Distinct neural processing of real versus unreal optic flow in the human brain

Abstract

Visual perception of self-movement relies on optic flow, a global motion pattern generated by self-movement in a rigid 3D environment. Such optic flow conveys both 2D features (e.g., radial velocity fields and centers of motion) and 3D structural information (e.g., motion perspective). However, many previous neuroimaging studies have used simplified 2D radial motion as "real" optic flow, leaving it unclear whether such "unreal" stimuli adequately capture how the brain processes ecologically valid self-movement information. Previous behavioral work has shown that sensitivity to expansion and contraction reverses depending on whether optic flow is ecologically valid (L. Li et al., 2025), raising the question of how such differences are represented in the human brain. We conducted two functional magnetic resonance imaging (fMRI) experiments to systematically compare cortical processing of real versus unreal optic flow. The two stimulus types were strictly matched in 2D global features and local motion signals, differing only in whether they preserved 3D structure consistent with self-movement. To minimize task-related influences, participants performed a task-irrelevant fixation point color-change detection task during scanning. Experiment 1 used a block design in which participants viewed expanding and contracting optic flow at different motion coherence levels. Experiment 2 presented real and unreal optic flow within the same scanning session to directly test neural sensitivity to ecological validity. Multivoxel pattern analysis (MVPA) revealed that, among multiple visual and optic flow-responsive regions, the human dorsal medial superior temporal area (MST) exhibited robust and consistent sensitivity to the ecological validity of optic flow. Specifically, under real optic flow, decoding accuracy for motion coherence in MST was significantly higher for expansion than for contraction, whereas this preference was reversed under unreal optic flow. Neural threshold estimates derived from fMR-metric functions further supported this pattern reversal, indicating systematic modulation of MST sensitivity by ecological structure. Experiment 2 further demonstrated that, regardless of motion pattern, MST reliably distinguished real from unreal optic flow, with decoding accuracy increasing as motion coherence increased. These findings indicate that MST encodes not only motion patterns themselves but also whether optic flow conforms to the ecological structure of self-movement. Together, these results indicate that MST responses are not determined solely by 2D radial motion features, but are modulated by whether optic flow preserves 3D structure consistent with self-movement. This work clarifies a long-standing conflation between simplified radial motion and real optic flow and highlights the importance of ecological structure in shaping dorsal-stream motion representations related to self-movement perception.