Week 7: Spatial Vision and Perception of Movement

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Week 7 Spatial Vision and Perception of Movement
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Week 7 Outline

• Contour and Contrast Perception
• Gratings, Gabors, and Spatial Frequency Analysis
• Contrast Sensitivity
• Selective Adaptation
• Perception of Movement
• Motion Aftereffects
• The Aperture Problem
• Structure from Motion, Optical Flow and
  Biological Motion

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Contour and Contrast Perception

• Contours
• Typically defined by luminance boundaries
  (differences in brightness)
• But sometimes defined by iso-luminant
  (equal-brightness) boundaries of
• Color
• Texture
• Binocular disparity

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Contour Enhancement Lateral Inhibition

• Lateral Inhibition the limulus (the horseshoe
  crab) http//www.mbl.edu/animals/Limulus/vision/
• Lateral Inhibition in human vision
  Center-surround receptive fields

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Visual Phenomenon Related to Center-Surround
Structure of Receptive Fields

• Hermann Grid Mach Bands
• Lightness Contrast

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Perception Responds to Changes

• Contours, edges, and borders are defined by
  changes in luminance, texture, color, etc.
• Stimuli with no changes
• Ganzfield Textureless field of uniform
  brightness
• Stabilized Images

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Quantifying Changes in Luminance Spatial
Frequency

• Spatial Frequency the number of variations in
  luminance (light-dark cycles) over a given space
• Gratings stimuli
• where
• A is the amplitude,
• f is the frequency,
• f is the phase, and
• x is horizontal position
• Left high amplitude, Right low amplitude
• Top vs. Bottom phase change

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Quantifying Changes in Luminance Spatial
Frequency (2)

• Differences in Frequency
• Left grating lower spatial frequency
• Right grating higher spatial frequency
• Complex Spatial Frequency Gratings
• Where N is the number of sine wave components

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Gabor Patch Stimuli
Sine wave grating convolved with Gaussian Gabor
Patch


Brightness


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Fourier Analysis and Synthesis Frequency vs.
Space Representations
X ? X ? Frequency (Hz)
Fourier Analysis ? ? Fourier Synthesis
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Vision and Spatial Frequency Analysis

• Neurons in area V1 are spatial-frequency tuned
  they respond to stimuli of a particular spatial
  frequency band
• Response is similar to the frequency structure of
  a Gabor patch

X ? X ? Frequency (Hz)
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Contrast Sensitivity

• Contrast Sensitivity
• variation in sensitivity to contrast (luminance
  boundaries) as a function of spatial frequency
• Operationally, this is defined as the ability of
  the human observer to perceive the lines in a
  grating as a function of the gratings frequency
  and contrast

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Selective Adaptation Spatial Frequency Channels

• How do we know that the visual system codes the
  spatial properties of a scene by frequency?
• The phenomenon of selective adaptation
• After clicking to advance to the next (more
  clear) slide
• close one eye
• move your open eye back and forth on the black
  rectangle between these two gratings for 1 min to
  adapt
• click to go on to the test stimulus

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Selective Adaptation Adapting Stimulus
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Selective Adaptation Test Simulus
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Selective Adaptation Discussion

• When you viewed the test stimulus, for which
  grating did the bars appear larger? Top or
  bottom?
• This slide repeats the test stimulus to the
  right. Do the bars look the same size now that
  your adaptation has worn off?
• Adaptation can occur with non-grating stimuli as
  well. You can repeat the demonstration with the
  next two slides.

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Selective Adaptation Dots
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Selective Adaptation Words
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Selective Adaptation How does it Work?
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Movement Perception

• Functions of movement perception
• Can you see the object in this scene?

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Movement Perception

• Now you probably can see the object!
• Movement helps parse an object from a noisy
  background

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Movement Perception

• Types of movement
• Real
• Induced
• Autokinetic
• Stroboscopic (apparent)

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Stroboscopic (apparent) Movement

• When two stationary lights are alternately
  flashed, a perception of movement (apparent
  movement) occurs
• Max Werteimer whole is greater than the sum of
  its parts
• Apparent movement depends on the inter-stimulus
  interval (ISI)

off
on
off
on
off
Light status
Time? ISI
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Stroboscopic (apparent) Movement

• Apparent movement depends on the inter-stimulus
  interval (ISI)
• Long ISI (400 msec) two flashing, but
  stationary lights are perceived

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Stroboscopic (apparent) Movement

• Medium ISI (200 msec) phi movement movement
  perceived but object not actually seen to move
  through intermediate locations

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Stroboscopic (apparent) Movement

• Short ISI (50 msec) beta movement movement of
  object through intermediate locations is
  perceived (classic apparent movement)

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Movement Perception

• Motion and the eye-head movement system
• Perception of movement can result from
• an image moving across the retina
• a stationary image but the eyes moving to track
  an object
• Some combination of both eye and retinal movement

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Corollary discharge theory

• Because the motor areas of the brain control the
  movement of the eyes, these movement signals can
  be fed back to the visual system (a corollary
  discharge)
• The feedback signal allows us to perceive
• a stable world even as it flows across our retina
  due to an eye movement
• a moving object flowing across retina when eyes
  are stationary
• a moving object, even when its image is
  stationary on the retina but the eyes are moving
  to track it

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Movement Perception Bilocal detectors

• Retinal Motion Detection Mechanisms Bilocal
  detectors
• First proposed by Reichardt (see figure?)
• Luminance boundary first detected by receptor A,
  then as boundary moves to the right, receptor B
  detects it.
• Boundaries moving with speed Ds/Dt will result in
  maximum output signal (R)

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Movement Perception Bilocal detectors
Moving Object

• A simplified bilocal detector
• Object moving at velocity (V) stimulates receptor
  A, then after traveling distance s, receptor B
• Signal from A is delayed (Dt) by interneuron
• Comparator neuron sums the signals from A and B
• Maximum output if V Ds/Dt

Ds
Receptor B
Receptor A
Signal Flow
S
Dt
Output
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Motion Aftereffects

• Prolonged exposure (adaptation) to motion stimuli
  produces aftereffects
• Viewing a neutral test stimulus after adaptation
  results in perceived motion in the direction
  opposite the direction of the adapting motion
• Similar to perception of color and spatial
  frequency
• Observations of motion aftereffects
• Fast moving streams Aristotle (330 B.C) and
  Lucretius (56 B.C.)
• Long parades, moving spokes of a wheel Purkinje
  (1820)
• Waterfall Illusion Addams (1834)
• Traveling on Railways Brewster (1845)
• Rotating Spiral Plateau (1849)
• Motion Aftereffects online (http//www.ski.org/Car
  andini/demo/DizzySpiral/motion_aftereffect.html)

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Motion Aftereffects Ratio Model

• Motion is coded by the relative amount of
  activity across a group of neurons (population
  coding)

Physical Stimulus
Neural Events
Perception
Balanced outputs no movement signal overall
U
stationary
Pretest
D
U
D signal dominant
moving downwards
Adaptation
D
U
U signal dominant
moving upwards
Post-test
D
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The Aperture Problem

• The receptive fields of motion detectors have a
  limited size or aperture
• If the boundaries of a contour extend beyond the
  aperture, only the motion component perpendicular
  to the aperture is sensed
• Individual neurons do not accurately code motion
  direction

Perceived motion direction
actual motion direction
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The Aperture Problem
Perceived motion direction
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The Aperture Problem
Perceived motion direction
actual motion direction
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How Does the Visual System Solve the Aperture
Problem?
component direction

• Adelson and Movshon (1982)
• Intersection of constraint lines defines the true
  direction
• Requires two levels
• Detect component movement (constraint lines)
• Integrate components to determine direction of
  overall pattern motion
• Movshon et al. (1985)
• Cells that respond to component motions are found
  in V1 and MT
• Cells that respond to the pattern motion are
  found in MT

true motion direction
Constraint lines
component direction
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Structure from Motion and Optical Flow

• Kinetic Depth Effect stationary objects might
  appear flat or unformed, but once they start
  moving they depth and 3D structure are defined
• Example biological motion
• http//www.bml.psy.ruhr-uni-bochum.de/Demos/BMLwa
  lker.html

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Motion Contrast
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Motion Contrast
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Lateral Inhibition and Motion Contrast

• Middle Temporal Area(MT)
• Some units have inhibitory surrounds
• Motion shear detectors
• Inhibition tuned to direction halved at 60 deg,
  disappears at 90 deg
• Inhibition tuned to speed of surrounding movement
• Inhibition can explain motion contrast effect

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