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
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