Psych 121 Lecture 10

8/13/2019 Psych 121 Lecture 10

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LECRURE 10Introduction to Motion PerceptionA. Motion is closely related to perception.1. Complex perceptual systems are found only in organisms that guide their own motion in space. (c.f.tunicates/sea squirts)

A. Motion is closely related to perception.1. Complex perceptual systems are found only in organisms that guide their own motion in space. (c.f.tunicates/sea squirts)2. Perception and self-motion probably co-evolved.3. Two equally important aspects of visual motion perception are:1. seeing moving objects2. seeing and guiding one's own motion

B. Motion perception is incredibly sensitive and accurate. examples: Returning a 120 mph tennis serve Hitting a 110 mph fastball

example: Returning a 120 mph tennis serve Ball moves 176 ft. / sec 17.6 feet in 1/10 sec almost 2 feet in 1/100 sec

Need to contact ball in right place with about 3 sq. in. area of racket

A. We can characterize motion sensitivity in various ways.- Slowest perceptible motion


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- Fastest perceptible motionSensitivity in central and peripheral viewing( --important in driving)

B. Focus on: Slowest perceptible motion How do we characterize this scientifically?Threshold: The answer is in terms of a velocity .

For example: velocity at which motion is detected 50% of the timeRetinal velocity: Change in visual angle per unit of time

Empty field, no background referencesObject must move 10-20 min/secSubject -relative motion Definition : Subject-relative is motion of a single visible object with no background reference.

What is your motion threshold?With background referencesObject must move 1-2 min/secObject -relative motion Definition : Object-relative is motion of a visible object relative to some other object or visible

background.

How would you build a motion detector?Involves a change in position over timeSo start with two adjacent receptors separated by fixed distance

Neural Circuit for Detection of Rightward Motion

What are basic requirements for velocity detection?What algorithm could work?

Register : CHANGE AT LOCATION 1-- elapsed time --

Register : CHANGE AT LOCATION 2


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Reichardt DetectorsArranged to detect in all different directionsEvidence suggests opponent process arrangement to determine net motion

A Neural Circuit for Detection of Rightward Motion

You noticed a problem?This detector will also sometimes activate when no real motion is provided

APPARENT MOTION


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occurs when images flash on and off in separate locations with certain timing relations. Althoughnothing really moves between the flash locations, motion is seen.It is also called stroboscopic motion.It is also called beta motion .

Apparent Motion -- TimingInterstimulus Interval (ISI) is the time between the end of one flash and the start of another.For a specific spatial disparity: When ISI < 60 ms: SIMULTANEITY is seen. When ISI is 60-200 ms (beta motion): OPTIMAL MOVEMENT is seen. (Movement appears smooth and continuous.) When ISI >200: SUCCESSION, not movement, is seen.

General Theories of Motion PerceptionPosition-change over time vs. Motion

INDIRECT Perception Theory : Motion is not a basic perceptual quality; it is derived from other things. DIRECT Perception Theory : Motion is a basic perceptual quality. Your system is wired to perceive it.Exner and Wertheimer did experiments offering support for the direct view.

Exners Experiment Threshold for perceived succession BELOW 45ms, cant tell the temporal order of two successive flashes. BUT: can tell the direction of apparent motion w ith a low as 14ms

Wertheimers Experiment Phi motion (Wertheimer)

At some ISIs, one sees simultaneous lights but also sees something moving between themTwo simultaneous perceptsOFF AND ON IN FIXED LOCATIONS + MOTION"OBJECTLESS" MOTIONIndicates that motion perception mechanism is triggered independently of the perception of a single objectfrom the two flashes

General Theories of Motion PerceptionDIRECT Perception Theory is correct:Motion is a basic perceptual quality.We are wired to perceive it.

What is the special wiring? M ganglion cells M cell layer in LGN all V1 cells MT


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Lesion to MT Patient LM

Reports that sees position of object change (jumps) Sees no motionTrouble with pouring coffeeApproaching cars

Induced motion (contrast)

Information for Motion PerceptionINDUCED MOTION involves an object and a surrounding reference frame. When the surround or framemoves, the object appears to move.

Induced motion may also make the observer feel she or he is moving. Direction contrast


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Remember color (negative afterimages)?

Works because of opponent color mechanisms

Reichardt Detectors opponent process arrangementResulting motion perception is the vector sum of opposite direction detectors

Perceiving motion in one direction fatigues detectors for that direction.Afterwards, looking at a stationary scene, there is a reduced response from fatigued detectors.

We call it the Motion aftereffect

Computation of Visual Motion First -order motion: The motion of an object that is defined by changes in luminance Second -order motion: The motion of an object that is defined by changes in contrast or texture, but not

by luminanceTexture-defined (contrast-defined) object: An object that is defined by changes in contrast or texture,

but not by luminance

Second-order motion

Close-ups of one section of the frames in Figure8.10, illustrating the changes from frame to frame


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Psych 121 Lecture 10