Position shifts of fMRI-based population receptive fields in human visual cortex induced by Ponzo illusion.

Ponzo illusion is a well-known perceptual phenomenon in which the perceived sizes of visual objects are altered by visual depth cues created by converging lines at the horizon. One possible neural mechanism of the Ponzo illusion is the receptive field position shifts of V1 neurons, as supported by a recent monkey electrophysiological study (Ni et al. in Curr Biol 24(14):1653-1658, 2014). Here, we used fMRI-based population receptive field (pRF) mapping technique in combination of psychophysics to investigate this idea. We found that, relative to the close apparent depth in a 3D scene, the far apparent depth in the scene caused the pRF positions of voxels in V1-V3 to shift toward the fovea, in line with subjects' percept of the Ponzo illusion. Moreover, the pRF position shift in V1 significantly correlated with the magnitude of the Ponzo illusion across individual subjects. Our findings thus provide evidence for the close association between the perceived object size and the pRF position shift in human visual areas, especially in V1, lending further support for the receptive field position shift explanation for the Ponzo illusion.