You've looked at the same two pictures for two minutes. You've checked everything. You're sure the game is broken and there are only four differences, not five. Then you glance at the top corner and there's a whole lamp missing. Not a tiny lamp either.
Almost everyone who plays this game knows that moment. It has nothing to do with bad eyes or not paying attention. Psychologists have a name for it, change blindness, and they've been poking at it for decades. Some of the experiments are genuinely fun to read about, so here are the ones we find most interesting.
We don't see as much as we think
The idea behind change blindness is simple and slightly unsettling. Your brain doesn't keep a detailed photo of what's in front of you. It keeps a rough sketch: the layout, the general kind of place, and the few things you actually paid attention to. The rest is filled in when you need it. If something changes that you never really looked at, there's nothing in your memory to compare it with, so you don't notice. There's a good overview on Wikipedia if you want to go deeper.
The flicker experiment
In 1997, Ronald Rensink, Kevin O'Regan and James Clark showed people two versions of a photo that kept alternating on a screen (Rensink et al., 1997). The differences weren't subtle. In one example, a whole jet engine disappeared from a plane's wing.
When the two images switched directly, people saw the change almost immediately. The flicker of motion pulled their eyes straight to it. But the researchers put a short blank screen between the two images, and with that, people often needed many rounds to find the same change. That tiny interruption wiped out the motion signal, and they had to search the scene piece by piece.
That's pretty much what happens when you play. Every time your eyes jump from the left picture to the right one, that jump works like the blank screen.
The door
Our favorite is a study by Daniel Simons and Daniel Levin (Simons & Levin, 1998). A researcher stopped people on a university campus and asked for directions. While they were talking, two men carried a door between them, and behind the door the researcher swapped places with a different person.
About half of the people didn't notice. They kept giving directions to someone else entirely. They'd been busy thinking about the route, not about the face of the stranger asking.
The gorilla
You might know this one. Simons and Christopher Chabris had people watch a video of students passing basketballs and count the passes (Simons & Chabris, 1999). In the middle of the video, someone in a gorilla suit walks through the scene. Around half of the viewers didn't see it. Strictly speaking, that's a related effect called inattentional blindness, but the lesson is the same: what you're looking for decides what you're able to see.
Why some differences jump out
Research on visual search explains a lot about which differences are easy. Anne Treisman's feature integration theory (Treisman & Gelade, 1980) says that some simple features, like color, orientation, and size, are processed across your whole field of view at once. One red object among blue ones pops out right away, no matter how many blue ones there are.
Things get slow when you need a combination of features, like "the small blue cup with the handle on the left". You can't find that in parallel. You check one object after another, and the more objects there are, the longer it takes.
In game terms, it looks roughly like this:
| Kind of difference | Easy or hard? |
|---|---|
| Big object changes to a bright color | Easy. Color pops out on its own |
| Something added to an empty area | Easy. It stands out against the calm background |
| Something removed | Hard. There's nothing left to catch your eye |
| Small shape change in a busy area | Hard. You have to compare object by object |
| Something moved a little | Very hard. Positions are tough to compare across two pictures |
On top of that, your working memory is tiny. Steven Luck and Edward Vogel found that people hold about four objects in visual working memory at once (Luck & Vogel, 1997). So every time your eyes cross over to the other picture, you're only carrying a handful of details with you.
What we took from it
Knowing all this changed how we play, and also how we design levels. If you can only carry about four things across, it makes sense to compare a few objects at a time instead of whole areas. It helps to look for one kind of change at a time, like only missing things, because it gives your attention something concrete. The boring parts of a picture deserve a second look, because that's where change blindness is strongest. And when you're completely stuck, looking away for half a minute really does help. You come back and build a new sketch instead of running the same route again.
We turned that into a longer list in 10 ways to get better at spot the difference.
It's a bit funny when you think about it. The whole game only works because our brains are lazy about details. If we saw everything perfectly, nobody would bother playing.
Sources
- Rensink, R. A., O'Regan, J. K., & Clark, J. J. (1997). To see or not to see: The need for attention to perceive changes in scenes. Psychological Science, 8(5).
- Simons, D. J., & Levin, D. T. (1998). Failure to detect changes to people during a real-world interaction. Psychonomic Bulletin & Review, 5(4).
- Simons, D. J., & Chabris, C. F. (1999). Gorillas in our midst: Sustained inattentional blindness for dynamic events. Perception, 28(9).
- Treisman, A. M., & Gelade, G. (1980). A feature-integration theory of attention. Cognitive Psychology, 12(1).
- Luck, S. J., & Vogel, E. K. (1997). The capacity of visual working memory for features and conjunctions. Nature, 390.