An oblique effect for local motion: psychophysics and natural movie statistics.

Human perception of visual motion is thought to involve two stages--estimation of local motion (i.e., of small features) and global motion (i.e., of larger objects)--identified with cortical areas V1 and MT, respectively. We asked if poor discrimination of oblique compared to cardinal directions (the oblique effect for motion; OEM) reflects a deficit in local or in global motion processing. We used an equivalent noise (EN) paradigm--where one measures direction discrimination thresholds in the presence of directional variability--to quantify local and global limits. We report that the OEM diminishes with increasing directional variability, indicating that global motion processing (the number of local motion signals pooled) is equal across all directions and that the OEM is attributable to anisotropies in local motion processing. To investigate the origin of this effect, we measured local motion statistics from natural movies (filmed from the point of view of a walking observer). This analysis reveals that the distribution of local directional energy on the oblique directions tends to be broader, and frequently more asymmetric, than on the cardinal directions. If motion detectors are optimized to deal with our visual world then such anisotropies likely explain the local nature of the OEM.