An fMRI study of the reverse perspective illusion

"Reverse perspective" is a powerful visual illusion similar to the hollow mask illusion, but more interesting in producing the perception of an illusory motion in a stationary picture. It is caused by conflict between motion parallax and pictorial depth cues in 3D "relief" paintings built with depth inversion. Here we report the measurement of brain activation using fMRI in response to a reverse perspective (RP) object, as well as a normal perspective, 3D-relief object ("shadow-box", SB) and a 2D painting of the same architectural scene. The stimuli were presented to 10 subjects in static and rotating conditions, subtraction of which revealed strong activation of area MT in all three cases. Contrasts between the RP, SB and 2D conditions showed the strongest activation for RP and almost no difference between SB and 2D. The similarity of brain activation between SB and 2D stimuli was interpreted as indicating that observers perceive the illusion of realistic 3D depth in 2D pictures as entirely normal and not qualitatively different from the 3D structure of the shadow-box stimulus. Contrasts between the RP stimulus and either the SB or the 2D stimulus revealed activation of Brodmann Areas 7, 19 and MT (and cerebellar cortex), suggesting the usage of brain regions involved in mental rotation and depth perception in response to the reverse perspective illusion.