Waterfall Illusions: How does it work and what does it say about your brain?

03. 03. 2020
6th international conference of exopolitics, history and spirituality

People have been fascinated by the game of illusion since ancient times. We are fascinated by the mismatch between the real retina image and what we perceive. Before films, books and pictures on the Internet caught us, many illusions in nature were seen. Yes, nature has been fascinated by the game of illusion since ancient times, and it has not given sleep to such greats as Aristotle or Lucretius. They focused on the illusion of observing running water.

Water illusion

For some time Aristotle watched the pebbles under running water and noticed that the pebbles seemed to be visually moving. Lucretius looked at his horse's non-moving leg in the middle of a fast-flowing river, and seemed to move in the opposite direction as the river flowed. This is called induced motion. This type of apparent movement occurs most often when viewing a small, stationary object in relation to the background, which is formed by relatively large moving objects. Under these circumstances, the impression appears that a small object is moving against the direction of actual movement of large objects. You can see this beautifully when you look at the night sky, where there are clouds and moon, visually it seems that the moon is moving in the opposite direction to the clouds.

Robert Addams, a traveler and philosopher, first described this great illusion. In 1834 he watched the Falls of Foyers in Scotland. After a moment of observation, he found that the rocks seemed to move upwards. At one point he gazed steadily at one particular part of the façade, where curtains of running water were formed, then pointed his eyes to the left at the rocky outcropping, which optically began to move up at the same speed as the water had previously fallen. This phenomenon was later known as a waterfall illusion. It is a fact that if we look at something that moves in one direction for a while, when you change the view, another thing will move in exactly the opposite direction at the same speed.

Moving images

Later attempts of this phenomenon were performed on rotating spirals or disks that can be stopped after movement. When stopped, these shapes move optically in the opposite direction. Check out the video below. Watch the video right in the center and see your surroundings at the end of the video…

So Addams provided the basis for explaining this illusion. He claimed, however, that the optical movement of the rocks was the result of the subconscious movement of the eyes when observing falling water. That even if one thinks that he is looking at one place, in fact the eyes move involuntarily in the direction of the falling water and back. But this theory was wrong. The movement of the eyes cannot explain this phenomenon, as it would result in the whole scenery, not just a part of it, moving optically. This was pointed out in 1875 by physicist Ernst Mach.

Brain and movement illusions

So what happens in the brain when you observe this illusion? There are neurons behind everything. Many cells in our cortex are activated by movement in one particular direction. When we look at something that is stationary, the "up" and "down" detectors have almost the same activity. But if we watch the falling water, the "down" detectors will be more active and we say we see the movement down. But after a while, this activation tires the detectors and they don't respond as much as before. When we change the view of something stationary, for example, the activity of the detectors "up" is relatively high compared to the activity of the detectors "down" - therefore we perceive the movement upwards. The whole process is more complicated, but let's take this as a simplified explanation.

People have always been fascinated by illusions, but only in the last century did scientists make it clear how the brain works in such illusions. And with the continued development of neuroscience, we will certainly have many other discoveries about the functioning of perceptions, the subconscious, and other brain activities.

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