Visual Motion Aftereffect in an Excitable Brain Tissue: Explaining the Waterfall Illusion

by Ketan Bajaj

When ones looks at a moving stimulus for a long duration and immediately after that looks at a stationary stimulus, the stationary stimulus appears to be moving in the opposite direction. This perceptual aftereffect is known as the motion aftereffect or the waterfall illusion. This perceptual aftereffect has been discussed widely and there are numerous research publications available discussing the phenomenon and underlying mechanisms. Here I present an explanation for motion aftereffect that might be after all the true explanation. Brain tissue is an excitable tissue with excitation spreading via neural synaptic connectivity, diffusion of ions in the free extracellular space and glial mechanisms of potassium buffering and calcium responses. A prolonged spiking activity of neurons in a tissue can trigger spread of excitability. Within a tissue volume there exist neural cells that are selective to opposite stimuli, like orthogonal orientations and opposite directions of motion. Direction selective neurons would adapt to their preferred stimulus and exhibit a decrease in responsiveness. While doing so these would trigger a spread of tissue excitability towards neurons selective to opposite direction of motion. The opposite direction selective neurons would exhibit an increase in responsiveness due to an increase in surrounding tissue excitability. It is suggested here that this leads to motion aftereffect, that is, following adaptation when a stationary stimulus is viewed it gives rise to a perception of motion in opposite direction.

58 pages, Paperback

First published March 26, 2013