Visual illusions have fascinated mankind for thousands of years and the study of the mismatch between perception and reality helps people to better understand the creative nature of the human visual system and ultimately may lead to deeper insights of how the brain works.

The Pinna illusion is a striking example of complex-flow motion perception in the absence of physical motion. When people move towards or away from it, they experience vivid illusory counter rotation. The strength and nature of this illusion critically and reliably depend on local micro-patterns such as the orientation and edge polarity of the small rhombi.

Previous studies conducted by the Institute of Neuroscience of Chinese Academy of Sciences discovered what part of the brain illusory activity was most closely linked to. However, the neural basis of the transformation from objective reality to illusory percepts remained unknown. This is a central question in vision tackling how sensory information is integrated from local visual cues to globally perceived representations across different brain regions.

In a recent study published in The Journal of Neuroscience, researchers at the Institute of Neuroscience continued to study this issue from a novel perspective by studying the transformation from physical to four illusory flow motions induced by a carefully parametrized Pinna figure at both the perceptual and neuronal level.

They first showed psychophysically that non-human primates (macaque monkeys) have psychometric functions similar to humans, suggesting that monkeys perceive illusions like human observers.

Subsequent neurophysiological recordings in two brain areas (MT and MSTd) critical for serial motion processing revealed that MSTd neurons represent both physical and illusory complex-flow motion equivalently through global integration of MT local-motion signals. However these neurons in MSTd required an extra time of about 15 ms for reliably representing the illusory motion.

By taking advantage of the unique characteristics of this stimulus, this study provided the first detailed analysis of the neuronal mechanisms underlying complex forms of illusory motion integration from MT to MSTd.

Specifically, the same motion-specific neural groups in MSTd that normally encode physical flow motions also mediate illusory flow motions through a bottom-up cascade of integration of non-veridical local motion signals, yet reliable discrimination is temporally delayed for the illusion.

This study provided contemporary neural-level support for the idea Jan Purkinje stated so concisely 150 years ago, that “illusions contain visual truth”. It was selected for promotion to the media with the title of ‘‘Brain Represents Optical Illusion as Delayed Reality’’.