1/29/2002 PSYC202-005, Term 2, Copyright 2002 Jason Harrison

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Perception of Motion and Movement

1/29/2002 PSYC202-005, Term 2, Copyright 2002 Jason Harrison 2

Quiz: List five ways to make a spot of light appear to move:1. Move the light2. Apparent Motion

• Turn off the light, turn on another3. Induced Motion

• Move a large framing object4. Auto kinetic effect

• View a dim light in a very dark room5. Movement aftereffect

• Move something for a long time - then look at the light

6. (Change the intensity of the light)

1/29/2002 PSYC202-005, Term 2, Copyright 2002 Jason Harrison 3

Say at V1 the neurons detected this pattern

Positions across retina

Ori

enta

tion

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and a moment later the neurons detected this pattern

Positions across retina

Ori

enta

tion

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What could be the source of this motion?

Positions across retina

Ori

enta

tion

Positions across retina

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tati

on

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Sources of movement for the eye-brain system • motion across the retina

– object moving or eye or head or body moving?

• eye movement– tracking an object or just looking around?

• head movement• body movement

1/29/2002 PSYC202-005, Term 2, Copyright 2002 Jason Harrison 7

World in motion:

Eyes inmotion:

Categorize into four possibilities

No Yes

NoStatic Vision

Retinal Motion

YesEye/Body moveme

ntTracking

Gregory: Inflow/Outflow

1/29/2002 PSYC202-005, Term 2, Copyright 2002 Jason Harrison 8

Retinal (image) motion• movement of light across retina• sensitivity found in all animals

– not all animals see static images– all animals see motion

• motion detected by some animals in retina– house fly, frog

• motion detected by some animals in brain– cats, humans

1/29/2002 PSYC202-005, Term 2, Copyright 2002 Jason Harrison 9

Uses of motion information1. Relative velocity of observer and

environment• direction of heading, time to contact

2. Segmentation of figure from ground• disruption of camouflage techniques

3. Recovery of 3D parameters• motion parallax (3D depth)• kinetic depth (3D shape)

4. Object identification• humans, friend/foe

1/29/2002 PSYC202-005, Term 2, Copyright 2002 Jason Harrison 10

Centre of expansion ------> = direction of heading

1. Relative velocity of observer and environment

• speed and direction of objects/observer• optic flow - pattern of motion in image

when observer moves

1/29/2002 PSYC202-005, Term 2, Copyright 2002 Jason Harrison 11

Opticflowdemo

Optic flow• used to determine

– direction of heading– time to collision– approach/avoidance

• types of optic flow caused by global motion– expansion/contraction (collision?)– translation– rotation

• Gibsonian approach uses optic flow a lot

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2. Segmenting figure from ground

• Figure: object that draws our attention• Ground: non-moving dots/contours/blobs• Gestalt law of Common Fate

– items that move together belong together

From http://www.human.pefri.hr/~bsremec/figure_motion.html

Commonfatedemo

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3. Recovering three-dimensional shape

• Retinas can only record 2D projection of world• depth dimension is lost

• Use motion to recover some 3D information• different information from different views• combine into a single 3D model (hypothesis)

• Two methods:a. motion parallaxb. kinetic depth

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Position 1

Observer sees cube to the right of sphere:

(Overhead view)

• a. Motion parallax:• different viewpoints recover depth

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Position 2

Observer sees cube to the left of sphere:

(Overhead view)

• a. Motion parallax:• different viewpoints recover depth

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b. Kinetic depth• different views recover shape

QuickTime™ and aAnimation decompressor

are needed to see this picture.

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4. Object identification• motion pattern identifies object• visual form masked by “stuff”, such as

• trees, leaves• smoke• bad vision• distance• low lighting

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From http://www-white.media.mit.edu/~jdavis/MotionTemplates/motiontemplates.html

QuickTime™ and aVideo decompressor

are needed to see this picture.

Davis

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QuickTime™ and aPNG decompressor

are needed to see this picture.

From my research!

PLD

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Point light displays (Johansson)• separate visual form and motion pattern• robust detection even with distractors

– separation of figure and ground• accurate determination

– sex, friends, animals, emotional affect, action• humans only really good at recognizing

humans– relies on human motor control system– “if I could move like that then it could be like me”

• but humans are pretty good at recognizing everything

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Finally• What was the purpose of this

presentation?

• Which question remains unanswered?

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Detecting image motion

It moved! It moved!

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Motion detection versus perception• detection: low level process

– low level, like edge detection

• perception: higher level process– high level, like object identification

• how do we know that we have specific neurons for motion detection?

• how do we know that we have specific centers for motion perception?

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Specific neurons for motion detection• motion after effect

– similar to colour after effect (“negative” after starring at image)

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Bang! The Canadian Flag!

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Motion aftereffect

• shape not affected• adaptation to

motion independent of shape

• separate systems for motion and shape!(and colour!)

QuickTime™ and aAnimation decompressor

are needed to see this picture.

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Neurons specialized for motion detection• 1960-1970s

– frogs (“bug”, “predator” detectors)– house fly (optic flow)– rabbit– cat (string, yarn -- just kidding)– monkey

• model proposed: Reichardt detector

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Input 1 Input 2

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Motiondetected

Reichardt detector• correlator + two different inputs

– one input has a delay

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Input 1 Input 2

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Directionally sensitive• fires for change in one direction, not in other

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Temporally sensitive• fires for change over particular interval• change too fast: first spike arrives too late• change too slow: first spike arrive too early

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Take home message

only a particular speed and directionsets off any particular Reichardt detector

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Apparent Motion (NOT phi motion)1. A visible item suddenly disappears2. A new item appears soon afterwards at

neighboring location• Perception:

the original item “moves” to a new location

• Where have you seen this?

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Apparent Motion (NOT phi motion)1. A visible item suddenly disappears2. A new item appears soon afterwards at

neighboring location• Perception:

the original item “moves” to a new location

• Where have you seen this?

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Apparent motion constraints1. not influenced by cognition - pops out2. short spatial range (< 0.25°)3. short temporal range (< 80ms)4. many objects at once5. very much like “real” motion detection

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MotiondetectedInput 1 Input 2

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Motiondetected

Explanation of apparent motion• Reichardt detector

– can detect continuous motion

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Input 1 Input 2

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Motiondetected

Explanation of apparent motion• Reichardt detector

– can detect sudden “jumps”– cannot distinguish them

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Applications of apparent motion• Movies and Television

– Reichardt detectors cannot distinguish “real” from “artificial” motion

• Present a sequence of frames/static images– seen the same as real motion

• Requires proper timing– separation between frames < 80ms

• Movies: 24 FPS = 1/24 seconds = 41ms• TV: 30 FPS = 1/30 = 33ms

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Importance of context• Reichardt detectors do not completely

account for motion perception• Just as lateral inhibition does not

completely account for brightness perception

• Context counts!– Apparent motion of the human body– Perception of motion can occur (or be

misperceived) -> induced motion

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Apparent motion of the human bodyShiffrar and Freyd (1990)1. image of human in one pose2. delay3. image of human in second pose4. chose from one of four motion pathsresults: physically plausible paths require

delays approximately equal to real world motion

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Shiffrar and Freyd: Implications• brain takes into account typical

movement patterns• brain “knows” how long a movement

should take• brain uses motor control and perception

centers to form perceived motion path

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Downing et al (2001)• EBA: extrastriate body area• active when viewing human bodies

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Is object/world moving or eye??• see Gregory pp 99-105

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Finally• What was the purpose of this

presentation?

• Which question remains unanswered?