1. Motion and Ambiguity Russ DuBois

2. Ambiguity = the possibility to interpret a stimulus in two or more ways Q: Can motion play a part in our interpretation of ambiguous stimuli?

3. Examples of static ambiguous figures

4. Some quick terms Reversal time – the time it takes for an ambiguous figure to ‘flip’ from one interpretation to another Kinetic Depth Effect (KDE) - a three-dimensional structural form of an object viewed in projection can be perceived only when the object is rotating. http://www.lifesci.sussex.ac.uk/home/George_Mather/Motion/K DE.HTML http://www.lifesci.sussex.ac.uk/home/George_Mather/Motion/K DE.HTML

5. Experiment #1 Q: Will adding a Kinetic Depth Effect change the reversal time of ambiguous figures Necker cube and ‘spinning dancer’ (Kayahara 2003) were used

6. Conditions Condition 1: static stimulus (freeze frame) Condition 2 (control) : stimulus rotated back and forth around the line of sight but with no KDE Condition 3: Movement with KDE

7. Procedures Each stimuli presented randomly for 180 seconds w/ a one minute break between trials Measured reversal time by hitting a key when a switch occurred. Then, take the average of the reversal times of all three conditions to get a mean reversal time for each condition

8. Why a control? The assumption is that a KDE will greatly decrease reversal time of ambiguous figures. Simply motion alone (change in position on retina) will not have this same effect. Control condition accounts for motion with no KDE

9. Necker Cube Results

10. Dancer Results

11. Condition 1: reversed frequently in perceived depth (~7s) Condition 2: same reversal time as condition 1 Condition 3: strong perceived depth which reversed vary rarely

12. Discussion The perceived depth was ambiguous in all the stimuli that were used The two possible directions of depth were always equally probable; no cues biased the interpretation either way The KDE did not shift the bias; it added stability to whichever interpretation was observed first

13. “Think of the visual system as a judge listening to a set of witnesses (cues) in order to arrive at a verdict (interpretation). What we have shown is that adding a strong new cue, in the form of a kinetic depth effect, does not bias the verdict toward one interpretation or the other, since the cue is designed to be fully ambiguous. Instead, it means that the additional testimony provided by the KDE satisfies the judge that the first interpretation reached is correct, so that once the judge’s mind is made up it takes much longer for him or her to reverse his opinion and change to the opposite interpretation. Thus, whichever of the two competing depth interpretations happens to be perceived first, the KDE will act as additional confirming evidence for its correctness, delaying the onset of the opposite depth percept.” (Anstis 2010)

14. Experiment #2 Local versus global perception of ambiguous motion displays Q: Can motion influence the way we perceptually organize information?

15. A cheetah in a bare cage at the zoo stands out like a spotted fur coat, but in its natural habitat of shrubs and tall grasses its spots provide camouflage. While motion can break this camouflage, from the prey’s point of view the cheetah may still be partially concealed by the undergrowth, and appear as disconnected groups of ambiguously moving spots. In order to perceive the cheetah, the prey must overcome these potential ambiguities and perceive a unified moving entity. Here, we examine some factors that influence the perceptual grouping of moving spots into local or global motion. (Anstis, Kim, 2011)

16. Navon and global perception “the forest is seen before the trees” A large global figure will tend to be seen before its local elements Global before Local Can motion have an effect on local vs. global perception?

17. Constructed various or moving spots or lines that could be perceptually linked together in different ways Can be interpreted either as local motion occurring at small parts of the display or as global motion occurring across the whole display http://www.journalofvision.org/content/11/3/13.long

18. Procedures Measured the time course of changes between local and global motion perceptions Both the number of dots in each local group and the number of local groups in the display, were varied

19. Results Adding independent motion groups increased global motion coherence Increased disks within the local groups decreased global motion A greater number of coherently moving features better defines the edges of local discs rather than global shapes

20. The configurations of moving spots used in Experiment 1 (a). Anstis S, Kim J J Vis 2011;11:13 ©2011 by Association for Research in Vision and Ophthalmology

21. Discussion There may be two separate systems at work, one low level system for local motion, and one high level ‘object driven’ system for global motion driven by attention “Raymond (2000) has reviewed the modulation of global visual motion perception by attention. Consistent with earlier researchers (Ullman, 1979), she concluded that there are probably two levels of motion processing, a motion data level and an object-relevant level. The motion data level, primarily involving V1, uses image filtering mechanisms to extract motion signals from information in the stimulus (low level)…. The object-relevant level is needed to account for motion perception of complex stimuli, such as our own ambiguous displays which contain multiple motion vectors. This level may segment and integrate information collected from the motion data level into discrete object representations” (Anstis, Kim 2011)20001979

22. Local motion grouping = early, fast, and pre-attentive Global motion grouping = slow, high-level, attention driven task Reverting to our cheetah example, it is a modest visual achievement to group some of the moving spots locally into legs or a tail, but a prey's actions and survival will ultimately depend on organizing them globally into the percept of a whole cheetah. (Anstis, Kim 2011)

23. Anstis, S., & Kim, J. (2011). Local Versus Global Perception of Ambiguous Motion Displays. Journal of Vision. http://www.journalofvision.org/content/11/3/13.long#F2