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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. 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.
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. 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. 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. 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 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. 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).
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Hack 28. Motion Extrapolation: The "Flash-Lag Effect"
 
If there's a flash of light on a moving object, the flash appears to hang a little behind.
How quickly we can act is slow compared to how quickly things can happen to usespecially when you figure that by the time you've decided to respond to something that is moving it will already be in a new position. How do you coordinate your slow reactions to deal with moving objects? One way is to calibrate your muscles to deal with the way you expect things to be, so your legs are prepared for a moving escalator [Hack #62], for example, before you step on it, to avoid the round-trip time of noticing the group is moving, deciding what to do, adjusting your movements, and so on. Expectations are built into your perceptual system as well as your motor system, and they deal with the time delay from sense data coming in to the actual perception being formed. You can see this coping strategy with an illusion called the flash-lag effect.1
2.17.1. In Action
Watch Michael Bach's Flash Lag demo at http://www.michaelbach.de/ot/mot_flashlag1 (Flash). A still from it is shown in Figure 2-23. In it, a blue-filled circle orbits a crosshold your eyes on the cross so you're not looking directly at the moving circle. This is to make sure the circle is moving across your field of view.
Figure 2-23. In the movie, the circle orbits the cross and flashes from time to time
Occasionally the inside of the ring flashes yellow, but it looks as if the yellow flash happens slightly behind the circle and occupies only part of the ring. This is the flash-lag illusion. You can confirm what's happening by clicking the Slow button (top right). The circle moves slower and the flash lasts longer, and it's now clear that the entire center of the circle turns yellow and the lag is indeed only an illusion.
2.17.2. How It Works
The basic difficulty here is that visual perception takes time; almost a tenth of a second passes between light hitting your retina to the signal being processed and reaching your cortex (most of this is due to how long it takes the receptors in the eye to respond). The circle in Bach's demo moves a quarter of an inch in that time, and it's not even going that fast. Imagine perpetually interacting with a world that had already moved on by the time you'd seen it.
So we continuously extrapolate the motion of anything we see, and our brain presents us with a picture of where the world most likely is now, rather than where it was a fraction of a second ago. This applies only to moving objects, not to stationary ones, and that's why the disparity opens up between the moving blue circle and the static yellow flashone is being extrapolated; the other isn't.
Straightforward extrapolation of the path of moving objects is one way in which this effect can take place, and this happens as early as the retina itself during visual processing. The cells in the eye compensate for its slow response by being most active at the front edge of a moving object. (Without this, the most active cells would be the ones that had been exposed to the object the longest, that is, the ones at the back.2)
That's one way in which the flash-lag effect could come about, because the delay for visual processing is compensated for with moving objects, but flashes still pay the penalty and are seen later. But that doesn't explain the demonstration movies constructed by David Eagleman and Terrence Sejnowski (http://nba.uth.tmc.edu/homepage/eagleman/flashlag; QuickTime). Essentially the same as Bach's demo, these movies have an erratically moving ring that should confuse the brain's motion prediction.
In Experiment 1 (http://nba.uth.tmc.edu/homepage/eagleman/flashlag/r1.html; QuickTime), the ring abruptly changes direction at the same time as the flash. Still we see the flash lag behind the moving ring, even though prediction of the future motion of the ring could not have occurred.
Eagleman and Sejnowski's explanation is that vision is postdictive. They argue that the brain takes into account changes in the scene that occur after the flash, for a very short time (less than a tenth of a second), and the motion preceding the flash isn't relevant at all. This is similar to the way two flashing dots can appear to be a single dot apparently moving [Hack #27] smoothly from one position to another, if the timing is right. Your brain must have filled in the interim motion retrospectively, because you can't know what in-between would be before the second dot appears. Similarly, the circle in this flash-lag experiment and the following fraction of a second comprise a period to be assembled retrospectively. The ring is moving smoothly after the flash, so you have to see it moving smoothly, and the flash appears slightly behind, because by the time you've mentally assembled the scene, the ring has moved on.
The situation is muddied because flash lag isn't unique to motion. One experiment3 found the same effect with color. Imagine a green dot slowly becoming red by passing through all intermediate shades. At a certain point, another dot flashes up next to it, with the same color for that time. Looking at it, you'd see the flash-lag effect as if the changing dot were moving along the color dimension: the flashed dot would appear lagged. That is, the flashed dot would appear greener than the changing dot.
That flash lag appears for phenomena other than motion supports that postdiction position. It could be the case that we don't see the world at an instant, but actually as an average over a short period of time. The moving ring appears to be ahead of the flash because, over a very short period, on average it is ahead of the flash. The colored dot appears to be redder than the flashed dot because it is redder, over the averaged period.
2.17.3. In Real Life
The effect was first noticed with the taillights of a car in the dark (the car being invisible except for the rear lights). A flash of lightning lets you see the car, and the lights appear to be halfway along it: the carwhich is flashedlags behind the taillightswhich are extrapolated.
It should also be evident in reverse. If you're photographed from a moving car, the flash of the camera should appear a little behind the car itself.
2.17.4. End Notes
1. Nijhawan, R. (1994). Motion extrapolation in catching. Nature, 370, 256-257.
2. Berry, M. J. 2nd, Brivanlou, I. H., Jordan, T. A., & Meister M. (1999). Anticipation of moving stimuli by the retina. Nature, 398 (6725), 334-338.
3. Krekelberg, B., & Lappe, M. (2001). Neuronal latencies delay the registration of the visual signal. Trends in Neurosciences, 24(6), 335-339.
2.17.5. See Also
· Flash lag may also contribute to controversial offside decisions in soccer. The offside rule is notorious for its opaqueness, so it's best, if you're a follower of the game, to read the paper yourself. It's about how a linesman observes both a moving player and the ball being played forward (which acts as the flash). The percept of the flash can lag behind the moving player, leading to an incorrect call of offside. Baldo, M. V. C., Ranvaud, R. D., & Morya, E. (2002). Flag errors in soccer games: The flash-lag effect brought to real life. Perception, 31, 1205-1210. (http://fisio.icb.usp.br/~vinicius/Public_pdf/Baldo_Ranvaud_Morya.pdf)
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Date: 2015-12-11; view: 936
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