Abstract
We live in a 3-dimensional world and motion perception plays a critical role in organisms’ survival. Past neuroimaging studies have shown that motion direction can be reliably decoded from BOLD activity within the motion processing pathway (V1 to hMT). However, the majority of experimental paradigms limit the motion stimuli to the fronto-parallel plane (i.e., 2D motion). Here, we examine if additional areas in visual cortex are involved in decoding 3D motion. To this aim, we presented random dots drifting in eight different motion directions (left/right, toward/away, and four intermediate directions). The stimuli produced distinct retinal motion velocities in the two eyes. For example, motion directly toward or away from the observer produces horizontally opposite retinal motion in the two eyes. In a control experiment, we instead presented vertical retinal motion, which contains virtually identical motion energy but produces transparent 2D, rather than 3D motion percepts. We decoded the presented stimuli from BOLD activity using a probabilistic decoding algorithm (TAFKAP; van Bergen & Jehee, 2021). We found that 3D motion direction can be decoded throughout the canonical motion hierarchy. In V1, horizontal (3D) and vertical (transparent) motion directions are decoded equally well. In hMT, however, 3D motion decoding performance is consistently superior to decoding of transparent motion. Critically, we found that 3D motion direction (but not the vertical control) could be decoded at an equal or greater accuracy when equating for number of voxels in IPS0 compared to in V1 and hMT. Decoding performance was much poorer in areas IPS2-5 and other regions in the ventral pathway. Our results suggest a role for IPS0 in 3D motion processing in addition to its sensitivity to 3D object structure and static depth. Our paradigm can be applied in the future to investigate the transformation from sensory input to perception in the visual pathway.