The Secret Life of the Brain
History of the Brain 3-D Brain Anatomy Mind Illusions Scanning the Brain The Episodes:

Mind Illusions
Movement Color Angles Form
Movement Study

A rotating spiral appears to be expanding or contracting depending on the direction of rotation. If you stare at the spiral and then avert your eyes, a stationary object will appear to be moving in the opposite rotation. This happens because in the visual nervous system there are cells that respond to specific directions of motion. So, when a person looks at an object moving downward such as a waterfall, that person's downward receptors are in action. Let's say he stares at the downward motion long enough for those cells to become fatigued and then looks at a stationary object, like the grassy hill. The grass will appear to be moving upwards. The upward receptors compensate for the fatigue of the downward receptors. This phenomenon is known as an afteraffect.
Receptor fatigue and the illusion of motion

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