Two-stage analysis of the motion of 2-dimensional patterns, what is the first stage?

The sum of two differently orientated moving sinusoidal gratings of similar spatial frequency, contrast, and velocity appears as a single coherent "plaid" pattern. The visual system is thought to analyse the motion of plaids in two stages, first analysing the motion of the (1-D) components, and then calculating a speed and direction which is consistent with those 1-D motions. We find that the direction of motion of a plaid (components 1.6 c/deg orientated +60 degrees and -60 degrees) can be discriminated at velocities so low that the direction of motion of its components is not discriminable. This finding is not consistent with the "two-stage" hypothesis in the form that it is usually expressed. We suggest that mechanisms sensitive to the motion of local elements in the pattern, such as edges, could also contribute to the first stage of the analysis of plaid motion.