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Motion Perception
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Why is motion perception important?
• Orientation and navigation
– calculating direction of heading
– avoiding/catching approaching objects
• Separation of figure and ground
– introduction of movement defines an object
• Defining object shape
– successive views can be integrated into a full percept
• Attracting attention
– detection of moving object is easier than detecting same target when stationary
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What is movement and how is it coded?
• Naïve view is that it is the motion of an image across the retina
• Much evidence to show that this is not always the case
– When do we perceive movement?
– Consider these cases:
1) something moves in the world- image moves across our retina
2) something moves and we track it with our eyes - doesn’t move across our retina
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What is movement and how is it coded?
Several different types of movement
A.“Real” movement - retinal image motion
B.Apparent movement - seen in the absence of physical motion
C.Induced movement - inappropriate attribution of movement
D.Optic flow
• Also need to consider how to distinguish external from self-motion
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A. The perception of real motion
• This is the most fundamental capacity
• Relies on successive stimulation of receptors
• Served by direction selective neurons in visual cortex
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Mechanisms for perception of real movement:
Direction selective neuron
A directionally sensitive neural circuit:
• Direction and speed sensitive
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• “Reichardt detector”
• delay and compare
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Higher-level neural processing of motion
• Cells in the medial temporal area (MT) of the cortex can extract “global” motion
information i.e. motion independent of form
• Demonstrated through use of random dot kinematograms (RDKs) measuring coherent
motion perception
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RDKs
50%
Plaids
5%
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Damage to MT: the motion blind patient
• Patient with damage to MT lost ability to perceive motion. Can recognise
change in position, but not the motion itself
• Similar consequences seen in monkeys with specific lesions in MT area
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“She had difficulty, for example, in pouring tea or coffee into a cup because the fluid appeared to
be frozen, like a glacier. In addition, she could not stop pouring at the right time since she was
unable to perceive the movement in the cup (or a pot) when the fluid rose. . . .
In a room where more than two other people were walking she felt very insecure and unwell, and
usually left the room immediately, because "people were suddenly here or there but I have not
seen them moving." . . . She could not cross the street because of her inability to judge the
speed of a car, but she could identify the car itself without difficulty. "When I'm looking at the
car first, it seems far away. But then, when I want to cross the road, suddenly the car is very
near.” (Zihl, von Cramon, and Mai, 1983)
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B. Apparent motion
• Apparent motion refers to any situation in which motion is perceived in the absence of
physical movement in the stimulus
• Several different kinds of apparent motion:
1. Stroboscopic motion - movies, signs
2. Illusory movement - the phi phenomenon
3. Motion aftereffects
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1. Stroboscopic movement:
• Mimics real movement
• Based on sequential presentations of objects in slightly different locations
• Basis for movies, TV images, moving sign displays etc.
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• Because the visual system integrates information over time, the sequences of still
pictures is seen as a continuous stream
• Can demonstrate limits of stroboscopic movement by viewing real movement under
stroboscopic illumination
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2. Illusory Movement
• Occurs whenever movement is seen, even though nothing is physically
moving
• Several different kinds of illusory movement
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Apparent motion: Phi phenomenon
• Wertheimer, 1912
• perceiving continuous motion from ‘snapshots’
• Illusory movement can occur when 2 or more lights are flashed in sequence
• Depending on the interstimulus interval and the spatial arrangement of the lights,
different forms of illusory movement are seen
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Simultaneous flickering
(< 30 ms -- no motion)
Partial movement
(~ 30 -60 ms)
Phi motion
(beginning at ~60 ms)
Sequential alternation
(successive-- no movement
above 300 - 300 ms)
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Korte’s Laws
1.Separation vs intensity
• Large separations need higher intensity
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2.Rate vs intensity
• Slower rates need higher intensity
3.Separation vs rate
• Large separations need slower rates
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Correspondence problem in apparent motion: which
objects go with which?
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Colour
Shape
• Orientation, size, shape do matter in single trial event
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Shortest path-- straight line assumption
3. Motion aftereffects: the Waterfall illusion
Robert Addams (1834), Foyers, Scotland
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More motion aftereffects
Physiological explanation of MAE
• Explain with reference to directionally-selective neurons
• In resting state, all neurons are firing at low spontaneous rates
• During adaptation, one group of these neurons is stimulated and eventually
fatigued
• After adaptation, when a stationary stimulus is shown, level of activity in the
non-adapted neurons is higher, so movement in that direction seen
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Physiological explanation of MAE
C. Induced movement:
• the incorrect attribution of movement
• e.g. sitting next to train that moves, moon in clouds, etc.
• Occurs typically when larger object moves relative to a smaller one
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Vection is induced self motion
– Occurs when large part of visual field (especially the periphery) is filled with a moving texture.
Very strong sense of self-motion
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Swinging room
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• Toddlers placed in a swinging room may lose their balance
• The visual information overrides their vestibular systems and they adjust their posture
to match the visual input
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Information in the optic array
• An alternative to the physiological analysis is an information processing approach
• J.J. Gibson argued that the “optic array” provided all the necessary information for
movement perception
• Recent physiological studies have provided evidence for mechanisms that process this
kind of information
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Local disturbances in optic array
• Provides a way of separating internal (eye movement, self-movement from other object
movement)
• Local movement
– Background will stay stationary when object moves
• Accretion and deletion
– Object moving in front will cover and uncover background
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D. Optic flow:
• J.J. Gibson regarded optic flow as the essential cue for navigation
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Self motion and optic flow
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• Depending on heading and speed, optic flow field will change in a systematic way
• This information can be used for steering oneself in the environment
• Can also be used for judging “time to impact”
– based on rate of angular expansion
• Physiological evidence available for expanding and contracting motion
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Disentangling self-produced from external movement:
• If movement is defined in terms of image motion across the retina, how can you
explain the phenomena shown above?
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The corollary discharge model:
• MS - motor signal
• IMS - image movement signal
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• CDS - corollary discharge signal
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•
•
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CDS sends a copy of its motor signal to the comparator
Comparator compares image movement with CDS
If they agree, then attribution of external motion is cancelled
If they disagree, then external motion is perceived
Motion as a way to organise perception:
• Gestalt principles also seem to apply to moving objects
• Movement can separate out an object from its background
• Movement can define biological organisms
• Movement can give depth information
• Motion capture
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Biological motion
– If small lights are attached to joints, we are able to extract information about what a person is
doing if the individual is moving
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Biological motion
• example of integration of local motion to give a global motion percept
• can easily judge gender of walker
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Structure from motion:
• The kinetic depth effect
– When the shadow of a rotating 3D object is cast on a screen, it looks threedimensional, even though the shadow is flat.
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Kinetic depth
• depth conveyed by velocity and direction of each dot
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Depth information from motion
• rigidity heuristic: if we can interpret the motion of an object as rigid, we will
perceive it that way
• this may lead to the impression of depth
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Stereokinetic effect
Structure-from-motion-- camouflage
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Motion capture
• Texture within a moving object will tend to move with the object
• Serves to solve the correspondence problem
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Top-down motion processing
• Visual system seems to apply heuristics to interpret the meaning of certain
movements
• “Movement tends to continue in one direction”
• “Movement obeys occlusion rules”
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