No second-order motion system sensitive to high temporal frequencies

It has been shown that the perception of contrast-defined motion (i.e., a second-order stimulus) at high temporal frequencies cannot be explained solely by global distortion products (i.e., luminance artifacts due to preprocessing nonlinearities) processed by the first-order system. However, previous studies rejecting the first-order pathway hypothesis have assumed that the preprocessing nonlinearities are identical for all first-order motion units. If this is not the case, then introducing a nonlinearity within the stimulus could neutralize the global (i.e., mean) distortion product but would leave residual distortion products. We neutralized either global only or both global and residual distortion products by superimposing a luminance modulation onto the contrast modulation. At a temporal frequency too high for features to be tracked (15 Hz), we found a substantial texture (i.e., contrast-modulated) contribution to motion when neutralizing only global distortion products but not when neutralizing both global and residual distortion products. Furthermore, we found that the texture contribution to motion at this high temporal frequency, when it was not completely neutralized, depended on the phase difference between luminance and contrast modulations, which implied some common processing before the motion extraction stage. We concluded that the texture contribution to motion at high temporal frequencies was due to nonuniform preprocessing nonlinearities within the visual system, enabling first-order motion units to process distortion products, and not due to a dedicated second-order motion system.