Motion Aftereffect Explained!

In an earlier post (Turtles All The Way Down: Deep Fractal Zoom), I noted that when I stopped the animated zoom on a Mandelbrot Set fractal, the stationary image appeared to shrink, and I asked if anyone knew why. Apparently, no one did, but now they do.

The motion aftereffect illusion, was known, apparently, to the ancient Greeks, but only now has anyone claimed to know its cause. In this week's Proceedings of the US National Academy of Sciences, Davis Glasser, Duje Tadin, James Tsui and Christopher Pack of the Montreal Neurological Institute, report that:

...25 ms of motion adaptation is sufficient to generate a motion aftereffect, an illusory sensation of movement experienced when a moving stimulus is replaced by a stationary pattern. This rapid adaptation occurs regardless of whether the adapting motion is perceived. In neurophysiological recordings from the middle temporal area of primate visual cortex, we find that brief motion adaptation evokes direction-selective responses to subsequently presented stationary stimuli. A simple model shows that these neural responses can explain the consequences of rapid perceptual adaptation. Overall, we show that the motion aftereffect is not merely an intriguing perceptual illusion, but rather a reflection of rapid neural and perceptual processes that can occur essentially every time we experience motion.

Although any motion, even motion so brief as to be imperceptible, can create an aftereffect, the magnitude of the aftereffect varies greatly with the stimulus. In the following video, the magnitude and persistence of the motion aftereffect seems to vary according to where the sequence is halted. One sees striking aftereffects on halting the motion between about 4 min 00 sec and 4 min 30 sec, and at many other places too.