New illusions of sliding motion in depth

Apparent sliding motion in the so-called Ouchi illusion has been attributed to the global integration of local motion vectors arising from the aperture effect (Fermüller et al, 2000 Vision Research 40 77- 96; Mather, 2000 Perception 29 721-727). In a number of variants of the Ouchi illusion, we here demonstrate that sliding motion will also arise without a directional motion bias from local elements. Specifically, we show that in a disk-annulus pattern made from wiggly lines, sliding motion occurs although the local orientations within the disk and annulus are the same. We then argue that in an array of square-shaped checks, sliding motion originates from the interaction between the explicit orientation of the checks and the implicit orientation of the invisible diagonals. Finally, we demonstrate that a central array of filled black circles surrounded by a grey edge appears to slide relative to a surround of empty circles. We tentatively account for sliding motion in this figure by differences in speed signals, figure-ground segregation and apparent depth due to contrast polarity, edge blur, demarcation by a frame, and difference in shape.