Important: Do NOT watch this if you’re prone to seizures.
What happens is that the nerve cells that signal motion in a single direction begin to get tired after several seconds. According to the IllusionWorks website:
Your brain represents a sensory quality, such as motion, brightness, color, or depth, not in terms of the firing of one group of neurons, but in terms of the electrical activity of one group of neurons relative to the electrical activity of another group of neurons. For example, the activity of neurons coding for clockwise motion relative to the activity of those coding for counterclockwise motion.
Clockwise motion is signaled by the fact that the neurons coding for clockwise movement fire more strongly than neurons coding for counterclockwise motion. If the clockwise neurons now fire less strongly because their electrochemical batteries run down after several seconds or more of continuous activity, the balance between these two groups is disrupted.
It's also known as the Loch Ness effect.
Over 150 years ago a scientist named Robert Addams looked at a waterfall on the side of the loch, which lies in the Scottish Highlands. He noticed that if he stared at it for long enough, it appeared as if the ground next to it was moving upwards.
Cinaed_KSM/Cinaed_KSM