When you spin your head around in circles, the world appears to continue to spin.
- Why do one's surroundings still seem to rotate when you stop whirling around in a circle?
- What is happening in the brain?
On each side of your head, in the middle ear, there is a system of canals in which displacement of a group of hair cells measure the movement along the 3 axes (the canals are tilted a bit back from 90°, so the coordinate system is not identical with that of the head).
So, when you move your head around and around as your body "whirls", the cupula of the horizontal canal (HC), which encases the hair cells, moves in the opposite direction and displaces the hair cells, causing the firing of action potentials. Once your head stops, the viscosity of the fluid in which the cupula is suspended prevents it from snapping right back to it's "default" position, so action potentials continue to be fired, and your nervous system (falsely) senses that your head is still moving.
Courtesy of Scholarpedia Vestibular System
You can find similar aftereffects when you stare out the front window of a moving vehicle for a couple of minutes and then stop: the landscape suddenly seems to slowly move away from you for some time; or when you stare out the widow of a train: when it stops the landscape appears to slowly move in the opposite direction. Aftereffects of this type can have any direction: when you stare at a rotating image and the rotation stops, the image appears to rotate in the opposite direction. Depending on the stimulus, this can be a smooth motion or it "jumps" forward and back.
This is called the direction aftereffect (DAE), "whereby prolonged exposure to a moving pattern affects the perceived direction of subsequent motion" (Clifford, 2002). This is an effect of perceptual adaptation, where the brain compensates for a supposed error in perception.
The mechanism behind this phenomenon was tested by Schrater and Simoncelli (1998):