Topic 8: Motion Perception

Question 1

Akinetopsia

Answer 1

a condition in which damage to an area of the cortex involved in motion perception causes blindness to motion

Question 2

Event

Answer 2

a segment of time at a particular location that is perceived by observers to have a beginning and an ending

Question 3

Event Boundary

Answer 3

the point in time when one events ends and another begins

Question 4

Point-Light Walkers

Answer 4

a biological motion stimulus created by placing lights at a number of places on a person’s body and having an observer view of the moving-light stimulus that results as the person moves in the dark

Question 5

Real Motion

Answer 5

the physical movement of a stimulus

contrasts with apparent motion

Question 6

Illusory Motion

Answer 6

perception of motion when there actually is none

Question 7

Induced Motion

Answer 7

the illusory movement of one object that is caused by the movement of another object that is nearby

Question 8

Motion Aftereffects

Answer 8

an illusion that occurs after a person views a moving stimulus and then sees movement in the opposite direction when viewing a stationary stimulus immediately afterward

Question 9

Waterfall Illusion

Answer 9

an aftereffect of movement that occurs after viewing a stimulus moving in one direction, such as a waterfall

viewing the waterfall makes other objects appear to move in the opposite direction

Question 10

Optic Array

Answer 10

the structured pattern of light created by the presence of objects, surfaces, and textures in the environment

Question 11

Local Disturbance In The Optic Array

Answer 11

occurs when one object moves relative to the environment, so that the stationary background is covered and uncovered by the moving object

this local disturbance indicates that the object is moving relative to the environment

Question 12

Global Optic Flow

Answer 12

information for movement that occurs when all elements in a scene move

the perception of global optic flow indicates that it is the observer that is moving and not the scene

Question 13

Reichardt Detector

Answer 13

a neural circuit proposed by Werner Reichardt, in which signals caused by movement of a stimulus across the receptors are possessed by a delay unit and an output unit so that signals are generated by movement in one direction but not in the opposite direction

Question 14

Output Unit

Answer 14

a component of the Reichardt detector that compares signal received from two or more neurons

according to Reichardt’s model, activity in the output unit is necessary for motion perception

Question 15

Delay Unit

Answer 15

a component of the Reichardt detector proposed to explain how neural firing occurs to different detectors of movement

the delay unit delays teh transmission of nerve impulses as they travel from the receptors toward the brain

Question 16

Middle Temporal (MT) Area

Answer 16

brain region in the temporal lobe that contains many directionally selective neurons

Question 17

Coherence

Answer 17

in research on movement perception in which arrays of moving dots are used as stimuli, the degree of correlation between the direction of the moving dots

zero percent coherence means all of the dots are moving independently

100 percent coherence means all of the dots are moving in the same direction

Question 18

Transcranial Magnetic Stimulation (TMS)

Answer 18

presenting a strong magnetic field to the head that temporarily disrupts the functioning of a specific area of the brain

Question 19

Microstimulation

Answer 19

a procedure in which a small electrode is inserted into the cortex and an electrical current passed through the electrode activates neurons near the tip of the electrode

this procedure has been used to determine how activating specific groups of neurons affects perception

Question 20

Aperture Problem

Answer 20

occurs when only a portion of a moving stimulus can be seen, as when the stimulus can be seen, as when the stimulus is viewed through a narrow aperture or through the “field of view” of a neurons’ receptive field

this can result in misleading information about the direction in which the stimulus is moving

Question 21

Shortest Path Constraint

Answer 21

in the perception of apparent motion, the principle that apparent movement tends to occur along the shortest path between two stimuli

Question 22

Biological Motion

Answer 22

motion produced by biological organisms

most of the experiments on biological motion have used walking humans with lights attached to their joints and limbs as stimuli

Question 23

Implied Motion

Answer 23

when a still picture depicts an action that involves motion, so that an observer could potentially extend the action depicted in the picture in his or her mind based on what will most likely happen next

Question 24

Representational Momentum

Answer 24

occurs when motion depicted in a still picture continues in an observer’s mind

Question 25

What information is provided to us by motion?

Answer 25

1. attracts attention
2. provides information about the object’s three-dimensional shape
3. provides information that helps us segregate figure from ground
4. provides information that enables us to interact with the environment

Question 26

What was the Ramachandran & Gregory (1978) study on how the magno handles motion?

Answer 26

used isoluminant stimuli: contains lines and forms which differ from the background only on the basis of hue

there are lightness differences

stimuli were moved against the background

difficult to perceive motion

conclusion: magno handles motion; parvo not responsible

Question 27

What is the Reichardt motion-detector circuit?

Answer 27

receptors A-F are pointed at different locations in space

cells G, I, K inhibit neighbors when motion is to the right

cells H, J, L pass on activity when motion is to the left

cell M signals leftward motion (does not signal motion to the right b/c of lateral inhibition)

Question 28

What are the problems with the Reichardt motion detector circuit?

Answer 28

following a moving object with your eyes (pursuit): object is perceived as moving, even though retinal position doesn’t change

sweeping you eyes across a stationary scene (saccade), it appears to remain motionless

Question 29

What is the solution to the problems of the Reichardt motion detector circuit?

Answer 29

take motion of eyes into account

Question 30

What are the components of corollary discharge theory?

Answer 30

image displacement signal (IDS): detects motion across retina

motor signal (MS): muscle-movement info sent to eye muscles

corollary discharge signal (CDS): copy of motor signal

comparator: determines perception of motion; occurs when IDS present or CDS present (but not both)

Question 31

What is motion perception with CDS?

Answer 31

1. observing afterimage in a dark room (retinal image stationary, eyes moving)
2. pushing on eyeball but keeping eyes stationary (muscles push back, causing CDS)
3. pursuit: moving object, eyes move with it (e.g., tracking a car driving past: no IDS)
4. eyes paralyzed, try to move your eyes (retinal image stationary, but CDS sent)

Question 32

What is the interstimulus interval (ISI)?

Answer 32

apparent motion perception depends on timing

important variable is the ISI: time between offset of frame 1 and onset of frame 2

< 30 ms: simultaneously
60 ms: continuous movement
>200 ms: succession (mind won’t stitch together)

Question 33

What does the perception of apparent motion depend upon?

Answer 33

distance between stimuli

frame duration

stimulus onset asynchrony (SOA)

SOA law: changes in ISI and frame duration do not affect motion perception unless there is also a change in SOA

Question 34

What is the motion correspondence problem?

Answer 34

which elements in frame 1 correspond with which elements in frame 2

for N elements, there are N! possible interpretations

Question 35

What is the solution to the motion correspondence problem?

Answer 35

nearest-neighbor principle: select alternative that minimizes overall “traveling distance” of elements

match is made on intensity; not necessarily shape

in absence of intensity differences, texture is used

Question 36

What is the Wagon Wheel illusion?

Answer 36

occurs when a rotating spoked wheel is perceived as rotating backwards; often seen in films or on TV

due to a discrepancy between the speed of rotation and the rate of filming

with small angular displacement (i.e., slow rotation speed), a spoke in one frame is correctly marched to the nearest neighbor (itself) in the next frame

however, at higher speeds there is greater angular displacement, and the nearest neighbor in the next frame may cause misinterpretation of rotation direction

interestingly, this effect may obtain in continuous light (not strobscopically, as on film)

this may be due to adaption of certain motion-sensitive neurons as in motion aftereffects

Question 37

What is a “point-light walker”?

Answer 37

attached 12 light bulbs to joints of a co-worker

filmed “point light walker” in a dark room

result is clearly identifiable as a person, even after only 200 ms

Question 38

What can a “point-light walker” be used for?

Answer 38

to identify dancing couples

to recognize one’s own walk (or that of friends)

to determine sex, within 4.4 seconds of viewing (males have greater shoulder than hip movement; vice-versa for women)

by cats (to detect other cats)

even when display is “masked” by randomly moving dots (albeit with decreased accuracy)

Question 39

How could the recognition of “point-light walkers” be due to evolution?

Answer 39

biological predisposition to recognize humans and animals

is preferred by infants by 4 months of age (but usually develops between 6-9 months)

associated with increased activity in superior temporal sulcus

Question 40

What is the uncanny valley?

Answer 40

as robots appear more humanlike, their shinwakan (affinity) increases until a valley is reached, in which subtle imperfections create a strangeness

motion creates a more intense reaction than a still entity

implication: robots should not be made into humanlike androids, but should be stylized to avoid entering the uncanny valley

has been applied to “creepy” computer graphics characters in movies and videogames

Question 41

What are the pros of the uncanny valley?

Answer 41

may provide a framework for making robots/CGI characters more humanlike

may contribute to an understanding of social interaction

Question 42

What are the cons of the uncanny valley?

Answer 42

shinwakan may be a confounding of strange/familiar and eerie/nor eerie

criticized as pseudoscientific

evidence equivocal

Question 43

What are the two requirements for motion pictures to be perceived correctly?

Answer 43

on-screen image must be flicker-free

slightly different frames must be shown in succession

Question 44

Why does an on-screen image have to be flicker-free for a motion picture to be perceived?

Answer 44

minimum 10 frames per second (fps) –> flickering movement

critical flicker fusion: frequency at which a succession of stimuli appear to be on continuously; ~50 fps

light projected through the shutter, which has rotating blades

blade covers the period of pulldown; film the stops momentarily

blade covers the period of pulldown; film then stops momentarily

film projectors show 24 fps with 2 blades –> 48 fps

Question 45

Why must slightly different frames be shown in succession for a motion picture to be perceived?

Answer 45

motion perception not governed by “persistence of vision”, which implies a positive afterimage that would cause frames to “bleed” together

persistence would lead to a smearing of successive images, and not necessarily to motion

“fine-grained” apparent motion over small distances (as in motion pictures) may activate the short-range motion system, and be treated as real motion

Question 46

What two motion detection systems were proposed by Braddick (1974)?

Answer 46

short-range: low-level; occurs within small visual angles (15-20’); governs real motion of stimuli

long-range: higher-level; occurs in larger visual angles (20’-6 degrees); mechanism for apparent motion