Topic 8: Motion Perception Flashcards
Akinetopsia
a condition in which damage to an area of the cortex involved in motion perception causes blindness to motion
Event
a segment of time at a particular location that is perceived by observers to have a beginning and an ending
Event Boundary
the point in time when one events ends and another begins
Point-Light Walkers
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
Real Motion
the physical movement of a stimulus
contrasts with apparent motion
Illusory Motion
perception of motion when there actually is none
Induced Motion
the illusory movement of one object that is caused by the movement of another object that is nearby
Motion Aftereffects
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
Waterfall Illusion
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
Optic Array
the structured pattern of light created by the presence of objects, surfaces, and textures in the environment
Local Disturbance In The Optic Array
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
Global Optic Flow
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
Reichardt Detector
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
Output Unit
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
Delay Unit
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
Middle Temporal (MT) Area
brain region in the temporal lobe that contains many directionally selective neurons
Coherence
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
Transcranial Magnetic Stimulation (TMS)
presenting a strong magnetic field to the head that temporarily disrupts the functioning of a specific area of the brain
Microstimulation
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
Aperture Problem
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
Shortest Path Constraint
in the perception of apparent motion, the principle that apparent movement tends to occur along the shortest path between two stimuli
Biological Motion
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
Implied Motion
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
Representational Momentum
occurs when motion depicted in a still picture continues in an observer’s mind
What information is provided to us by motion?
- attracts attention
- provides information about the object’s three-dimensional shape
- provides information that helps us segregate figure from ground
- provides information that enables us to interact with the environment
What was the Ramachandran & Gregory (1978) study on how the magno handles motion?
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
What is the Reichardt motion-detector circuit?
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)
What are the problems with the Reichardt motion detector circuit?
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
What is the solution to the problems of the Reichardt motion detector circuit?
take motion of eyes into account
What are the components of corollary discharge theory?
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)
What is motion perception with CDS?
- observing afterimage in a dark room (retinal image stationary, eyes moving)
- pushing on eyeball but keeping eyes stationary (muscles push back, causing CDS)
- pursuit: moving object, eyes move with it (e.g., tracking a car driving past: no IDS)
- eyes paralyzed, try to move your eyes (retinal image stationary, but CDS sent)
What is the interstimulus interval (ISI)?
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)
What does the perception of apparent motion depend upon?
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
What is the motion correspondence problem?
which elements in frame 1 correspond with which elements in frame 2
for N elements, there are N! possible interpretations
What is the solution to the motion correspondence problem?
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
What is the Wagon Wheel illusion?
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
What is a “point-light walker”?
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
What can a “point-light walker” be used for?
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)
How could the recognition of “point-light walkers” be due to evolution?
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
What is the uncanny valley?
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
What are the pros of the uncanny valley?
may provide a framework for making robots/CGI characters more humanlike
may contribute to an understanding of social interaction
What are the cons of the uncanny valley?
shinwakan may be a confounding of strange/familiar and eerie/nor eerie
criticized as pseudoscientific
evidence equivocal
What are the two requirements for motion pictures to be perceived correctly?
on-screen image must be flicker-free
slightly different frames must be shown in succession
Why does an on-screen image have to be flicker-free for a motion picture to be perceived?
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
Why must slightly different frames be shown in succession for a motion picture to be perceived?
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
What two motion detection systems were proposed by Braddick (1974)?
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
