How do humans perceive illusory motion in some stationary images?
Neuroscientists from Yale University have finally found the answer to why people perceive illusory motion in some stationary images by studying the eyes of fruit flies (Drosophila). They found that fruit flies too perceive motion in stationary images that evoke illusory motion percept in humans (PNAS, August 2020). They found that illusory motion percept is generated by unbalanced contributions of direction-selective neurons’ responses to stationary edges. In the case of flies, perceiving motion in static images is an artefact of the brain’s strategies for efficiently processing motion in natural scenes. Perceptual tests in humans suggest that our brains may employ similar mechanisms for this illusion. The scientists presented flies with optical illusions and then measured the flies’ behaviour to check if they perceived motion in the optical illusions much the same way as humans do. They found that flies turned their bodies instinctively towards any perceived motion. The direction the flies turned, the scientists found, was the same as that of the motion that humans perceive in the pattern. The perception was abolished when the elementary motion detector neurons T4 and T5 were silenced. They could also manipulate both the magnitude and direction of the fly’s illusory percept by selectively making either T4 or T5 neurons non-functional (silencing). In human volunteers, the scientists were able to manipulate the magnitude and direction of their perception by moving light edges or dark edges.
