There's a chance you've already seen this perception test video in a psychology class (it's a pretty popular one). But if you haven't, give it a try:
There are people in white shirts passing a basketball back and forth among themselves. Your job is to count the number of passes they complete. At the same time, people in black shirts will be passing another basketball among themselves. Their passes don't count.
Okay, ready? Here's the link. Watch the video.
~Don't read further until you've come up with a total number of passes!~
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Did you get sixteen? That's what I got. Maybe sixteen and a half if you count the second half of the pass that the movie opens with. Did you wonder about what the S's on the walls were about? So did I. I have no idea what they're about. Did you see the girl in the gorilla suit walk into the middle of the group, beat her chest with her fists, and walk back out of the scene? No? If not, you might want to watch the movie again.
Maybe you picked up on it the first time you saw the video. If so, pat yourself on the back. But it's okay if you didn't see it- most people don't, says Dan Simons '91, Carleton graduate and head of the Visual Cognition Laborarory at UIUC.
Dan Simons gave a talk at Carleton recently about change blindness and inattentional blindness- if you didn't notice the gorilla, that was inattentional blindness. You weren't looking for the gorilla- you were counting passes, not gorillas- and so you didn't see it.
Inattentional blindness can be deadly just as often as funny: Dan Simons says that many motorcycle crashes at intersections are caused simply because some motorists in cars are looking for other cars and not for motorcycles, so they simply don't see the motorcycle coming when making a left turn, and they make the turn and drive right into the motorcyclist. A famous example of the deadliness of inattentional blindness was when Commodore Scott Waddle surfaced his submarine with a Japanese fishing boat directly overhead, killing nine people. He had looked through the periscope before surfacing, and the ship had been right there, but since he hadn't expected it to be there, he didn't see it.
Another visual shortcoming related to inattentional blindness is "change blindness"- I'll let you experience this one for yourself again. Here's a video in which part of the scene is gradually changing. Try to figure out what it is before reading further.
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DID YOU SEE THE GORILLA THIS TIME?
No?
That's good, because there wasn't a gorilla. If you did see a gorilla, go lie down for a while. What was changing was the right edge of the cornfield. Look at the first frame of that video, and then look at the last frame. Now you probably see the difference. But when it's changing veeeery slowly, it's hard to notice that anything is changing at all.
Not only does slowing down a change prevent us from seeing it, but so does an interruption while the change is taking place. For instance, here is some footage of an experimenter approaching an unsuspecting man, asking for directions, and then being interrupted by some people carrying a table. When the confusion clears, the man is giving directions to a different person – and he doesn't notice!
Dan Simons has carried out many variations of this trick, such as standing behind a booth and asking someone to sign up for an experiment, then ducking behind the booth. His friend then would stand up from behind the booth and continue the conversation as if he were Dan Simons. Few subjects noticed.
Oddly enough, people just don't seem to notice changes or objects that they aren't expecting to see. Which is enough to make me wonder if one of my friends is actually four or five people who have been acting like the same person, or if the Loch Ness Monster has been swimming around Lyman Lakes and none of us have noticed it because, hey, who expects to see the Loch Ness Monster at Carleton?
Something to think about, anyway.