Watch something in motion, say, a waterfall or scrolling text on a video game, then look away at a rock, a wall, or anything stationary. Briefly, the stationary object will appear to move in the opposite direction.

This visual illusion has been recognized for a very long time; Aristotle first noted it. Now, a new study has found that even a very brief glimpse of motion — for as little as 1/40 of a second — can trigger the brain mechanism responsible for the illusion. 

"It all comes back to a process called adaptation, the idea you change the sensitivity of your senses based on the environment you are in, and you do this constantly," said Davis Glasser, the lead researcher and a graduate student at the University of Rochester.

For example, when you put your clothes on in the morning, you only feel them against your skin for a short time, or when you walk into a room with a noisy air conditioner, you only hear it for a short time, Glasser explained. It is theorized that adaptation allows us to ignore a constant stimulus so we can detect other things, he said.

The visual illusion, called a Motion Aftereffect, is visual evidence that our brains have adapted to the motion we see. By looking at a stationary object, we can "read out" this adaptation, which appears as the illusory motion, according to Glasser. [See the visual illusion]

Using tests in which participants responded to videos, Glasser and his colleagues found that after only a very brief exposure to an image in motion, the brain responds to stationary objects as if they are actually moving. They found a corresponding pattern of activity in tests of individual brain cells from a visual brain region important for perceiving motion. 

The motion needed to elicit this response in the human participants was so brief that the human subjects could not consciously tell the direction in which it was going. However, it still affected their perception of a stationary image.

The brevity of exposure to motion needed to stimulate these responses indicates this process is an automatic adaptation and can happen anytime you see something moving, according to Glasser. [Eye Tricks: Gallery of Visual Illusions]

Glasser said he is now exploring the long-standing theory that adaptation to a particular stimulus, such as motion in our visual field, improves our sensitivity to other stimuli.

This article was reprinted with permission from LiveScience.

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