The Brain Mechanisms Underlying The Processing Of Visual Size Illusions

Accurately perceiving the size of visual objects is crucial in our daily life.Visual size perception depends not only on the features of objects,but also relies on their surrounding context.For example,a visual object would appear larger when surrounded by small items compared to large items,which is known as the Ebbinghaus illusion.Similarly,as illustrated in the Ponzo illusion,a visual object would be perceived larger when placed at an apparently far location compared to at an apparently near location.Although several brain regions including the occipital,parietal and temporal cortex have been found to be involved in the processing of visual size illusions,it is largely unknown how these brain regions interact with each other to form the functional connection network to shape the experienced visual size illusions.In study 1,we used resting state fMRI technique together with the method of dynamic causal modeling（DCM）to investigate the neural networks associated with the Ebbinghaus illusion and the Ponzo illusion.In study 2,repetitive transcranial magnetic stimulation（r TMS）was used to investigate the causal contribution of a key node（i.e.,the right superior parietal lobule,SPL）to the processing of visual size illusions.DCM results showed that when the neural network consisted of right primary visual cortex（V1）,lateral occipital cortex（LOC）,and SPL,the bilateral connection between V1 and LOC,as well as the feedforward connection from V1 to SPL were significant.Notably,the inhibitory connection strength from V1 to SPL was negatively correlated with the magnitude of the Ebbinghaus illusion.In the neural network composed of right V1,V3 v and SPL,we found that the feedback connection from SPL to both V3 v and V1,as well as that from V3 v to V1 were significant.Moreover,the self-connection strength of right SPL was negatively correlated with the magnitude of the Ponzo illusion.The r TMS results revealed that the temporary disruption of right SPL increased the Ebbinghaus illusion effect instead of the Ponzo illusion effect.To sum up,we can draw the following conclusions:1.The results showed that cortical excitability in the absence of explicit tasks could predict the perceived size illusion effect,in favor of the heritability of visual size illusions.2.The findings also provide direct evidence in supporting of the distinct neural mechanisms underlying the processing of the Ebbinghaus and the Ponzo illusions.