Long-range interactions in the spatial integration of motion signals

When a sinewave grating is moving within a cross-shaped aperture, a strongly multi-stable phenomenon is perceived. The percept switches between the coherence of an extended surface moving in a single direction and the segragation of two patterned strips sliding across each other in directions parallel to the branches of the cross. The possible effect of the length of the arms and the shape of their ends on the balance between these two percepts is determined. In particular, it is demonstrated that human observers report the segragation into two surfaces more often when the branches of the cross are extended, and when the small sides of the arms are oriented parallel to the grating.