Light the Eye and the Visual System

1
(No Transcript)
2
Motion detection versus perception

• What is the difference between Motion
  -Detection, Motion-Perception and
  Motion-Sensation?
• Detection low level process
• low level, like edge detection
• Perception higher level process
• high level, like object identification
• Sensation ?

3
Quiz List five ways to make a spot of light
appear to move

• Move the light
• Apparent Motion
• Turn off the light, turn on another
• Induced Motion
• Move a large framing object
• Auto kinetic effect
• View a dim light in a very dark room
• Movement after-effect
• Move something for a long time - then look at the
  light
• (Change the intensity of the light)

3
4
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

5
Perceived Motion is 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

6
(No Transcript)
7
Categorize into four possibilities
World in motion
Eyes in motion
Gregory Inflow/Outflow
8
Uses of Motion Information

• Relative velocity of observer and environment
• direction of heading, time to contact
• Segmentation of figure from ground
• disruption of camouflage techniques
• Recovery of 3D parameters
• motion parallax (3D depth)
• kinetic depth (3D shape)
• Object identification
• humans, friend/foe

9
Examples for Using Motion

• Segmentation of Figure-Ground
• Figure object that draws our attention
• Ground non-moving dots/contours/blobs
• Using Gestalt law of Common Fate
• items that move together belong together
• Use motion to recover some 3D information
• depth dimension is lost
• different information from different views
• combine into a single 3D model (hypothesis)
• Two methods
• motion parallax
• kinetic depth

10
Using to Motion recover 3D information The
Motion Parallax

• Different viewpoints recover depth

11
Using to Motion recover 3D information The
Motion Parallax

• Different viewpoints recover depth

12
Visual Perception of Real Motion

• In the real world, static perception is rare,
  therefore is motion perception very important.
• Velocity detection thresholds are influenced by
  several factors
• uncertaincy about motion direction
• presence of stationary backgound
• We percieve the world as stable when moving.
• Optic flow gives us great information about
  motion.

13
Movement of the Observer

• Several question emerge when you consider a
  person moving and looking at the world
• Why does the world seem stationary when moving?
• What information allows people to detect their
  direction when moving?
• Can we get confused with motion?
• When do we perceive motion better without
  ego-motion or with?

14
Movement of the Observer

• Perceiving a Stable World Despite Motion
• The world remains stable in spite of your
  movement through the world, -gt we attribute the
  retinal change to our own movements!
• But how? -gt Several processes evaluate the visual
  input in comparison with own body movements
  (Wallach).
• Perceiving the Direction of the Observer Motion
• Direction is determined by the optical flow
  field.
• Gibsonian approach uses optic flow a lot(What
  was the Gibsonian approach about??)

15
Movement of the Observer

• Speed and direction of objects/observer
• Optic flow - pattern of motion in image when
  observer/object moves

Source http//www-sop.inria.fr/robotvis/personnel
/pkornp/These/FO/cube.html
16
Using Optic Flow

• Used to determine
• direction of heading
• time to collision
• approach/avoidance
• Types of optic flow caused by global motion
• expansion/contraction
• translation
• rotation
• Gibsonian approach uses optic flow a lot(What
  was it the Gibsonian approach is about??)

17
Movement of the Observer

• The Self-Motion Illusion
• We said that human can usually determine
  self-motion from optic flow.
• Sometimes you perceive that you move when you are
  not(more likely when moving objects are in the
  peripheral vision)
• Perception of the Speed of Movement
• In reality, under normal conditions, people are
  usually good at judging their speed of movement
• In virtual environments, there is markedly
  different performance

18
Movement of the Observer

• Experiment on Speed Estimation
• Participants walk on a treadmill wearing a HMD
  with simulated optical flow and try to match the
  speed of flow to the speed of the treadmill
• Participants looked straight ahead
• Treadmill ran at 3 mph ( a fast walk )
• Results
• Subjects chose an optical flow corresponding to
  4.6 mph to match the speed of walking
• Straight-ahead optical flow in the HMD is
  perceived to be too slow for the actual movement
  speed

19
(No Transcript)
20
Illusory Movements

• In illusory movements the observer misperceives
  an object's motion.
• Some illusions lead us to misperceive
  trajectories of objects that are in motion.
• Examples ball games, airplanes
• Focus are on stationary objects that seem to move
• Stroboscopic Movement
• Autokinesis
• Induced Motion
• Movement Aftereffects

21
Stroboscopic Movement

• Is the Illusion of movement produced by a rapid
  movement of stimulation on different types of the
  retina.
• Same object moves by disappearing and appearing
  in at a different location within 80-100 ms.
• 100-200 ms no real perceived motion -gtassumed
  motion Phi motion
• above 200 ms no motion is detected/perceived

22
Stroboscopic Movement

• A visible item suddenly disappears
• A new item appears soon afterwards at neighboring
  location
• Perception ?
• Where have you seen this?

23
Stroboscopic Movement

• A visible item suddenly disappears
• A new item appears soon afterwards at neighboring
  location
• Perception The original item moves to a new
  location
• Where have you seen this?

24
Autokinesis

• Autokinesis occurs when stationary objects with
  no clear background appear to move.
• Probably produces by tiny spontaneous eye
  movements.
• some researcher disagree (Mack, 1986)
• test using sensors to track eye movements
• discovered relationship in direction between
  eye-movement and apparent motion.
• Cognitive factors play also an important role
• experience
• expectations

25
Induced Movement

• Appears when a visual frame of reference moves in
  one direction.
• Then a stationary target is moving in the
  opposite direction
• Example
• Dark room with two luminous shapes a triangle and
  a circle.When the triangle move to the right the
  observer will automatically perceive the circle
  to move to the left.
• Explanation
• motion is relative, also larger object more
  likely to be not moving.
• Self Experiment
• The moon is considered stationary, do the moving
  clouds produce the illusion that the moon
  slightly moves?

26
Motion Aftereffect

• Motion after effect
• similar to color after effect (negative after
  starring at image)

27
Motion Aftereffect
28
Motion Aftereffect

• Shape is not affected
• Adaptation to motion independent of shapelt
• Separate systems for motion and shape!(and
  color!)

29
(No Transcript)
30
Theoretical Explanations

• Motion perception is a basic, accurate and
  complex perceptual process.
• can perceive self-motion (aka. Ego-Motion)
• can perceive motion of object around us
• can differentiate between Ego-Motion and
  Independently-Moving-Objects (IMO)
• How do these processes work? Well several
  theories
• Corollary Discharge Theory
• Direct Perception Approach
• Computational Approach.

31
Corollary Discharge Theory

• Recording this theory the visual system compares
  the movements registered on the retina with the
  eye movements.
• tries to explain why we don't perceive motion on
  eye movement.
• Argues that by sending signal for movement to the
  eye muscles, it sends a copy of this message to
  the Visual System.
• The copy is called the Corollary
  Discharge.(corollary means related)
• The structure to hold the corollary discharge has
  not been identified, yet.
• Researchers refer is as the comparison structure.

32
Corollary Discharge Theory
33
Corollary Discharge Theory

• The comparison structure then compares the input
  from the retina image with the expected input to
  be produced by the corollary-discharge.
• Four scenarios, What do these explain
• No corollary discharge, no retinal change
• No corollary discharge, retinal change
• Corollary discharge, no retinal change
• Corollary discharge, retinal change

34
Direct Perception Approach

• Direct Perception Approach (Gibson), praise the
  stimulus and the richness of information it
  provides.
• Compared to the Corollary Discharge Theory which
  praises the processing done by the visual
  system.
• Theory concentrates on six source of information
  provided by motion stimuli.
• Relative Movement
• Occlusion/discclussion
• Image Size
• Motion Parrallax
• Motion perspective
• Binocular cues

35
Direct Perception Approach

• Relative movement
• We can tell if we or anm object is moving by
  noticing the motion in relative to it's
  backgound.
• Occlusion
• Eases segmentation between object and background.
• Tell us direction of movement.
• Image Size (object size)
• motion changes the percepted size of objects.
• example Cartoons, expanding backgrounds
• Motion Parrallax
• With a opposite argument since we have seen
  different persepectives, something is moving..

36
Direct Perception Approach

• Motion Perspective
• Is similar but more general to the motion
  parrallax
• Motion perspective occurs as images of objects
  flowing across the retina at different rates.
• On ego-motion (as in a car), stationary objects
  close to the car move faster (only in the retina
  image) as objects more distant
• Binocular cues
• all other information works but additional
  information is provided by an additional eye as
• the difference in motion between the left and
  right eye.(direction, speed)

37
Computational Approach

• Like the direct perception approach ackknowledges
  the richness of the stimuli, but states the
  problem of processing this information.
• Known problems are
• Correspondance problem
• Apperture problem
• Several solution proposed, but no solution is
  general accepted.

38
The Aperture Problem

• Each neuron in the visual system is sensitive to
  visual input in a small part of our visual field.
• Each neuron is looking at the visual field
  through a small window or aperture.
• Individual neurons early in the visual system
  respond to motion that occurs locally within
  their receptive field.
• the estimates from many neurons need to be
  integrated into a global motion estimate
• One way in which the aperture problem could be
  solved is empirically, through experience of the
  world.

39
Correspondance Problem

• Some problem as with stereopsis
• finding the same features of an object in
  different images(stereo left-gtright) in time
  (t0 -gt t1).

40
The Aperture Problem

• Can tell the motion if only looking on the inner
  circle.
• So the contextual information is required to
  percieve the correct motion

Orginal
Motion 1
Motion 2
41
PE-Questions

• Someone new to vision, might argue that we
  perceive motion on retinal change (when something
  move in our visual field). Why is this an
  inadequate statement and how should it be stated
  correctly?
• Look at the four scenarios about corollary
  discharge
• What can these scenarios explain?