Psy280: Perception

Psy280: Perception Prof. Anderson Department of Psychology Vision 7 Motion

Optional papers: QuALMRI • Question/hypothesis • Alternative • Logic • Method • Results • Inferences • Detailed description on website

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 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?

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

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)

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

Motion and the retinal image • Change in image intensity (luminance) over time • Dark to light • Light to dark Difference image

Illusory movement:Apparent motion • Luminance change • No physical continuity • Infer motion where none is present • Critical temporal/spatial parameters • Simultaneous flicker • <10 ms interval • Perceive 2 events • Motion • ~60 ms interval • Perceive 1 event

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

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

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

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 Perceived motion

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

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 • Don’t perceive either • Create common directional signal • Like force vectors • Down & left moving plaid

Motion detection as an opponent process • Like colour vision: Red-green, blue-yellow • Motion • Up-down • Left-right • Spiral in-out • Enhances “motion contrast”

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

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)!

Direction repulsion: Lateral inhibitory influences in motion • 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” Actual Perceived

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

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

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 doesn’t move that fast

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

Motion from structure • Not only can motion induce shape perception • Shape can induce motion perception • Top-down influences • FFA/IT —> MT

How does the brain represent motion?

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

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

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

MT motion processing:Random dot stimuli • How do we know MT supports motion perception? • 0%, 30%, and 100% coherence • Use to determine monkey/human detection of directional motion

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

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

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

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 doesn’t world appear to shake? • Would get pretty nauseating • Vision needs to “correct” for eye movements • How does it do it?

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!

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 Visual cortex MS C CDS IMS Eye

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

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

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

“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

The End

Psy280: Perception