When you've been driving down a long, straight road, and you suddenly come to a stop, does the road seem to rush away from you? When you look at a waterfall, then glance away, does it seem as though the scenery is flowing upwards? It's called motion aftereffect, and it's a kind of visual adaptation.

Motion Aftereffect and the Brain

There are neurons in the brain that sense motion. They are not active or inactive, they are competitive. When we look at something that isn't moving, the motion neurons are all still firing, but the neurons sensing different directional motions are firing at the same rate, canceling each other out. When we look at steady downward motion, like a waterfall, the downward motion neurons go into overdrive, shouting out the rest of the neurons. After some time, the shouts don't need to be as loud as they were at first. The brain adapts, and it takes fewer neurons to make us aware of the same effect.

When we turn our heads away from the motion, those few firing "downward motion" neurons shut down. The other neurons are still firing at the same rate they always were. Instead of all the directional motion neurons firing equally, the downward motion neurons are lazy, and the upward motion neurons are as active as they've ever been, leading us to see upward motion even if there is no actual motion in front of us.

Seeing Motion That Isn't Happening

There's a fun twist to this. People can see motion aftereffect for motions that they've never seen in the first place. All they need to do is picture the motion in their minds. Tell a person to close their eyes and picture a long train moving from left to right and they can, upon ceasing the visualization and opening their eyes, see the scenery in from of them moving right to left. If they were to picture dust getting sucked upwards into a vacuum, they'd open their eyes to see downward motion.


But what if they didn't consciously picture anything?

That was the question settled by an interesting experiment. The experiment had people read a passage of prose that described either upward or downward motion. The downward motion, for example, might be about a person throwing buckets of ping pong balls off a roof. It would include a sentences like, "The first wave of walls fell down, bouncing briefly on the fire escape, but still falling. Showers of ping pong balls zoomed down past people's windows, down towards the parking lot. They reached the mid-point of the building, still careening down." Anyone reading the passage would get the message; down, down, down.


The subjects of the experiment moved, immediately after reading their passages, to stare at a screen. The screen showed dots, which moved in a way pre-programmed by the scientists. Sometimes the dots' movement could be completely random, showing no overall motion. If their movement was altered the dots began, more and more, moving in a certain direction. At a certain threshold, people would be able to perceive an aggregate motion of the dots.

If the motion aftereffect could be brought on by prose, people looking at the dots after reading about downward motion would see upward motion in the dots. This upward motion might not happen when the dots were moving randomly, but it would occur, fore people experiencing motion aftereffect, at a much lower threshold than that of people who weren't experiencing the effect.

The results were mixed. Some people did see motion aftereffect in the dots. The aftereffect people were all of a certain type — the type that relies on visualization. These people weren't non-readers, or uninterested in words. They simply engaged with the written word much more intensely if it contained visual descriptions. They weren't as interested in dialog or wordplay. People who tended to skim visual description, and enjoyed reading about two characters talking, were more word-oriented and less likely to see the motion aftereffect in the dots.


They were, actually, not likely to see motion aftereffect at all. This seems to be an illusion that relies, to a certain extent, on our ability to visualize. I have never seen the motion aftereffect in the car, or while walking, and I certainly can't see it when I visualize motion in my head. Perhaps if there were a waterfall in the area I would go take a look. It probably would work if I concentrated deeply on continuous motion, but it's not something that I ever see accidentally. What about you, readers? Do you tend to see this visual illusion often? Can you try picturing motion in your mind to see if the illusion kicks in after? And, if so, do you like descriptive prose or are you more verbal?

Waterfall Image: Louise Docker. Ping Pong Balls: Michael Knowles.

[Via Louder Than Words]