The existence of a second-order motion system distinct from both the first-order and the feature tracking motion systems remains controversial. We recently suggested that the first-order motion system could process contrast-defined motion due to non-uniform preprocessing nonlinearities introducing different distortion products for different first-order motion units (Allard & Faubert, 2008; 2012). At temporal frequencies too high for features to be tracked, we found no texture (i.e. contrast-modulation) contribution to motion when non-uniform preprocessing nonlinearities were neutralized by adding a high-contrast luminance modulation drifting with the contrast modulation. The texture contribution to motion was measured by superimposing another luminance modulation drifting in the opposite direction and asking observers to adjust its contrast until they perceived no net motion. The texture contribution to motion was defined as the contrast difference between the two luminance modulations when no net motion was perceived. The goal of the present study was to investigate if we have a dedicated second-order motion system operating at low temporal frequencies (1.875 Hz). To neutralize the feature tracking motion system, we presented the motion (radial modulations rotating either clockwise or counterclockwise) in the near periphery (annulus spatial window between 5 and 9 degrees of eccentricity) and varied the spatial frequency of the modulations (1 to 64 cycles per circumference). In the periphery, where attention resolution is low, it should be impossible to attentively track bars if they are too close to one another (i.e. crowding) to be individually selected. Results showed no texture contribution to motion when both the non-uniform nonlinearities were neutralized and the bars of the gratings were too close to one another for attention to individually select and track them (more than 16 cycles per circumference, which corresponds to about 0.38 cpd). We conclude that there is no dedicated second-order motion system operating in the periphery.
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