An apparently effortless task, the opening of our eyes, engages an extensive and anatomically complex network of brain areas. Visual illusions have fascinated mankind for thousands of years, and the study of the mismatch between perception and reality helps us to better understand the creative nature of the human visual system and ultimately may lead to deeper insights of how our brain works. The Pinna illusion is a striking example of rotary motion perception in the absence of physical motion. When approaching or receding concentric rings composed of static rhombi slanted in opposite directions, observers experience vivid illusory counter rotation (Figure 1). Although this phenomenon is well known, where and how does this happen in the human brain?

Figure 1.The striking Pinna illusory rotation. Observers perceive vivid illusory counter rotation between the red and the blue rings when fixating on the central black dot while moving their head forward and backward. This figure of a Pinna variant is for illustration only and was produced by graduate student JunxiangLuoin Wei Wang’s Lab.

The laboratories of Drs GU Yong, WANG Zheng, and WANG Wei at the Institute of Neuroscience, Shanghai Institutes for Biological Sciences, CAS, have collaborated closely to solve the above ‘where’ question, i.e. which visual brain mediates the Pinna illusion, a prerequisite for gaining further insights intothe ‘how’ question, i.e. how neurons in the visual brain integrate local visual cues to form a global representation of a compelling movement illusion. The researchers employed human psychophysics and fMRI methods (Siemens Tim Trio 3.0 T scanner), to examine the representation of Pinna illusion in early and intermediate visual areas. Among these visual areas, they found that in the dorsal visual stream, the subarea MST of hMT+ was predominantly activated by the Pinna figure compared to subarea MT (Figure 2). By contrast, no significant activations in earlier visual stages and the ventral stream including V1-V4v were related to the rotation per se. Thus, their results for the first time provide direct evidence demonstrating that illusory rotation is represented as if it were real rotary motion in human MST. These findings imply that the Pinna illusion is represented by neurons capable of encoding physical rotation in MST, and thus might help us to gain insights into how our brain perceives the visual illusion and the reality.

Figure 2.fMRI results from left hemisphere of one subject. A-C, Activation maps for three stimulus conditions (given on top). D-F, The corresponding difference maps after subtraction of the two stimulus conditions specified underneath. Both the physical and Pinna illusory rotations contained expansion components. Color bar indicates –log10 (P) value of paired t-test (P < 0.05). Areas MST and MT are outlined in white and black, respectively. The blow-up images are for better visualization.

This work was published online in Human Brain Mapping on March 4, 2016 , and it is the first human fMRI work published from the Institutional Functional Brain Imaging Platform (FBIP)headed by Dr. WANG Zheng. This work was supported by National NSFC Grant 31571078 (WW), the Recruitment Program of Global Youth Experts (GY),  and the Chinese Academy of Sciences.