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 us—especially 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 ground 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
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 cross—hold 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.
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 ...
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