second after the first object appears and lasts about a second. If the second object appears sooner than that, we actually notice it quicker. Subsequent research revealed that the effect became progressively smaller at greater and greater distances from the spot where the first object appeared—surprisingly, we’re quicker to spot other objects appearing farther away from the original object.
Arthur Samuel and Schuyler Weiner of SUNY Stony Brook wanted to know if the effect also held for objects disappearing. They noted that though there was extensive research on this phenomenon, called inhibition of return, for objects appearing in the visual field, there was very little on objects disappearing. So, they designed an impressive set of experiments to determine if disappearing objects caused the same effect (“Attentional Consequences of Object Appearance and Disappearance,” Journal of Experimental Psychology: Human Perception and Performance, 2001).
In their first experiment, they used more complex display of 8 circles arranged around a central focal point and replicated Posner’s work showing inhibition of return for objects appearing more than 300 milliseconds after the first object appeared, but no difference in reaction time when the second object appeared sooner. In the second experiment, instead of having two objects appear, first an object disappeared and then a different object appeared. Surprisingly, reaction time was significantly slower when the second object appeared in the area around where the first object disappeared, even as close as 80 milliseconds later. This suggested that there is a different mental process occurring when objects disappear compared to when they appear.
So Samuel and Weiner tried two additional tasks. In the third experiment, they reversed the pattern of experiment two and had an object appear and then another object disappear. This time the results matched experiment 1. Next they had two objects disappear in a row. The results matched experiment 2. So it seemed that the different results occurred only when an object disappeared first, not simply because a disappearance occurred at some point during the experiment.
Speculating that perhaps a disappearance was less noticeable than an appearance, they repeated experiments 2 and 4, but with larger objects disappearing. Again, the results were the same. Whenever an object disappeared first, the pattern of the data was different from the standard inhibition of return: inhibiton occurred both immediately after the disappearance, and also later on, when it normally occurred with appearing objects.
So why would we respond to disappearing objects differently from appearing objects? Samuel and Weiner have a guess. Remember that we can’t possibly pay attention to all the information in the visual field at once. So, for the sake of effiency, it makes sense that when we notice an object appearing, we pay attention to it for an extended period of time—about a third of a second—so as to be able to identify it and determine whether it is threatening. Then we direct our attention elsewhere to locate additional possible threats. It is not likely that another object will appear in the same area as the first one, so we end up taking longer to notice something appearing near the first object than farther away.
When something disappears, by contrast, there is no need to linger at all—once it’s gone, there’s nothing there to identify. We immediately direct our attention elsewhere, which is why we are slower to recognize another object appearing or disappearing near the spot where the original object disappeared, and why this slower response is not delayed at all.