Evidence for a Feature Tracking Explanation of Why Type II Plaids Move in the Vector Sum Direction at Short Durations

When two moving sinusoidal gratings, with similar spatial frequency, contrast, phase, but different orientation are combined to form a plaid, their perceived direction of motion has been predicted by the intersection of constraints rule (IOC) (Adelson & Movshon, Nature, 300, 523-525, 1982). However, at short durations (60 msec) the direction of perceived motion has been predicted by the vector sum direction for "Type II" plaids (Yo & Wilson, Vision Research, 32, 1, 1992). Type II plaids are the set of plaids where the components are both located on one side of the resultant computed using the IOC rule. Yo and Wilson suggest that the vector sum direction is observed for Type II plaids at short durations because non-Fourier information is not available and direction is computed from Fourier information only. The first experiment in this study replicates the original Yo and Wilson result using similar stimuli but a simpler task; perceived direction was measured using a direction discrimination task instead of the method of adjustment used by Yo and Wilson. The second experiment provides evidence against generalizing the result to all Type II plaids. A systematic set of type II plaids that varied only in terms of the orientation of the second component provided an ideal set because their predicted motion direction followed very different patterns when predicted by the IOC and vector sum computations. The results obtained were predicted more accurately by the IOC than the vector sum. Experiment 3 provides further evidence that movement in the vector sum direction is not a general property of type II plaids. A small change to the velocity of one of the components of a plaid previously perceived in the vector sum direction had the effect of shifting the perceived motion in the IOC direction, despite increasing the difference between the IOC and VS predictions. This result is not consistent with Yo and Wilson's hypothesis that Type II plaids move in the vector sum direction because of a temporal delay between Fourier and non-Fourier information. Computational analysis of the stimuli used in both the current and original experiments revealed a possible explanation of the results in terms of a contribution from local feature tracking rather than a vector sum operation.