Abstract
Optic flow patterns induced by forward/backward self-motion in a rigid scene contain 3D structure information as well as 2D features such as a radial velocity field. Previous studies often confuse radial motion patterns that do not contain any structural information about self-motion in a rigid scene the same as optic flow. Here we examined behavioral and neural responses to non-rigid radial motion versus optic flow stimuli with perfectly matched 2D features. We first conducted a psychophysical experiment in which we varied the noise level in the motion stimuli and asked participants to indicate whether they perceived any coherent motion patterns. We found that the detection sensitivity was higher for contraction than expansion patterns for non-rigid motion stimuli, but the opposite trend was found for optic flow stimuli. We then conducted an fMRI experiment to find the cortical areas whose responses can relate to the behavioral performance. Participants viewed the same stimuli of the psychophysical experiment in a block design and made a task-irrelevant judgment (color discrimination) during scanning. For each participant, we identified their visual ROIs and the ROIs previously reported to respond to radial motion stimuli (e.g., V1, V2, V3d, V3a, V3b/KO, MT, MST, V6, V7, VIP, CSv, and Pc) using standard localizers. We performed an ROI-based multivoxel pattern analysis (MVPA) and found that only area MST showed a significantly higher decoding accuracy for contraction than expansion patterns for non-rigid motion stimuli, but this trend was reversed for optic flow stimuli, similar to the behavioral performance. We conclude that the visual system does not consider non-rigid radial motion patterns the same as optic flow. Although previous studies have reported that many cortical areas respond to radial motion patterns, only area MST is specialized to process optic flow in the human brain.