Identifying cortical areas that underlie the transformation from retinal to world motion signals

Accurate motion perception requires that the visual system integrate the retinal motion signals received by the two eyes into a single representation of 3D (i.e., world) motion. However, most experimental paradigms limit the motion stimuli to the fronto-parallel plane (i.e., 2D motion) and are thus unable to dissociate retinal and world motion signals. Here, we used stereoscopic displays to present separate motion signals to the two eyes and examined their representation in visual cortex using fMRI. Specifically, we presented random-dot motion stimuli that produced percepts of various 3D motion trajectories. We also presented control stimuli that contained the same retinal motion energy in the two eyes but were inconsistent with any 3D motion trajectory. We decoded the stimuli from BOLD activity using a probabilistic decoding algorithm. We found that 3D motion direction can be reliably decoded in three major clusters in the human visual system. In early visual cortex (V1-V3), we found no significant difference in decoding performance between the 3D motion and control stimuli, suggesting that these areas represent retinal motion rather than world motion signals. In voxels in and surrounding hMT and IPS0 however, decoding performance was consistently superior for 3D motion compared to control stimuli. Our results reveal the parts of the visual processing hierarchy that are critical for the transformation of retinal into world motion signals and suggest a role for IPS0 in the representation of 3D motion signals, in addition to its sensitivity to 3D object structure and static depth.