TRANSPARENT MOTION PERCEPTION AS DETECTION OF UNBALANCED MOTION SIGNALS .3. MODELING

In the preceding two companion articles we studied the conditions unde r which transparent motion perception occurs through psychophysical ex periments, and investigated the underlining neural mechanisms through physiological recordings. The main finding of our perceptual experimen ts was that whenever a display has finely balanced motion signals in a ll local areas, it is perceptually nontransparent, and that transparen t displays always contain motion signals in different directions that are either spatially unbalanced, or unbalanced in their disparity or s patial frequency contents. In the physiological experiments, we found two stages in the processing of transparent stimuli. The first stage i s located primarily in area V1. At this stage motion measurements are made and V1 cells respond well to both the balanced, nontransparent st imuli and the unbalanced, perceptually transparent stimuli. The second stage is located primarily in area MT. MT cells show strong suppressi on between opposite directions of motion. The suppression for the unba lanced, transparent stimuli is significantly less than that for the ba lanced, nontransparent stimuli. Therefore, the activity in the second, MT stage correlates better with the perception of motion transparency than the first, V1 stage, which does not distinguish reliably between transparent and nontransparent motion. The above experiments suggest a two-stage model of motion perception with a motion measurement stage in V1 and an opponent-direction suppression stage in area MT. In this article we explicitly test this model through analysis and computer s imulations, and compare the response of the model to the perceptual an d physiological results using the same balanced and unbalanced stimuli we used in the experiments. In the first stage of the computational m odel, motion energies in different spatial frequency and disparity ran ges are extracted from each local region. Similar to V1, this stage do es not distinguish between the balanced and unbalanced stimuli. In the subsequent stage motion energies of opposite directions but with same spatial frequency and disparity contents suppress each other using su btractive or divisive inhibition. This stage responds significantly be tter to the transparent stimuli than to the nontransparent ones, in ag reement with MT activity.