by George Wilkinson
The Motion Aftereffect illusion, or Waterfall illusion, occurs when prolonged viewing of motion in one direction makes a subsequently viewed stationary item appear to move in the opposite direction. An example is seen in this short video. The viewer fixates on a stationary blue point near a pattern moving toward the top of the screen. When the pattern is replaced with a fixed photograph of a waterfall, the water appears to flow toward the bottom. We see motion because our visual system adapted to the upward motion of the first stimulus, and that adaptation carries over for a few seconds affecting our perception of the stationary image.
This illusion is thought to reflect adaptation of neurons in the visual cortex responsible for detecting motion. In this model, overall motion perception results from a comparison of the relative firing patterns among neurons sensitive to diverse specific directions of motion. A stationary scene results in balanced outputs for all directions. In the classical Motion Aftereffect illusion, prolonged stimulation with one direction of motion leads to decreased output by the neurons sensitive to that direction of motion. When the stimulus is removed, output from that set of neurons remains lowered. But because the output from the other motion-sensitive neurons is higher by comparison, the resultant imbalance creates the perception of motion opposite to the original moving stimulus.
Althoug classical studies of the Motion Aftereffect illusion used a conditioning stimulus lasting several seconds, it is known that neuronal adaptation can occur much more quickly– on a timescale of milliseconds. Could perceptual adaptation be observed at brief timescales, relevant to the timescales in which neurons adapt? A recent study from the lab of Duje Tadin finds that the Motion aftereffect illusion can be observed even after very brief exposure to a moving stimulus. Subjects shown a video of a pattern that is moving for only 1/40 of a second (25 milliseconds) – so short that they can’t consciously distinguish its direction of motion – will then perceive motion in the opposite direction of the briefly presented background motion when viewing a stationary object. The implication of their results is that the cortical processes involved in the Motion Aftereffect illusion could affect our perception virtually every time we view motion. The work of this lab and others suggest that there may be more than one brain region, and more than one neuronal circuit, contributing to this class of illusion.