Hack25.See Movement When All Is Still

Hack 25. See Movement When All Is Still

Aftereffect illusions are caused by how cells represent motion in the brain.

Why, when the train stops, does the platform you are looking at out the window appear to creep backward? The answer tells us something important about the architecture of the visual system and about how, in general, information is represented in the brain.

The phenomenon is the motion aftereffect. Just as when you go from very bright sunlight to the indoors, everything looks dark, or if you are in a very quiet environment, loud noises seem even louder, so continuous motion in a certain direction leaves us with a bias in the otheran aftereffect.

2.14.1. In Action

Watch the video of a waterfall (http://www.biols.susx.ac.uk/home/George_Mather/Motion/MAE.HTML; QuickTime) for a minute or so, staring at the same position, then hit pause. You'll have the illusion of the water flowing upward. It works best with a real waterfall, if you can find one, although pausing at the end is harder, so look at something that isn't moving instead, like the cliff next to the waterfall.

The effect doesn't work for just continuous downward motion. Any continuous motion will create an opposite aftereffect; that includes spiral motion, such as in the Flash demo at http://www.at-bristol.org.uk/Optical/AfterEffects_main.htm.

The effect works only if just part of your visual field is moving (like the world seen through the window of a train). It doesn't occur if everything is moving, which is why, along with the fact that your motion is rarely continuous in a car, you don't suffer an aftereffect after driving.

2.14.2. How It Works

Part of what makes this effect so weird is the experience of motion without any experience of things actually changing location. Not only does this feel pretty funny, but it suggests that motion and location are computed differently within the architecture of the brain.

Brain imaging confirms this. In some areas of the visual cortex, cells respond to movement, with different cells responding to different types of movement. In other areas of the visual cortex, cells respond to the location of objects in different parts of the visual field. Because the modules responsible for the computation of motion and the computation of location are separate, it is possible to experience motion without anything actually moving.

The other way is to be able to perceive static images but be unable to experience motion, and this happens to some stroke victims whose motion module is damaged. Their life is experienced as a series of strobe-like scenes, even thoughtheoreticallytheir visual system is receiving all the information it would need to compute motion (that is, location and time).

You don't need brain imaging to confirm that this effect takes place at the cortex, integrating all kinds of data, rather than being localized at each eye. Look at the movie image of the waterfall again but with one eye closed. Swap eyes when you pause the videoyou'll still get the effect even with the eye that was never exposed to motion. That shows that the effect is due to some kind of central processing and is not happening at the retina.

To understand why you get aftereffects, you need to know a little about how information is represented in the brain. Different brain cells in the motion-sensitive parts of the visual system respond, or "fire," to different kinds of motion. Some fire most for quick sideways motion, some most to slow motion heading down to the bottom left at an angle of 27 degrees, and so on for different angles and speeds. Each cell is set to respond most to a different type of motion, with similar motions provoking almost as much response, and they won't respond at all to motions with completely different angles and speeds.

The kind of motion we perceive depends on the pattern of activation across the whole range of motion-sensitive cells. Relative activation of the cells compared to one another matters, not just how much each one individually is activated. But if some cells fire continuously, their level of response drops (a process called adaptation). So as you watch the waterfall, the cells coding for that particular motion adapt and stop firing so much.

Pausing the waterfall means normal service is resumed but not for the adapted cells. Relatively, they're responding much less than the cells looking for motion in the opposite sense, which haven't been firing. Usually these two groups of cells should balance each other out, but now the cells for the opposite direction are firing more. Despite a stationary input, overall your brain interprets the response pattern as movement occurring in the opposite direction.

2.14.3. Hacking the Hack

Originally some people thought that adaptation in the motion aftereffect may have been caused by simple fatigue of the motion-sensitive cells. We know now that this isn't the case. Instead, the mechanism is far more interesting and far cleverer. To demonstrate, simply try the original waterfall effect, but before watching the static pattern, close your eyes for 20 seconds. Now if the effect were due to fatigue and the effect itself lasted for 10 seconds, a wait of 20 seconds should remove the effect completely. But instead, you get an aftereffect nearly as long as you would have if you hadn't waited for 20 seconds with your eyes closed. The motion-sensitive neurons should have had time to recoverwhy are they still adapted?

They are still adapted because your baseline for motion perception hasn't been reset (because you've had your eyes closed). Adaptation worked as a kind of gain control, adjusting the sensitivity of your motion perception to the new expected level of input provided by the constant motion of the waterfall.

Aftereffects are common illusions; they don't occur just for motion. The relative activation and habituation of neurons are general features of the brain. The reason aftereffects are built into neural processing is to adjust our sensations to cancel out continuousand therefore uninformativeinformation. It operates to make us sensitive to changes around the adapted-to baseline, rather than being overwhelmed by one dominant level of input. Think about how your eyes adjust to the dark for a good example of useful adaptation that can result in an unpleasant aftereffect. Adaptation is discussed further in [Hack #26] .

2.14.4. See Also

• A motion aftereffect with scrolling text (http://www.naturalhighs.net/waterfalls/illusion.htm).

• A good demo and good explanation of the effect (http://psylux.psych.tu-dresden.de/i1/kaw/diverses%20Material/www.illusionworks.com/html/motion_aftereffect.html).

• Mather, G., Verstraten, F., & Anstis, S. (1998). The Motion Aftereffect: a Modern Perspective. Cambridge, MA: MIT Press.

• Grunewald, A., & Mingolla, E. (1998). Motion after-effect due to binocular sum of adaptation to linear motion. Vision Research. 38(19), 2963-2971.

• Rees, G., Frith, C. D., & Lavie, N. (1997). Modulating irrelevant motion perception by varying attentional load in an unrelated task. Science, 278(5343), 1616-1619.