Monday, September 26, 2011

The Stopped Clock Illusion

Everyone can relate to this. You're in class, at work, waiting for something... you look at the clock and that first second that goes by seems to take forever. Then every second after that appears to progress normally. Why is that?

This is due to a phenomenon called saccadic masking or saccadic suppression. A saccade is the rapid movement of your eyes from one point of attention to another. To demonstrate this to yourself, hold out your two thumbs in front of you, and try to move your eyes smoothly from your right thumb to your left. You'll notice that your eyes don't move smoothly. Instead, they jump to points between your two thumbs along the way.

Saccades allow us to make a mental map of our surroundings and all the points of interest within it.  This is because the part of your retina directly behind your pupil, the fovea is packed with receptors that enhance visual acuity.

Cross-section of the eye, Wikimedia.

When you look around, your eyes don't span smoothly across the scene in front of you. Instead they move in saccades, quickly directing your fovea from one object of interest (the sharp corner of that table that you don't want to walk into) to another (that cute guy/girl waiting for you at the other end of the room). Saccades also occur when focusing on the details of a single image, as demonstrated by these eye movement traces of subjects as they examined the bust of Nefertiti.

Source: MIT

What you don't notice in between these saccades is well, anything.  The saccade itself is so fast that your brain doesn't have enough time to process the information coming to it to make a clear image.  A blurry image isn't too helpful and would probably just give you motion sickness.  In fact, the shaky "hand-held" camera effects in the movie Cloverfield did just that to many of its viewers.

So then what does your brain do with the information sent to it during a saccade?  Nothing; this is what is meant by the terms, saccadic suppression and saccadic masking mentioned earlier.  Even though the eyes are sending information to the brain, the brain does not process the information, leaving you effectively blind during a saccade. However, saccades are not so fast that you wouldn't notice the lapse in vision. What's going on? You don't actually perceive being blind during a saccade but you also don't see the blurry image, so what are you seeing?

It turns out that once you've fixed your fovea on an object, your brain actually tells you that you've been looking at it from the beginning of the saccade.  You don't notice this difference in timing at all, unless of course that object of your gaze actually keeps time.  So even though you're focused on the clock for only a second, your brain is telling you that you've been focusing on the clock for the 1,000 milliseconds it takes the second hand to move plus the time it took for your eyes to move to the clock.

On average, a saccade takes about 100 milliseconds, or about 10% of one second. So if you happen to look at a clock right at the beginning of a new second, it will appear to take 10% longer than normal, resulting in the famous illusion known as Chronostasis, or the stopped clock illusion.


  1. Congrats on the blog. I'll be reading and sharing with arty/sciency kids.

  2. Very interesting blog! Its fun to read about visual illusions because the more you learn about how your brain receives and processes information the more you realize you're working with a very strange almost hobbled together system. It is almost like our brains have been kludged together to gloss over and trick us into thinking they work perfectly when really we're just getting by enough to not get eaten by lions. One of the most interesting illusions I've see is the following:

    Which way does the dancer rotate for you? I'll tell you at the end so I don't influence you one way or the other. The trick is in getting her to change direction. It takes a bit of concentration but after awhile I was able to do it.

    P.S. - Clockwise, which is what the majority of people see.

  3. Thank you!

    I agree, it's amazing the little tricks our brains pull on us every day! It's especially amazing when you can learn to take advantage of some of those little hiccups in perception. I can't seem to find it now, but there are websites (which you're probably already aware of) that have little hints and tricks on how to hack your brain for all sorts of stuff.

    The dancer spins clockwise for me, too. I've gotten her to spin the other way a couple times in the past, but I seem to have forgotten how to do it now.

  4. I don't get it... Are you saying, the brain can alter the past? ;)
    Why can't we notice this 100ms delay?

  5. In a sense, that is what your brain is doing. It's altering your perception of the past. The reason we don't notice the delay is still unknown to science but the most common theory is that the information sent to the brain during the saccade would be useless to you, so it gets filtered out. If it didn't, you would just be seeing a lot of blurry images that would make you sea sick. So in order to preserve a sense of continuity between saccades, the brain extends your perception of how long you've been looking at the new object back in time to the beginning of the saccade.

    Hope that helps!

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