Psy280: Perception

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Psy280 Perception

• Prof. Anderson
• Department of Psychology
• Vision 7
• Motion

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Optional papers QuALMRI

• Question/hypothesis
• Alternative
• Logic
• Method
• Results
• Inferences
• Detailed description on website

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Motion Frames of reference

• What does the term "at rest" mean?
• Can you cite an example of an object at rest?
• Is the room at rest?
• Room has at least three types of motion
• Motion due to earth 24000 miles / 24 hours
  1000 miles/hr
• Earth circles the sun2 pi 93,000,000 miles /
  8760 hours 66700 miles/hr
• Sun circles the galaxy (30,000 light year r)
  every 1 / 4 billion years 1.76 x 1017 miles /
  2.19 x 1012 hr 80400 miles/ hr
• Is there anything that is not moving?
• Must be careful about our description of motion
• Moving relative to what reference frame?

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Animism Worshiping the light

• Divides living organisms
• Animals vs plants
• Capacity for voluntary movement
• vs phototropism
• Co-evolution
• Organisms that move
• Evolution of a capacity to sense movement

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Invisible motion Morning glory

• 5 AM to 7PM
• Open in morning
• Pollination by diurnal insect
• Dies in afternoon
• Motion too slow to notice even dramatic change
• Our visual system are tuned to events that move
  more quickly
• E.g., Animals (fast) not plants (slow)

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Motion and change detection

• Visual motion is sensing change in retinal image
  (sort of)
• As duration between changes increases perception
  of motion decreases
• Motion is a perceptual adaptation for detection
  of change, otherwise invisible to the eye

Can tell difference across time
Cant tell difference across space
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Motion and the retinal image

• Change in image intensity (luminance) over time
• Dark to light
• Light to dark

Difference image
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Illusory movementApparent motion

• Luminance change
• No physical continuity
• Infer motion where none is present
• Critical temporal/spatial parameters
• Simultaneous flicker
• lt10 ms interval
• Perceive 2 events
• Motion
• 60 ms interval
• Perceive 1 event

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Not just simple luminance change 2nd order motion

• First-order motion
• Change in luminance boundary
• Luminance change doesnt explain all motion
• Second-order motion
• Motion but no luminance boundary
• Not net luminance change
• Object disappears when motion stops

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Second order motionIllusory shapes and motion

• No luminance boundary for low-level motion
  detectors to use
• Motion perception must rely on other
  top-down/higher-order influences
• Simple luminance based motion detectors cant
  explain all of motion perception

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Simple luminance detectors wont do The aperture
problem

• Narrow view of world through small receptive
  fields (RF)
• Ambiguity of direction of motion
• Need additional info for accurate motion sensing
• Edges or texture

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The aperture problem

• Looking at motion through the window of one
  neuron
• RF represents horizontal motion
• Global scene has different motion
• Local computations dont necessarily explain
  motion
• Need to share information across neurons

Perceived motion
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Motion perception More than the sum of its parts

• The underlying mechanism involves signals at
  different retinal locations being integrated to
  arrive at global motion signals

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Motion integration at the same retinal location
Plaids

• First order low-level motion detectors
• Respond to each component of motion (horizontal
  and vertical)
• Motion integration
• Dont perceive either
• Create common directional signal
• Like force vectors
• Down left moving plaid

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Motion detection as an opponent process

• Like colour vision Red-green, blue-yellow
• Motion
• Up-down
• Left-right
• Spiral in-out
• Enhances motion contrast

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Motion after effect

• Reversing waterfall
• Fatigue your direction sensitive neurons
• See opposite motion where there is none
• Explanation
• No motion
• Direction selective cells produce equal responses
• No longer equally oppose each other
• E.g., Adapt to redgtperceive green

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Spiral motion after effect Disfiguring Brad

• Fatigue neurons representing radial expansion
• Induces radial contraction due to lessened
  inhibitory influence
• Motion (perception) is a perceptual/neural
  process, not necessarily a property of the world
  (object movement)!

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Direction repulsion Lateral inhibitory
influences in motion
Actual
Perceived

• Vertical and 45
• degree movement
• Interact to enlarge directional disparity
• Evidence of lateral inhibitory interactions
  between motion detectors
• Enhancement of directional contrast
• Motion mach bands

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Perceptual organization Structure from motion

• Motion perception not used just to assess
  stimulus movement
• Can define objects
• Laws of organization
• Common fate
• Things that move together belong to same object
• A camouflaged animal is difficult to see until it
  moves
• Not just knowledge based
• Can see novel objects

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Structure from motion Kinetic depth

• Can define depth
• What motion cues define depth?
• Parallax
• Differing dot velocity
• Track single dot
• See velocity change
• Infer depth from motion

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Kinetic depth Shadow motion

• Moving shadows are also strong cue for depth
  change
• Heuristic
• Ambiguous info
• Shadow might reflect light source movement
• Assume light source is constant
• Sun doesnt move that fast

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Experience and motion perception Biological
motion

• Dot walkers
• We each have our own motion signature
• Recognition by motion
• Experience influences motion perception

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Motion from structure

• Not only can motion induce shape perception
• Shape can induce motion perception
• Top-down influences
• FFA/IT gt MT

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Motion from structure

• Not only can motion induce shape perception
• Shape can induce motion perception
• Top-down influences
• FFA/IT gt MT

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How does the brain represent motion?
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V1 Simple motion detectors

• Directionally selective
• E.g., right ward and up
• Small receptive fields
• Local not global motion
• Thus, respond to components of a plaid, not
  perceived direction
• Higher level info must override V1 simple motion

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Designing a directionally selective V1 neuron

• Temporal component
• Built in delays
• Neuron to neuron communication takes time
• Timing of inhibition is critical
• Results in neuron liking right to left motion
• Not left to right

Delayed inhibition
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The brains motion eye Area MT (V5)

• Middle temporal area (MT)
• Dorsal stream
• 90 of cells are directionally selective
• Organized in directional columns
• Like V1 orientation or IT shape columns
• Stimulation of column increases directional
  motion perception
• 100 times larger than V1 RFs
• Wide view of world
• Good for composite motion

Human MT
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MT motion processingRandom dot stimuli

• How do we know MT supports motion perception?

• 0, 30, and 100 coherence
• Use to determine monkey/human detection of
  directional motion

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Psychophysical and neural motion response profiles

• Neurons response relate to perceptual experience
  of motion?
• MT neuron firing rate parallels perception

Neuron and observer motion detection
Random dots
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Stimulation of MT and motion

• Neurons response correlated with perceptual
  experience of motion
• Causally related?
• Stimulation of MT increases propensity to
  perceive motion in certain direction

Proportion seen right directed motion
Right
Left
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After MT Increasing complexity/specificity

• Medial superior temporal (MST)
• More specific patterns
• Expansion/
• contraction
• Superior temporal sulcus (STS)
• Biological motion
• Higherarchical organization and sepcificity
  coding extends to motion

Neuron 1
Neuron 2
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Keeping the world still

• Given examples of motion w/out retinal change
• E.g., motion after effects
• What about retinal change w/out motion?
• Eyes constantly make small fast movements
• Remember World fades without these movements
• Why doesnt world appear to shake?
• Would get pretty nauseating
• Vision needs to correct for eye movements
• How does it do it?

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Corollary discharge theory

• Integration of retinal stimulation and eye
  movements
• Use motor signals to stabilize vision
• Head movement
• Eye movement
• How about movement without motor signal?
• (keep one eye closed) Push your open eye. Gently
  please!
• World moves!

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Corollary discharge theory

• 3 signals
• Motor (MS)
• Image movement (IMS)
• Corollary discharge (CDS)
• Comparator (c)
• Eye (IMS) and motor signals (MS) need to be
  compared
• CDS is a copy of motor signal
• CDS and IMS cancel each other
• When both are present no signal sent to visual
  cortex
• gt No perception of motion

Motor cortex
Visual cortex
MS
C
CDS
IMS
Eye
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Corollary discharge theory

• Anytime CDS and IMS dont co-occur gt perceive
  motion
• IMS alone gt perceive motion
• Veridical movement
• Eyes still, stimulus moves
• Illusory movement
• Pushing your eye
• Move image on retina
• w/out MS/CDS
• This theory makes interesting predictions
• CDS alone should also result in motion

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CDS Moving after images!

• CDS without IMS
• Doesnt often happen
• No canceling of IMS and CDS
• Should result in motion perception
• After images
• No IMS
• Fatigued photoreceptors result in stationary
  stimulus
• MS/CDS without IMS
• After images move

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CDS alone results in motion perception

• Track a flying bird
• No IMS, stabilized on retina
• MS/CDS without IMS
• CDS activates motion perception in cortex
• Paralyze eye muscles
• Can send MS but no eye movement
• MS/CDS without IMS
• Stationary events appear to move

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Real movement neurons

• Higher order cortical neurons (e.g. V3)
• Bar moves through RF
• Move bar
• Move eyes
• Retinal stimulation held constant
• Respond most when not moving eyes

Real movement neuron
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The End