It’s a misty morning in 380 B.C. Greece, and Aristotle is taking his daily stroll along a flowing brook. Eventually he stops and observes the water, letting his eyes soften their focus on its motion. When he finally rips his gaze away, he notices that the stationary rocks around the brook seem to be moving in his gaze, as if they’re floating upstream! He quickly jots this observation down: “the senses are affected in this way when they turn quickly from objects in motion, e.g. from looking at a river and especially from looking at swiftly flowing streams. For objects at rest then seem to be in motion.”
What Aristotle had experienced so many years ago, was something we now call the motion-aftereffect (MAE). The MAE, sometimes called the waterfall effect because of its origin of discovery, is a visual illusion that occurs after our eyes focus on a moving visual stimulus for about 10 milliseconds to a minute, and when we look away, we observe stationary objects moving in the opposite direction of the moving stimulus.
You can try it yourself here!: The Motion-Aftereffect Illusion
Pretty trippy right? But what exactly causes this phenomenon? In 1880, German scientist Hermann Helmholtz discovered the biological mechanism behind this magic: neural adaptation. Neural adaptation is what happens when our brain “gets used” to a certain stimulus. Have you ever noticed that when you first put your hand on a table, you feel the surface and pressure of it on your skin, but after leaving your hand there for a few moments, you cease to feel the surface of the table? You may be asking how this has anything to do with optical illusions, but surprisingly this experience, along with motion-aftereffect, are two examples of neural adaptation.
When we are exposed to a stimulus for a prolonged period of time, our brain becomes less excited about it. In other words, the neurons responsible for signaling to our brain about a stimulus that is happening over and over have a tendency to become less responsive. For the motion-aftereffect, this occurs in the visual area of the brain.
Imagine that our neurons are in a constant tug-of-war. When our visual system detects motion in a certain direction, for example downward, the neurons that are responsible for viewing downward motion begin to “win” the tug of war and signal to our brain that we’re seeing downward motion. But after looking at the motion for a time, the downward viewing neurons “winning” the tug-of-war begin to get fatigued, while the brain begins to acknowledge that this motion is unchanging. When we finally rip our gaze from the movement, the neurons on the other side of our tug-of-war, which are responsible for viewing upward motion, pull the rope, and our poor fatigued neurons cannot even-out the signal, causing an overcompensation in upward movement. The result is our visual area of the brain signaling to us that stationary objects are moving up when in fact they are not moving at all!
The body’s ability to adapt new environments is astounding and allows us to pay attention to the changes in our world; the motion after-effect is just one of the wacky side-effects of this important evolutionary ability. So, next time you’re hiking along a creek or waterfall, dip your feet into ancient Greece and experience this age-old phenomena that befuddled even the most intelligent philosophers. But unlike, Aristotle, you’ll know that this is no product of sorcery, only our neurons playing tug-of-war!