Transcript Motion and Ambiguity

Motion and
Ambiguity
Russ DuBois
• Ambiguity = the possibility to interpret a stimulus in two or
more ways
• Q: Can motion play a part in our interpretation of ambiguous
stimuli?
Examples of static ambiguous
figures
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
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
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
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
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
Necker Cube Results
Dancer Results
• 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
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
• “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)
Experiment #2
• Local versus global perception of ambiguous motion displays
• Q: Can motion influence the way we perceptually organize
information?
• 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)
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?
• 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
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
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
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
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)
• 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)
• 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