Motion - Department of Psychology

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Transcript Motion - Department of Psychology

Psy280: Perception

Prof. Anderson Department of Psychology Vision 7 Motion 1

Optional papers: QuALMRI

 Qu estion/hypothesis  A lternative  L ogic  M ethod  R esults  I nferences  Detailed description on website 2

Part 2: Perceiving Size

  Not as simple as size of stimulus on retina Visual angle: retinal projection depends on distance  Different physical size  Same retinal Projection  Bigger stimulus further away = visual angle to closer smaller stimulus 3

Size constancy

 Perception of size remains constant  Despite different visual angle/retinal size 4

Size distance scaling

 Perceived size = retinal image size distance from object X 2 x distance but same retinal size = 2 x perceived size  Without depth information  Perceived size = retinal image size 5

Emmert’s law

 Perceived size of an after image depends on depth perception (consistent with S = R x D) 6

Size-depth illusions

   Moon appears larger on the horizon than the sky  Same retinal size  Difference in magnitude estimation Horizon provides depth cues Sky does not  Appear flattened 7

Motion

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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 sun: 2 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 10 17 miles / 2.19 x 10 12 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 Quic kTime™ and a Sorenson Video decompress or are needed to see this picture.

movement  vs phototropism  Co-evolution  Organisms that move  Evolution of a capacity to sense movement 10

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) QuickTime™ and a Sorenson Video decompressor are needed to see this pictur e.

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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 Can’t tell difference across space QuickT i me™ and a Graphi cs decom pressor are needed to see t his picture.

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Motion and the retinal image

 Change in image intensity (luminance) over time  Dark to light  Light to dark Difference image QuickT i me™ and a Graphi cs decom pressor are needed to see t his picture.

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Illusory movement: Apparent motion

    Luminance change No physical continuity Infer motion where none is present Critical temporal/spatial parameters   Simultaneous flicker  <10-30 ms interval  Perceive 2 events Motion  ~60 ms interval  Perceive 1 event QuickT ime ™ an d a GIF d ecomp res sor a re ne eded to se e th is p ic tu re.

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Not just simple luminance change: 2nd order motion

   First-order motion  Change in luminance boundary Luminance change doesn’t explain all motion Second-order motion  Motion but no luminance boundary   Not net luminance change Object disappears when motion stops QuickTime™ and a Animation decompressor are needed to see this picture.

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Second order motion: Illusory 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 can’t explain all of motion perception QuickTime™ and a Animation decompressor are needed to see this picture.

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Simple luminance detectors won’t 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 17

The aperture problem

   Looking at motion through the window of one neuron RF represents horizontal motion Global scene has different motion  Local computations don’t necessarily explain motion  Need to share information across neurons Quick Time™ and a Graphic s decompress or are needed to s ee this picture.

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Perceived motion Quick Time™ a nd a Animation de compr ess or ar e nee ded to see this pictur e.

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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 19

Motion integration at the same retinal location: Plaids

First order low-level motion detectors QuickTi me™ and a Graphi cs decom pressor are needed to see t his pict ure.

 Respond to each component of motion (horizontal and vertical)  Motion integration  Don’t perceive either  Create common directional signal  Like force vectors  Down & left moving plaid QuickT ime ™ an d a Grap hics dec ompr esso r ar e need ed to see this pictur e.

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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” 21

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 red — >perceive green QuickT i me™ and a A nim ati on decom pressor are needed to see t his pict ure.

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Spiral motion after effect: Disfiguring Brad

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• 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      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” Perceived QuickTime™ and a Graphics decompressor are needed to see this picture.

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Perceptual organization: Structure from motion

Motion perception not used just to assess stimulus movement Can define “objects” QuickTime™ and a Video decompressor are needed to see this picture.

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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 QuickTi me™ and a Y UV420 codec decom pressor are needed to see t his pict ure.

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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 QuickTime™ and a Animation decompressor are needed to see this pictur e.

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Kinetic depth: Shadow motion

 Moving shadows are also strong cue for depth change QuickT ime ™ an d a Grap hics dec ompr esso r ar e need ed to see this pictur e.

 Heuristic    Ambiguous info  Shadow might reflect light source movement Assume light source is constant Sun doesn’t move that fast Quick Time™ a nd a Cinepa k deco mpre ssor are n eede d to s ee this picture .

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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 QuickTime™ and a Animation decompressor are needed to see t his picture.

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Motion from structure

 Not only can motion induce shape perception  Shape can induce motion perception  Top-down influences  FFA/IT —> MT 29

Motion from structure

 Not only can motion induce shape perception  Shape can induce motion perception  Top-down influences  FFA/IT —> MT 30

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 32

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 33

The brain’s 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 processing: Random dot stimuli

 How do we know MT supports motion perception?

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 0%, 30%, and 100% coherence  Use to determine monkey/human detection of directional motion 35

Psychophysical and neural motion response profiles

 Neuronal response related to perceptual experience of motion?

Neuron and observer motion detection  MT neuron firing rate parallels perception Random dots 36

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 37

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After MT: Increasing complexity/specificity

Medial superior Neuron 1 temporal (MST) Neuron 2   More specific patterns Expansion/ contraction Superior temporal sulcus (STS)  Biological motion Higherarchical organization and sepcificity coding extends to motion 38

Keeping the world still

 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 doesn’t world appear to shake or move when we move our eyes?

 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  —> No perception of motion Motor cortex MS CDS Eye Visual cortex

C

IMS 41

Corollary discharge theory

 Anytime CDS and IMS don’t co-occur —> perceive motion  IMS alone —> 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 42

CDS: Moving after images!

 CDS without IMS  Doesn’t 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 44

Motion perception is more than movement across the retina

  Perception more than what retina tells us  So what’s new!

Can dissociate retinal change and motion perception   Retinal change without motion perception  Move eyes across stationary scene  World doesn’t move despite radical retinal shift Motion perception without retinal change  Track a moving object  No movement across retina: Powerful perception of motion 45

“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 V1?

Real movement neuron 46

The End

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MT and virtual motion

 MT responsive to virtual motion  Motion after effects or illusory motion  No retinal change  Stimulate MT —>voila! Motion Time course of MT activation follows motion after effect 48

3D motion: More motion heuristics

 Visual stimulus is ambiguous  Multiple interpretations  Which is visual system attracted to?

 Vision assumes movement of rigid objects QuickTime™ and a Graphics decompressor are needed to see this picture.

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Intelligence of motion perception

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1. Notice oscillation in direction of motion: Due to single reversal 2. Look at with and without blinking your eyes 50