Aristotle, Waterfalls, VanGogh...oh my!

I wanted to write a post about the psychology of Perception and look what ended up in my inbox!  As long as you do not have a medical reason to skip this demonstration - follow the directions in the video by staring at the center of the swirling circle for 30 seconds until it changes to an image of Vincent VanGogh's "Starry Night" painting: 

If the optical illusion worked correctly, the image of "Starry Night" should have appeared as it, too, was whirling and swirling even though it was not moving at all (for you doubters who think the painting was animated on Youtube, check out this version)!

This illusion happens thanks to sensory adaptation - what happens any time you get used to seeing something, hearing something, smelling something, etc.  The type of adaptation you experienced is called Rapid Motion Adaptation or RMA: your brain got used to perceiving motion and you could see evidence of that when the scene switched to something that was still.  The illusion is sometimes described as the Motion After Effect.

This after-effect was mentioned as early as 350 B.C. by the philosopher, Aristotle.  In "Parva Naturalia" he wrote, "...when persons turn away from looking at objects in motion, e.g. rivers, and especially those which flow very rapidly, they find that the visual stimulations still present themselves, for the things really at rest are then seen moving."

An interesting twist of the RMA is that the after-effect motion seems to move in the opposite direction than the actual movement that is first seen.  Record of this "Waterfall Effect" is found in Robert Addams' 1834 description of how the rocks next to a waterfall began to appear as if they were moving upwards after he had stared at the water plunging downwards.

In 2011, Glasser, Tsui, Pack, and Tadin did a series of studies on the RMA and the after-effect illusion.  Although their sample sizes were small, their findings suggest that the RMA can be triggered even faster than had previously been suspected:  it can be provoked after less than one second of watching movement. So even if the person was not aware of the direction of movement that they briefly saw, they were usually correct in reporting an after-effect motion in the opposite direction.

Using two macaques (monkeys), the researchers were also able to pinpoint neurons in the Medial (Middle) Temporal region of the brain that were activated when the RMA was triggered.  This region is related to the processing of perceptual information.  Even after a very brief view of an image in motion, the neuronal activity indicated that the monkeys' brains were responding to the follow up still images as if they were also moving.  The suggestion is that similar activity occurs in our brains, too.

So even though we might call "The Starry Night" demonstration an "Optical Illusion," it is not so much our eyes playing tricks on us - instead it is evidence that our brains are adapting to the world around us.

Further Reading:

A really wonderful website on the history of the Motion After Effect.  It includes larger sections from Aristotle and Addams, plus other early works that describe this effect.

Here is the Glasser et al. (2011) article if you would like to read in detail about their studies.  I especially enjoyed the part about plaid stimuli - had never thought of plaid in relation to perception!

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