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Infinite Pac-man

In the video below, you see an animation of the classic video-game Pac-man 1. But there is something weird about Pac-man. Can you spot what it is? (For the best effect, play the video in full-screen!)

In fact, as you probably noticed after a while, Pac-man is not doing anything weird at all. It’s the maze that is continuously changing. And not just a wall being added or removed here and there! The whole maze, with the exception of the tiles around Pac-man, is completely randomized with every new frame.

How can it be so easy to miss these large changes that are happening right in front of your eyes?

This video is a somewhat unusual demonstration of a classic phenomenon called change blindness, which was first described by Ronald Rensink, Kevin O’Regan, and James Clark. A typical change blindness experiment is similar to a “spot the differences” game, in which two slightly different images are presented side by side, and you have to, well, spot the differences. This can be very difficult!

Source: Wikimedia Commons

But spotting differences becomes very easy when the same two images are presented one after another at the same location. Now differences correspond to unique changes in the image. These unique changes grab your attention, and therefore really “pop out”.

Source: Wikimedia Commons

However, by adding a blank in between the alternating images, the differences again become difficult to detect. The reason is that, because of the blank, the whole image changes (from blank to image and vice versa) and the differences no longer correspond to unique changes. Without unique changes to guide you, you have to actively direct your attention to different parts of the image until, more-or-less by chance, you spot the difference.

Source: Wikimedia Commons

So how does change blindness relate to the Pac-man video? Change blindness experiments tell us that we need two things to make a change go unnoticed:

  • First, we need to prevent the change from being unique. In the video, this is accomplished by having the pac-dots blink, which introduces lots of change everywhere.
  • Second, we need to prevent the viewer from paying attention to the change. In the video, this is accomplished by having a moving Pac-man, which almost certainly diverts the viewer’s attention away from the background, at least initially.

With a well-crafted animation, you can get away with enormous changes! In this case, I probably got away, for a while, with changing pretty much everything. Whether you were fooled by the Pac-man video for a long time, or whether you quickly noticed that the background was changing, depends primarily on how focused you were on the Pac-man. Once you become bored or distracted, your attention is likely to fall on the changing maze and you immediately notice what’s going on.

I wonder what would happen if you were to turn this video into an actual game. When you control the Pac-man yourself, you are almost guaranteed to pay attention to it, and consequently to not pay attention to the maze. Perhaps gamers would spend many fruitless hours trying to clear the maze …


Rensink, R. A., O'Regan, J. K., & Clark, J. J. (1997). The need for attention to perceive changes in scenes. Psychological Science, 8(5), 368?373.

  1. In case you enjoy the tune in the background: The song is Goof by Binärpilot, and is available for free.