Perceptual motion transparency : the role of geometrical information

Perceptual motion transparency occurs whenever two or more patterns are seen moving at different depth levels, such that we can see one pattern move across the others, and perceptual motion coherence occurs when we see a single motion. We present here a model of perceptual motion transparency and coherence that consists of three stages: (i) measure the normal velocity along contours or the velocity of features such as corners or line end points; (ii) take the intersection, in velocity space, of all possible pairs of constraint lines associated with the normal velocity components; and (iii) combine the results of steps (i) and (ii) in the velocity histogram, which is the plot of the total number of votes for each velocity in velocity space. For two patterns we perceive motion coherence, transparency, or a mixture of both types of motion depending on whether the velocity histogram is unimodal, bimodal, or trimodal. According to our model we perceive motion transparency or coherence depending on the total number of prominent peaks of the velocity histogram, where each peak is located at the position corresponding to the velocity of one of the patterns or of coherent motion. We show that the number of prominent peaks in the velocity histogram depends on the error in the measurement of the local velocity and on the relative orientation of the local velocities along contours; this relative orientation encodes contour shape. Our model differs from current motion theories in that it describes the perception of motion not as a result of local velocity extraction but instead as the result of the integration of local velocity and geometrical shape information across different points of the image and across superimposed patterns. This model allows for the occurrence of mixed motion perception, which arises from the combination of the velocity information associated with motion transparency and coherence. We have tested this model through computational and psychophysical experiments done with line patterns. As a result of these experiments we conjecture that the human visual system may use at least three stages to process image velocity.