If one stares consistently at movement in a particular direction for
even a short time, subsequently viewed stationary scenes briefly appear
to move in the opposite direction. This phenomenon was known to the Ancient
Greeks, but the first modern report of it is often attributed to Robert Addams (1834), who observed the effect
while viewing a waterfall at Foyers in Scotland. This demonstration was
created from a view of the Falls of Foyers. After gazing at the movie for
about 30 seconds (keep your eyes still by fixating on the tree branch in
the middle of the falls), pause the movie (click on the pause button in
the lower left corner) while maintaining fixation. Do you see an after-effect
of movement?
The 'motion after-effect' can be explained by adaptation in visual neurones
that respond selectively to moving contours in the image (see Barlow
and Hill, 1963). In the absence of image motion, cells tuned to different
directions produce roughly equal responses. Exposure to a particular direction
of motion alters this balance in favour of cells tuned to the opposite direction,
leading to the illusion. A review of modern research on the phenomenon can
be found in Mather et al. (1998).