Transcript Ch. 2 the LGN and Striate Cortex

Sensation & Perception
Ch. 4: The Visual Cortex and Beyond
© Takashi Yamauchi (Dept. of Psychology, Texas A&M
University)
Main topics
Spatial layout
Columns, location, orientation & ocular dominance
Pathways for what, where and how
Modularity: faces, places & bodies
Evolution and plasticity
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Processing past the retina
• LGN (lateral geniculate nucleus)
• V1 (Primary visual cortex/striate cortex)
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General principles
• Functional segregation
• Divide and conquer
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Lateral Geniculate Nucleus
Image courtesy of Dr. Paul Wellman
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LGN
• Input & output
• Organization of LGN
– 2 types
– 6 layers in total
• Retinotopic map
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Input & output to and from LGN
T: thalamus
L: Other LGN neurons
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The organization of LGN
• LGN has 6 layers:
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LGN has 2 types of main layers:
• Parvocellular layers
• Magnocellular layers
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Why different layers?
• What distinguishes these layers?
• Any guess?
Functional differences:
Magnocellular Layers: carry information
necessary to detect motion
Parvocellular Layers: carry information
necessary to detect color, fine textures
and patterns.
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What does this tell?
• The human brain perform many specialized
tasks.
• The human brain is organized functionally.
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The Striate Cortex (V1)
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V1 (Striate Cortex)
Image courtesy of Dr. Paul Wellman
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V1 (Striate Cortex)
• Main characteristics:
– Functional specialization and columnar organization
• What functions? Any guess?
•Neurons in V1 are specialized to respond to
specific aspects of stimuli.
•Neurons in V1 can detect a line and its
orientation.
•Neurons in V1 are organized in columns.
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Single cell recording
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Detecting a line and its
orientation?
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Detecting a line and its
orientation?
A particular neuron(s) in V1 respond only
when the line has a specific orientation.
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Copyright © 2002 Wadsworth Group. Wadsworth is an imprint of the
Wadsworth Group, a division of Thomson Learning
Simple cortical cell receptive field
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• Video
• Single Cell Recording
– Hubel & Wiesel
• http://www.youtube.com/watch?v=IOHayh06LJ4
&feature=related
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Neurons in Striate Cortex - continued
• Complex cells
– Like simple cells
• Respond to bars of light of a particular
orientation
– Unlike simple cells
• Respond to movement of bars of light in
specific direction
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Neurons in Striate Cortex - continued
• End-stopped cells
– Respond to:
• Moving lines of specific length
• Moving corners or angles
– No response to:
• Stimuli that are too long
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3 types of neurons in V1
• Simple cells
– Detect lines and their orientations
• Complex cells
– Detect lines, their orientations and their
movement
• End-stopped cell
– Fire to moving lines of a specific length
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V1 (Striate Cortex)
Image courtesy of Dr. Paul Wellman
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Representing spatial layout
• Retinotopic map  preserving the spatial
layout of a stimulus
• LGN and V1 (primary visual cortex) show a
retinotopic map.
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Cortical magnification factor
• Fovea accounts for .01% of retina
• Signals from fovea account for 8% to 10% of
the visual cortex
Retina
V1 (primary
visual cortex)
fovea
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Fig. 2.11, p.53
Selective adaptation of neurons
• Neurons are like specialized feature
detectors.
– They detect particular perceptual properties of
stimuli
• E.g., brightness, direction, color, size, orientation,
• Neurons tuned to specific stimuli fatigue
• Neuron’s firing rate decreases
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• This causes a modification
of perception.
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Demonstration
• Contrast sensitivity
• Waterfall illusion
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Selective adaptation to size
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Narrow-bar
neurons
wide-bar
neurons
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Narrow-bar neurons
 fatigued
wide-bar neurons
 fatigued
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Selective adaptation to orientation
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Organization of V1
• Columnar organization
– Orientation columns
– Location columns
– hypercolumns
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V1
LGN
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Ocular dominance columns
Visual information
from different eyes
is stored in
separate columns
in an alternate
manner
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Higher-level visual processing
• The brain processes increasingly more complex
visual information separately.
• And then combine them together.
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Pathways for what, where, and how
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Two visual pathways (what &
where/how systems)
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Image from Neuroscience, 2nd Ed. (2000).
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Two visual pathways
• The information about “what” and the
information about “where” & “how” is
segregated.
• Video clip: “The Brain: Vision & Motion”
(1995)
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Lesion studies
• Removing a particular part of a brain.
• Test what happens after the removal.
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The monkey had to select the
correct object to get the food
reward
Object
discrimination
task
Landmark
discrimination
task
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The monkey had to select
the food well close to the
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cylinder.
Double dissociation
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Object
discrimination
task
Can’t do the object
discrimination task
but can do the
landmark
discrimination task
Landmark
discrimination
task
Can’t do the landmark
discrimination task
but can do the object
discrimination task
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Two visual pathways (where &
what systems)
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Image from Neuroscience, 2nd Ed. (2000).
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Two visual pathways (how &
what systems)
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Image from Neuroscience, 2nd Ed. (2000).
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Two visual pathways (how & what system)
Patient D. F had bilateral damage to
the ventral path.
Carbon monoxide poisoning at
age 35.
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Two visual pathways (how & what
system)
D. F
Control
Matching task:
match the orientation
of the card to that of
the slot.
Posting task:
The orientation
of the slot was
changed in
each trial.
Post the card into
the slot.
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• NPR: Blind man sees with subconscious
eye (12/23/08); Interview and video record
– http://www.npr.org/templates/story/story.ph
p?storyId=98590831
• A blind man who damaged the occipital
lobe can still navigate and walk without
bumping into objects.
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Modularity: structures for faces,
places, and bodies
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Feature detectors
• Neurons that fire to specific features of
a stimulus
• Pathway away from retina shows
neurons that fire to more complex
stimuli
• Cells that are feature detectors:
– Simple cortical cell
– Complex cortical cell
– End-stopped cortical cell
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Single cell recording of neurons
in the temporal lobe
An electrode is
inserted here, and
neural responses are
measured when
stimuli are changed
gradually
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Neurons in this area respond to
complex stimuli like those
shown on the left.
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Modularity: Structures for
Faces, Places, and Bodies
– Inferotemporal (IT) cortex in monkeys
• One part responds best to faces while
another responds best to heads
• Results have led to proposal that IT cortex is
a form perception module
– Temporal lobe damage in humans results in
prosopagnosia
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Figure 4.18 (a) Monkey brain showing
location of the inferotemporal cortex (IT)
in the lower part of the temporal lobe. (b)
Human brain showing location of the
fusiform face area (FFA) in the fusiform
gyrus, which is located under the temporal
lobe.
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Prosopagnosia
•
•
•
•
Episode from
“The man who mistook his wife for a hat”
Video: Mind Eye
Also see “The man who mistook his wife
for a hat” by Oliber Sacks.
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Can copy them but can’t
name them
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Copies of the black (A)
and
the white (B) vertical
contour.
Copies of the black (A)
and
the white (B) diagonal
contour.
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Copies of the left subfigure (A)
The right subfigure (B)
And the central subfigure (C)
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How do neurons become
specialized?
• Experience-dependent plasticity in humans
– Experience causes changes in how neurons are
tuned.
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Figure 4.24 (a) Greeble stimuli used by Gauthier. Participants were trained to name
each different Greeble. (b) Brain responses
ch 4to Greebles and faces before and after 77
Greeble training.