What is Motion Aftereffect?
Also known as the waterfall illusion, the motion aftereffect or MAE occurs because of motion adaptation. This means that your brain basically captures the effect of the moving body and then projects that image on a blank, unmoving object. Your brain is made up of many neurons. What happens is that they adopt the activity of an object in order for you to understand what is happening. When you continue to focus on the same activity for some time, say 60 second, the neurons adopt this activity. So when you suddenly move your gaze from the moving effect to a still effect, the neurons in action are the ones which were in motion. Hence, the object you are focusing on now also seems to be in motion. This effect lasts for a few seconds before the neurons for stationary object come alive.
In very simple words, when you look at a waterfall for some time and then switch your gaze to the rocks right next to it, it will feel like the rocks are moving too. But the most interesting thing about the whole scenario is that, unlike the waterfall which was moving in a downward direction, the rocks will seem to be moving upwards. This is what happens in the motion aftereffect, where you see motion in a stationary object, but it is reversed.
A Brief History
Research suggests that the motion aftereffect has been known since the time of the Ancient Greeks. Aristotle in 350 B.C. explained the effect once he continued to look at a moving object. But he did not distinguish the direction of the motion. But it was in 1820 that the first clear basis of motion aftereffect was laid by Jan Evangelista Purkyne, a physiologist and anatomist from Czech Republic. He gave the theory after checking out the movement of the cavalry parade. In 1834, Robert Adams came up with the term ‘waterfall illusion’ when he went to visit the Fall of Foyers in Scotland that is located near the Loch Ness. He was the one who explained how after looking at the moving water of the waterfall, it felt like the stones next to it were also moving.
Other scientitsts who tried to explain and understand the concept of motion aftereffect include Thompson (1880), and Gustav Adolf Wohlgemuth (1911).
Now the next time you look at such an illusion, you will be able to understand the science behind it too!