Basic Properties in Visual Perception

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Basic Properties in Visual Perception
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Todays
Lecture

• Summary of the previous lecture
• Brain systems involved in vision
• Theories of brain systems involved in vision
• Basic aspects of visual perception (color,
  motion, depth processing)

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Brain systems

• from retina to extrastriate cortex

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The eye - receiver of information

• Light sources sun, light bulbs, candles, moon
• Light reflects off of objects in
  environmentobjects effectively become light
  sources

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What do eyes do?

• why do most creatures have them?
• how do they do their work?
• what do they pass on to the brain?

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Photoreceptorreceptor of light photons

• photoreceptor cell transforms light into nerve
  impulses

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Photoreceptortransducer of light into neural
signal

• process transduction
• object transducer
• coding direct correspondence

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Human eye
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Human Eye
Human Eye

• lens eye
• array of photoreceptors retina
• rods and cones
• focusing cornea plus crystalline lens
• photoreceptors are backwards
• axons (nerves) leave through blind spot

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Retina

• Retina covered with light-sensitive receptors
• rods
• primarily for night vision perceiving movement
• sensitive to broad spectrum of light
• cant discriminate between colors
• sense intensity or shades of gray
• cones
• used to sense color
• sharpness of vision

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Retina

• Center of retina has most of the cones
• allows for high acuity of objects focused at
  center
• cones packed very tightly in fovea
• Edge of retina is dominated by rods
• allows detecting motion of threats in periphery

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Optic nerve

• axons of the ganglion cells
• 1 million optic nerves
• 120 million photoreceptors

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From light to vision
Lateral Geniculate Nucleus (LGN)
Striate Cortex
Geniculo-Striate Pathway
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Neural processing responsible for vision

• photoreceptors
• retina
• Rods and cons
• ganglion cells (optic nerve)
• optic nerves
• optic chiasma (X)
• lateral geniculate body
• striate cortex

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Parvocellular (P) and magnocellular (M) pathways

• The parvocellular or P pathway this pathway is
  most sensitive to color and to fine detail most
  of it comes from cones (in blobs region respond
  to color and in interblobs regions to location
  and orientation)
• The magnocellular or M pathway this pathway is
  most sensitive to information about movement
  most of it come from rods.

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Parvocellular (P) and magnocellular (M) pathways

• Neurons from P and M pathways mainly project to
  V1 (primary visual cortex).
• The P and M pathways are not totally segregated
  because there is an input from the M pathway into
  the P pathway

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Parvocellular (P) and magnocellular (M) pathways

• Figure 2.3

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Primary and Secondary Visual Cortex (V1 and V2)

• Retinotopic maps
• Receptive fields
• On-off cells Off-on cells
• Orientation sensitive cells (simple cells)
• Lateral inhibition

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Retinotopic maps in V1
Response in monkey primary visual cortex (V1)
measured by radio-active tracers
Stimulus pattern

• Retinotopic mapping locations on retina are
  mapped to cortex in orderly fashion. Note more
  of visual cortex is dedicated to foveal vision

Tootell, R. B., M. S. Silverman, et al. Science
(1982)
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Stimulus
Cortical Mapping Left Hemisphere
Cortical Mapping Right Hemisphere
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Revealing retinotopic maps with fMRI
From Geoff Boynton, SALK institute
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Revealing retinotopic maps with fMRI
From Geoff Boynton, SALK institute
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Single Cell Recording(usually in animal studies)
Measure neural activity with probes. e.g.,
research byHubel and Wiesel
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Receptive Fields (Hubel Wiesel, 1979)

• The receptive field (RF) of a neuron is the area
  of retina cells that trigger activity of that
  neuron

Simple cells (bar detectors)
On-off cells and off-on cells
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Receptive Fields (Hubel Wiesel, 1979)
Simple cells (bar detectors)

• Simple cells respond to bars
• Complex cells to straight line stimuli in a
  particular orientation plus to moving contours

• Hypercomplex cell respond to more complex
  pattern (e.g., line ending within a field)

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Hubel and Wiesel (1962)

• Studied LGN and visual cortex in the cat. Found
  cells with different receptive fields different
  ways of responding to light in certain areas

• What are cells 1, 2, and 3 doing ?
• detecting edges
• detecting oriented bars
• detecting movement in particular direction
• detecting cat faces
• What are likely locations for cells 1, 2, and 3?
• LGN
• V1 (primary visual cortex)
• V5

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A wiring diagram for building orientation-sensitiv
e cells out of on-off cells
Hierarchical organization of the brain by
aggregating responses over several on-off cells,
the brain can detect more complicated features
(e.g. bars and edges)
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Striate cortex(primary visual centre)

• Neurons are edge detectorsfires when an edge of
  a particular orientation is present

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Striate cortex(primary visual centre)

• Neurons are edge detectorsfires when an edge of
  a particular orientation is present

frequent output
vertical bar
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Striate cortex(primary visual centre)

• Neurons are edge detectorsfires when an edge of
  a particular orientation is present

infrequent output
diagonal bar
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Edge detection

• each cell tuned to particular orientation
• vertical
• horizontal
• diagonal
• cats only horizontal and vertical
• humans 10 degree steps
• edges at particular orientations and positions

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What is this cell coding for?a) any faceb)
monkey facec) human faced) eyese) hands
spike train each individual line represents a
neuron firing. The axis represents time
Bruce, Desimone Gross (1981)
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Information processed by neurons activating each
other in sequence
-output of one neuron input of
next -connection synapse
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But excitation is not the only way that neurons
interact
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Lateral inhibition

• If no activity in neighboring photoreceptors,
• output output of photoreceptor

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Lateral inhibition

• if activity in neighboring photoreceptors,
• output is decreased, possibly absent

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Lateral Inhibition

• Lateral inhibition sets up competition between
  neurons so that if one neuron becomes adept at
  responding to a pattern, it inhibits other
  neurons from doing so.

Light
On-Off Cells with lateral inhibition
Response ? Edge detection
DEMO APPLETS http//www.psychology.mcmaster.ca/4
i03/demos/lateral-demo.html http//serendip.brynma
wr.edu/bb/latinhib_app.html
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Lateral inhibition

• Enhances the contrast at the edges of objects
  thus making it easier to identify the dividing
  line between one objects and another

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Mach Bands and Lateral Inhibition
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Lateral Inhibition enhances edges
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Craik-Cornsweet-OBrien Illusion
Left part of the picture seems to be darker than
the right one. In fact they have the same
brightness.
The same image as above, but the edge in the
middle is hidden. Left and right part of the
image look to be equally dark
How is this different from mach bands?
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Another demo of the same effect
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Extrastriate cortex(beyond the striate cortex)
V1
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Extrastriate cortex

• Each area handles separate aspect of visual
  analysis
• V1-V2 complex Map for edges
• V3 Map for form and local movement
• V4 Map for colour
• V5 Map for global motion
• Each is a visual module
• connects to other areas
• operates largely independently

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Functional Specialization (Zeki, 1992, 1993)

• Spatially different areas are functionally
  specialized for processing visual attributes such
  as shape, color, orientation, and direction of
  motion
• Achromatopsia (damage to V4)
• cortical color blindness all color vision is lost
  and the world appears in shades of gray. And in
  achromatopsia, unlike as in blindness caused by
  damage to the eyes or optic nerve, even memory of
  color is gone
• Akinetopsia (damage to V5 or MT)
• or motion blindnessthe loss of the ability to
  see objects move. Those affected report that they
  perceive a collection of still images.

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Visual pathways

• path of visual information through brain
• starts with ganglion cells
• ends at cortical area
• maps at LGN, V1, etc

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Neuroscience approach Dissociation Between
Object vs. Spatial Visual Systems
(Jonides Smith, 1997 Kosslyn Koenig, 1992
Underleiger and Mishkin, 1982)
Spatial
Parietal
Prefrontal
Occipital
Visual-Object
Inferior temporal
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Dorsal and ventral stream

• Ungeleider Mishkin (1982)
• Lesion studies with monkeys
• Object discrimination
• Delayed non-matching to sample
• Monkeys with a bilateral lesion of the
  inferotemporal lobe are impaired on this task.

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Dorsal and ventral stream

• Ungeleider Mishkin (1982)
• Lesion studies with monkeys
• Spatial discrimination
• Landmark discrimination
• Choose the food well closer to the landmark.
• Monkeys with bilateral posterior parietal
  lesions are impaired on this task.

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Where parietal lobe -dorsal path

• LGNV5/MT, action, spatial vision, visually
  guided behaviour and action global movement

Parietal Lobe
V5/MT
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What temporal lobe - ventral path

• LGNV3 - object recognition through form and
  local movement

V3
Temporal Lobe
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What temporal lobe

• LGNV4V8
• object recognition through colour

V8
V4
Temporal Lobe
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Perception-action hypothesis

• Not where but how pathway
• Movie visual agnosia (problems with object
  recognition but not grasping) versus optic ataxia
  (severe impairment in visually guided reaching
  in the absence of perceptual disturbance)

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Summary of pathways

• Object structure (what I am seeing?)
• located in temporal lobe conscious?
• form perception
• colour perception
• Movement (how do I get this there?)
• located in parietal lobe unconscious?
• motion perception and planning

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Summary of pathways

• All pathways operate separately, in parallel
• Question how do we experience a unified world?

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Binding Problem

• If spatially different areas are functionally
  specialized for processing visual attributes such
  as shape, color, orientation, and direction of
  motion.
• then how does the brain then bind together the
  sensory attributes of an object to construct a
  unified perception of the object?
• ? Binding Problem

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Illusions

• tricking the processes that estimate properties
  of the world

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Four kinds of illusions

• Distortions
• Ambiguities
• Paradoxes
• Hallucinations

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Distortions

• Perception is not accurate
• e.g., incorrect size or shape
• Example Ponzo Illusion

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Distortions

• Perception is not accurate
• e.g., incorrect size or shape
• Example Ponzo Illusion

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Example 2 Mueller-Lyer illusion

• wings-out configuration seen as larger

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Example 2 Mueller-Lyer illusion

• wings-out configuration seen as larger

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Explanation of Mueller-Lyer illusion

• Inappropriate use of perspective and size
  constancy

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Explanation of Mueller-Lyer illusion

• Inappropriate use of perspective and size
  constancy

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How versus What pathways

• distortion illusions affect what pathway
• but not the How pathway
• e.g., perception confused, action not confused

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2. Ambiguities

• percept is not stable (alternates)
• Example 1 Necker cube

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2. Ambiguities

• percept is not stable (alternates)
• Example 1 Necker cube

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2. Ambiguities

• percept is not stable (alternates)
• Example 1 Necker cube

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2. Ambiguities

• percept is not stable (alternates)
• Example 1 Necker cube

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Explanation of Necker cube

• multiple high-level interpretations are
  compatible with image
• brain attempts to find (remember) structures
  compatible with data
• if more than one is found, the percept alternates
• not a blend of alternatives
• alternation much like binocular rivalry

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Example 2 Rabbit-duck
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Explanation of Rabbit-duck

• multiple high-level interpretations are
  compatible with image
• brain attempts to find (remember) structures
  compatible with data
• memory biased towards favorite interpretation
• if more than one is found, the percept alternates
• not a blend of alternatives
• alternation much like binocular rivalry

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3. Paradoxes

• No hypothesis possible -- no consistency
• Example 1 Impossible figure (Reuterswärd)

If interpreted as 3D, not possible for these
cubes to exist in the world
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Example 2 Impossible figure (McAllister)
If interpreted as 3D, not possible for this box
to exist in the world
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Example 3 Impossible figure (Escher)
If interpreted as 3D, not possible for this city
to exist in the world
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Explanation

• no hypothesis can account for the entire image
• brain can find local interpretations (e.g. cubes)
  based on rules such as T-junctions, shading, etc.
• interpretation dependant on local area and path
  of attention through image
• Result paradoxical percept
• different hypothesis for each part of the image

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4. Hallucinations (fictions)

• Hypothesis independent of reality
• e.g., seeing things that arent there
• Example 1 Illusory figure (Kanisza)

Perception of occluding triangle, even
though its not really there
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Explanation of illusory figure

• a triangle is imagined since it is the simplest
  account of image pattern
• visual completion
• brain hypothesizes such structures
• must be no evidence against the interpretation
• Note no replacement of image properties
• no filling in of triangular occluder

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Example 2 Vegetable Man (Arcimboldo)
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Explanation of illusory figure

• a man is imagined since it is the simplest
  account of image pattern
• abstract level -- overall form
• brain hypothesizes such structures
• even if details dont fit exactly
• day to day differences in your friends and family
• Note no replacement of image properties
• vegetables are still seen

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Four kinds of illusions

• Distortions
• Ambiguities
• Paradoxes
• Hallucinations
• One explanation Hypotheses formation via
• bottom-up information from images on retinas
• top-down knowledge from memory