Ecem Altan: High-level processing at play in Ponzo-like size illusions

Illusory size perception is usually an outcome of complex processing that incorporates the object and its spatial context. The primary visual cortex (V1) has been found to reflect illusory perception; however, the stage of processing leading to this outcome is not fully understood. To test the involvement of higher-level processing in a Ponzo-like size illusion, we measured the illusion magnitude in upright and inverted conditions, and with altered versions containing various amounts of depth cues. The results showed weaker illusion for inverted conditions compared to the upright ones, and weaker effects as the depth cues decreased. Moreover, using functional MRI, we found that the extent of neuronal activation in V1 was smaller in the inverted version in line with perceived size. Our findings suggest that the perceived size bias is linked to visual experience and expectations, indicating the involvement of a feedback mechanism from high-level stages of visual processing.