Hack27.Show Motion Without Anything Moving


Hack 27. Show Motion Without Anything Moving

Find out why static pictures can make up a moving image on your TV screen.

The motion aftereffect [Hack #25] shows that motion is computed in your brain separately from location. For instance, becoming accustomed to the moving surface of a waterfall causes you to see stationary surfaces as moving the other way, although they're quite still. In theory, motion can be calculated from position and time information, but that's not how your brain does itthere's a specialized brain region for detecting motion directly. Since location and motion are perceived separately, this can lead to some odd illusions, the motion aftereffect chief among them: you get the illusion of motion without anything actually changing position.

The motion aftereffect relies on an initial moving scene to set it up, but we can go one better and get an impression of movement when there's been no actual thing present, moving or otherwise. The effect is apparent motion, and even if you haven't heard of it, you'll have experienced it.

Look at two pictures one after the other, very rapidly, showing objects in slightly different positions. Get the timing right, and your brain fills in the gap: You get an illusion of the objects in the first picture moving smoothly to their position in the second. There's no single, moving object out there in the world, but your brain's filling in of the assumed path of movement gives you that impression.

Sound familiar? It should; it's the effect that all television and cinema is based on, of course.

2.16.1. In Action

The easiest way to experience this effect is, of course, to turn on your television or go to the cinema. Movie projectors show 24 frames (pictures) a second, and that's good enough for everyone to perceive continuous motion in the change from one frame to the next.

In the old days of cinema, the film had 16 frames a second, which were projected using a three-bladed shutter to increase the flicker frequency above the rate necessary for flicker fusion. Despite seeing the same frame three times, your brain would fill in the gaps between the images, whether they were the same or different, so that you'd get the impression of continuous motion.


Television and computer screens are more complex cases, because the refresh doesn't happen for the whole image at once as it does with cinema but the principle is the same.

To demonstrate the effect to yourself in a more low-tech way, try this old child's game. Take a notebook and in the page corners draw the successive frames of a moving scene. I'm not very good at drawing stickmen, so when I did it I just tried drawing small, filled circles moving up from the bottom corner to the top of the page. Alternately, you may find a flip book in your local bookshop.

Flip through the pages of the book using your thumb andat a particular speedyou'll see the scene come to life. They're not just single pictures any more; together they form an animation. In my case, I see the little dot shoot up the side of the page. If I flip through the pages more slowly, the dot moves more slowlybut still continuously, as if it moves through every position on its path. Then, as I slow down even more, there comes a certain point at which the feeling of watching a single moving circle disappears and I'm just looking at a bunch of pages populated with slightly different shapes in slightly different positions.

2.16.2. How It Works

This apparent motion effect is also sometimes called the phi phenomenon. The simplest form in which you've probably encountered it before is two lights flashing at such an interval that you see one light moving from the first position to the second, as on an LED ticker display. Imagine only two lights from such a display. If the delay between the lights flashing is too short, the lights seem to flash on simultaneously. If it is too long, you just see two lights flashing on, one after the other. But if just right, you'll be treated to some apparent motion.

Although the optimum time varies with circumstance, 50 milliseconds is approximately the delay you need between the first light blinking out and the second light flashing on, in order to get a strong illusion of a single light moving between the two locations. Note that that's 20 flashes a second, close to the rate of image change in cinema. (Just so you know, as the physical distance between the two light flashes increases, so does the optimum time delay.1)

The effect is most powerful when you see the light appearing at several locations, making a consistent movementexactly like LED tickers, on which a message appears to scroll smoothly across despite really being made out of sequentially flashing lights. In fact, it isn't just that we feel there's an illusion of movement: the apparent motion effect activates a region called MT (standing for middle temporal gyrus, a folded region on the temporal lobe) in the visual cortex, one primarily responsible for motion processing. Apparent motion is just as valid as real motion, according to the brain.

And this makes sense. The only difference with apparent motion, as far as visual perception is concerned, is that some of the information is missing (i.e., everything that happens in the locations between the flashing lights). Since there's no way to detect motion directlywe can't see momentum, for exampleand visual information is all we have to go on, apparent motion is just a legacy of our tolerance for missing data and our ability to adjust.

A visual system that wasn't susceptible to the effect would be overdesigned. The capacity to perceive apparent motion lets us see consistency in images that are moving too rapidly for us to comprehend individually.

2.16.3. In Real Life

The obvious benefit of the phenomenon is that we can sit back and watch television and movies.

It also explains why wheels can look as if they are going backward slowly when they are actually going forward extremely quickly. Remember that apparent motion is strongest when adjacent lights, or images, flash up approximately 50 milliseconds apart. Caught on film, a wheel rotating forward may be turning at such a speed that, after 50 milliseconds (or a frame), it's made almost a full turn, but not quite. The apparent motion effect is stronger for the wheel moving the short distance backward in that short time rather than all the way round forward, and so it dominates: We see the wheel moving slowly backward, rather than fast and forward.

2.16.4. Hacking The Hack

The phi phenomenon also seems to say something important about the relationship of real time to perceived time. If you show two flashing lights of different colors so as to induce the phi phenomenon, you still get an effect of apparent motion.2 For some people, the light appears to change from the first color to the second as it moves from the first spot (where the first light was shown) to the second spot (where the second light was shown).

Now the thing about this ishow did your brain know what color the light was going to change to? It seems as if what you "saw" (the light changing color) was influenced by something you were about to see. Various theories had been put forward to explain this, either about the revision of our perceptions by what comes after or about the revision of our memories. Philosopher Daniel Dennett3 says that both of these types of theory are misleading because they both imply that conscious experience travels forward in time along a single, one-step-forward-at-a-time-and-no-steps-back track.

Instead, he suggests, there are multiple drafts of what is going on being continuously updated and revised. Within an editorial window (of, some have suggested, about 200 milliseconds of real time), any of these drafts can out-compete the others to become what we experience .4

2.16.5. End Notes

  1. You can measure how the optimum timing of the flashes is affected by distance with the Apparent Motion Experiment maintained by Purdue University's Visual Perception Online Laboratory (http://www.psych.purdue.edu/~coglab/VisLab/ApparentMotion/AM.html; Java).

  2. You can see a demo of the changing color phi phenomenon here at Ken Kreisman's Phi Phenomenon Demo page (http://www.cs.tufts.edu/~kreisman/phi/index.html; requires Java).

  3. Dennett, D. C. (1991). Consciousness Explained. Boston: Little, Brown.

  4. Obviously there's a lot more to both Dennett's theory and to the philosophy of consciousness in general. "Multiple Drafts: an Eternal Golden Braid?" (http://ase.tufts.edu/cogstud/papers/multdrft.htm) by Daniel Dennett and Marcel Kinsbourn, and this summary of Chapter 5 of Dennett's book Consciousness Explained, "Multiple Drafts Versus the Cartesian Theater" (http://epmalab.uoregon.edu/writings/Chapter%205%20summary.pdf; PDF), both discuss the mental world as a parallel process that is edited down into a single experience for conscious consumption.

2.16.6. See Also

  • Greg Egan's science fiction short story "Mister Volition" (part of the excellent collection Luminous) is inspired by the multiple drafts theory of consciousness and, to understand the theory, a good a place as any to start. See Egan's bibliography for availability (http://gregegan.customer.netspace.net.au/BIBLIOGRAPHY/Online.html).



    Mind Hacks. Tips and Tools for Using Your Brain
    Mind Hacks. Tips and Tools for Using Your Brain
    ISBN: 596007795
    EAN: N/A
    Year: 2004
    Pages: 159

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