Feb. 17, 11:45, Wei Ji Ma. Optimality and Probabilistic Computation in Visual Categorization

Curved apparent motion induced by dynamic occlusion and the launching effect
Sung-Ho Kim, Rutgers University
Many aspects of amodal completion in static scenes have been studied, but
relatively little is known about how completion interacts with moving
structures in dynamic scenes. We examined whether amodal completion can
bias an apparent motion path towards longer curved paths behind an
occluder, which would violate the well-established principle that apparent
motion follows the shortest possible path. In a series of experiments,
observers viewed motion sequences of two alternating rectangular targets
positioned at the ends of a semicircular “tube,” with varying
inter-stimulus intervals (ISI: 100-500 ms). With short ISIs, observers
tended to report simple straight path motion, i.e. outside the tube. But
with longer ISIs, they became increasingly likely to report a curved
motion path occluded by the tube. Subjects also reported that at longer
ISIs straight motion became jerker while curved motion became smoother.

Other experiments investigated whether the motion path could be influenced
by a Michotte-style "launch" at the initiation of motion. We added two
more small objects which appeared to collide with the motion tokens at
offset, in the direction of either the straight path or the curved path.
Subjects in these experiments almost exclusively perceived a motion path
in the direction of the launch, suggesting a very strong bias in the
direction of perceived momentum.

In sum, our results suggest that (1) The amodal representation of a fully
hidden object behind an occluder can bridge the gap between two token
locations via a curved motion trajectory, (2) amodal completion in
space-time can make curved motion appear relatively smooth and continuous,
and (3) the launching effect strongly induces a motion path in the
direction of launching.