The Visual System

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PSY 3520 Sensation and Perception
The Visual System
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Introduction and Psychophysics Lecture Outline

• Properties of Light
• A. Particle Theory
• B. Wave Theory
• C. Current Theory
• 1. Perceptual Correlates
• 2. Properties of Light (Refraction, Reflection
  Absorption)
• Primary Structures of the Eye
• A. Fibrous Tunic
• B. Vascular Tunic
• C. The Retina
• Focusing Light on the Retina
• A. Cornea Lens
• B. Focusing Problems
• The Retinal Pathway
• A. Photoreceptors
• 1. Transduction
• 2. Pigment Regeneration/Dark Adaptation
• 3. Factors in Brightness Perception
• B. Retinal Distribution of Photoreceptors

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• D. Cells of the Retinal Pathway
• E. Responses of Retinal Ganglion Cells
• F. Ganglion Cells
• 1. Receptive Fields
• 2. Classification
• G. Introduction to Neural Circuits
• 1. Lateral Inhibition
• 2. Hermann Grid
• 3. Mach Bands
• The Visual Pathways
• A. Tectopulvinar
• B. Geniculostriate
• The Visual Cortex
• A. Striate Cortex
• B. Receptive Fields in the Striate Cortex
• 1. Simple Cells
• 2. Complex Cells
• 3. Hypercomplex/End-stopped Cells
• C. Organization of the Primary Visual Cortex

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I. Properties of Light

• Particle vs. Wave Theory
• __________ light is made up of particle packets
  (photons) that travel in a straight line (Sir
  Isaac Newton)
• __________ light travels in a wave-like manner
  (James Clerk Maxwell)

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Properties of Light

• Current View - Louis de Broglie (1924)
• Particle masses traveling at a given speed have
  an associated wavelength.
• Light is electromagnetic radiation that travels
  _________ miles per second (in a vacuum)

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Properties of Light

• Light is measured as a wavelength and is
  described in nanometers (nm)

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Properties of Light

• There is a very large spectrum of electromagnetic
  radiation the visible spectrum is a very small
  part of this

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Properties of Light

• There are perceptual correlates of the physical
  measures of light
• Physical Perceptual Correlate
• ____________ ______________
• ____________ ______________
• ____________ ______________

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Properties of Light

• Intensity is measured using a photometer
• Brightness is the perception of the intensity of
  light
• Brightness and intensity are not always equal

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Properties of Light

• For a given wavelength we can estimate what
  hue/color will be perceived

Violet
360-400 nm
450 nm
Blue-violet
475 nm
Unique Blue
500 nm
Blue-green
505 nm
Unique Green
550 nm
Yellow Green
580 nm
Unique Yellow
600 nm
Orange
650 nm
Reds
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Properties of Light

• __________ amount of white or black in a color

Saturated Red
Add White
Add Black
Desaturated Red
Desaturated Red
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Properties of Light

• One of three things can happen if light hits a
  medium

Absorption
Refraction
Reflection
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II. Primary Structures of the Eye

• Three concentric layers of the eye each has a
  different function
• __________ protective function
• __________ nourishes the eye
• __________ detects light and initiates neural
  messages to the brain

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Primary Structures of the Eye

SCLERA
CHOROID / RETINAL PIGMENT EPITHELIUM
PUPIL

VISUAL AXIS
BLIND SPOT

ZONIAL / ZONULE FIBERS
CILIARY BODY
OPTIC NERVE
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Fibrous Tunic

• __________ white of the eye
• __________ front of the eye where outer coat
  loses the white color

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Vascular Tunic

• Choroid / Pigment Epithelium
• Anterior Chamber
• Ciliary body
• Aqueous Humor

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Vascular Tunic

• Iris, Pupil, Lens
• Crystalline Lens
• Capsule
• Epithelial Layer
• Lens
• Vitreous Chamber
• Vitreous Humor

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The Retina

• Innermost of the 3 layers of the eye
• Direct extension of the CNS Central Nervous
  System
• The light stimulus is focused on the retina

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III. Focusing Light on the Retina

• Two main structures that focus light on the
  retina
• _______ major refractive surface of the eye
• 2 reasons
• Curvature
• Refractive index
• _______
• Accommodation

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Cornea

• Cornea accounts for 70-80 of the refractive
  power of the eye
• Curved structure

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Cornea

• Index of Refraction n c/v
• n index of refraction
• c speed of light in a vacuum
• v speed of light in the medium

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Cornea
Cornea 1.38

• Refractive Index
• Air 1.0
• Water 1.33
• Cornea 1.38
• Aqueous 1.34
• Vitreous 1.34
• Lens 1.41

Air 1.0
Water 1.33
Cornea 1.38
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Lens

• ___________ change in the refractive power of
  the lens

Distant Focus
Close Focus
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Lens

• With age the lens hardens and the muscles weaken
• This results in a decreased ability to
  accommodate, which affects the near point

Near Point the point at which the lens can no
longer Accommodate to bring close objects into
focus
Age (yrs)
10 20 30 40
50 60 70
Distance of Near Point (cm)
7 10 14 21
42 100 400
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Clinical Aspects of Vision

• Defining Blindness
• the definition can vary from state-to-state
• it is most commonly defined as 20/200 vision that
  can not be corrected.
• Measured using a standard acuity test (E-chart)
• it measures foveal vision
• 20/20 normal observer
• 20/200 at 20 ft. see what a normal observer
• sees at 200 ft.

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Focusing Problems

• Focusing problems can be corrected
• Two sources of focusing problems
• __________ related to the cornea
• __________ related to the length of the eyeball

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Focusing Problems
__________ Normal Vision
20/20 vision
__________ Near-sighted
Refractive cornea too bulged Axial eye is too
long
__________ Far-sighted
Refractive cornea too flat Axial eye is too
short
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Focusing Problems

• ____________ misshapen cornea

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Treatment of Focusing Problems

• Glasses or Contact Lenses
• Get a prescription from the optometrist
• Reading a prescription
• Sphere
• Cylinder
• Axis

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Sphere

• This indicates the severity of myopia or
  hyperopia
• 2 types of lenses
• Concave - myopia
• Convex hyperopia
• Prescription is in diopters
• Negative diopters - myopia
• Positive diopters - hyperopia

Concave
Convex
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Cylinder Axis

• Cylinder - describes the amount of astigmatism
• Axis this is the degrees from the horizontal
  axis of the astigmatism
• Astigmatism is described in degrees from the
  horizontal axis
• The cylinder curves more in one direction or
  another

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Treatment of Focusing Problems

• Surgery
• RK (radial keratotomy)
• PRK (photorefractove keratectomy)
• LASIK (laser assisted in-situ keratomileusis)

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RK (Radial Keratotomy)

• Surgeon makes 4-8 cuts to flatten the cornea
• Only effective for myopia

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PRK (Photorefractive Keratectomy)

• A UV pulse laser is used to sculpt the epithelial
  layer of the cornea
• Tissue is removed from the surface of the cornea

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LASIK (Laser Assisted In-situ Keratomileusis)

• 3 Steps
• Thin layer of the cornea is folded back
• Excimer laser removes small section of the cornea
• Corneal flap is returned to position

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IV. The Retinal Pathway

• Five layers to mammalian retina
• 3 cellular layers
• Outer Nuclear Layer
• Inner Nuclear Layer
• Ganglion Cell Layer
• 2 synaptic layers
• Outer Plexiform Layer
• Inner Plexiform Layer

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Photoreceptors Rods and Cones
The outer segment faces toward the pigment
epithelium The photopigment is located in the
outer segment of the rods and cones The
electrical response of the rods and cones is
generated in the photopigment
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Transduction

• Transduction the transformation of physical
  energy into electrical energy
• The process is different than a typical neuron
• Franz Boll (1876) was the first to isolate rod
  photopigment

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Transduction
In the dark there is a large influx of Na - the
membrane potential is kept depolarized (-40
mV) Exposure of the photoreceptor light causes
the cation channels of the outer segment to close
the membrane potential becomes hyperpolarized
(-70 mV)
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Transduction
Dark
The photosensitive pigment (________) is made up
of a visual pigment molecule (opsin) and a light
sensitive molecule (retinaldehyde
retinal) When rhodopsin absorbs light the
protein (_____) becomes active which causes a
conformational change The opsin and
retinaldehyde break apart and light can no
longer be absorbed This process is called
______________________________
Opsin
Retinaldehyde
Light
Opsin
Retinaldehyde
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Transduction
Once the conformational change occurs in the
photopigment, the amount of cGMP is reduced which
in turn closes the cation channels resulting
hyperpolarization A recovery process then occurs
where the intracellular levels of cGMP must be
restored and the opsin and retinaldehyde
molecules recombine This depends on enzymes in
the pigment epithelium The rod and cone
photopigment do not regenerate at the same rate
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Pigment Regeneration Dark Adaptation

• ____________ increase in sensitivity that
  occurs with more time being spent in the dark
• This increased sensitivity occurs in two distinct
  phases
• Initial rapid stage attributed to cones
• A later slower stage attributed to rods
• Experiments support this idea

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Dark Adaptation Experiments

• Experiment 1 measure the dark adaptation curve
• peripheral retina
• X
• fixation test
• point light

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Dark Adaptation Function
Dark Adaptation Function
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Dark Adaptation Experiments

• Experiment 2 repeat the first experiment with
  one variation
• The observer will look directly at the test
  light test the fovea only

Test stimulus
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Dark Adaptation Experiments

• Experiment 3 use the same method as Experiment
  1, but you use a different type of observer
• Use a rod monochromat because no normal observer
  has a rod-only portion of the retina

peripheral retina X fixation
test point light
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Dark Adaptation Function
high
low
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Dark Adaptation

• Conclude
• The rods are adapting during the cone phase
• See the rod adaptation only after the rods become
  more sensitive than the cones
• Rod-cone break
• Occurs approx. 7 min. after dark adaptation
• Rods take approximately 20-30 min. to adapt
• Cones take approximately 3-4 min.
• This information is used in experiments

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Factors in Brightness Perception

• Wavelength (Purkinje Shift)

Light-adapted
Dark-adapted
460 nm Blue 640 nm Red
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Factors in Brightness Perception

• Retinal Locus/Location
• Fovea cones only
• Periphery more rods, but also cones
• Sensitivity related to convergence of signals
• Fewer cones converge signals to receiving neurons
  than rods

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Retinal Distribution of Photoreceptors
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Retinal Distribution of Photoreceptors

• Fovea central fovea is rod free (optic axis
  goes through the fovea 0o)
• Blind Spot somewhere between 10-20o on the
  nasal part of the retina
• 3 reasons why the blind spot is not noticed
• Rods have their peak density at about 20o
• Beyond 10o the cone density remains fairly
  constant

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Duplicity Theory of Vision

• 1896 J. von Kries proposed the Duplicity Theory
  of Vision
• Duplicity Theory we have 2 different
  photoreceptors that operate under different
  levels of illumination and both have different
  properties
• _________ rod vision
• _________ cone vision

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Scotopic vs. Photopic
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Visual Acuity

• Visual acuity refers to the ability of the
  visual system to resolve details
• Similar to brightness perception, acuity can also
  be affected by different factors
• _________________________________
• _________________________________
• Measurement of Acuity and Visual Angle

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Retinal Locus / Location
Light
Light
Cones
Rods
Ganglion Cell(s)
G
G
G
G
G
G
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Adaptation Level

• Scotopic vs. Photopic
• Which has greater acuity?

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Measurement of Acuity and Visual Angle

• Measurement of acuity
• Introduced by Hermann Snellen (1862)
• E-chart
• 20/20 normal observer
• 20/40 at 20 ft. see what a normal observer sees
  at 40 ft.
• 20/200 legally blind if vision can not be
  corrected

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Calculating Visual Angle

• Visual Angle describes the size of the stimulus
  on the retina
• In terms of acuity the smallest size of a
  visual stimulus that can be resolved
• To calculate visual angle you need
• D distance of the stimulus from the observer
• S size of the stimulus (diameter)

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Calculating Visual Angle

• D 70 cm
• S 2.4 cm
• tan a S/D
• tan a 2.4/70 0.034
• a 0.034 tan -1
• a 1.964o

a
a
S
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Cells of the Retinal Pathway

• Five layers to mammalian retina
• 3 cellular layers
• Outer Nuclear Layer
• Inner Nuclear Layer
• Ganglion Cell Layer
• 2 synaptic layers
• Outer Plexiform Layer
• Inner Plexiform Layer

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Responses of Retinal Cells

• Retinal cells will respond differently to the
  same stimuli
• Main distinction I will make transient vs.
  sustained responses
• _________ response continues while the stimulus
  is present
• _________ the cell responds to changes in the
  stimulus

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Ganglion Cells

• Ganglion cells can have either a sustained or
  transient response
• They respond to a receptive field
• ______________ region of the retina when
  stimulated influences firing rate of the neuron

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Receptive Field

• 2 Parts of the Receptive Field
• Center corresponds with dendritic spread of
  ganglion cell
• Surround produced by lateral signals carried by
  amacrine and horizontal cells

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Responses of Ganglion Cells

• Two Types of Receptive Fields
• On-center, off-surround
• Off-center, on-surround
• The retina contains an approximately equal number
  of on-center and off-center ganglion cells

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Classification of Ganglion Cells

• There are 3 distinct groups of ganglion cells
• Parvocellular
• Magnocellular
• Koniocellular

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Classification of Ganglion Cells

Superior Colliculus ?
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Introduction to Neural Circuits

• Mach Bands
• Lateral Inhibition
• Hermann Grid

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Introduction to Neural Circuits

• Mach Bands psychophysical evidence from Ernst
  Mach (1870)

Our perception is different from the actual
physical distribution of intensity Mach bands do
not appear in the intensity distribution Mach
suggested a physiological explanation
reciprocal action of retinal elements
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Mach Bands Lateral Inhibition

• Hartline, Wagner Ratliff (1956) showed the
  reciprocal action proposed by Mach by studying
  the horseshoe crab (limulus)
• Physiological evidence came many years afer
  psychophysical evidence

Limulus eye is made up of hundreds of ommatidia
(singular ommatidium)
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Lateral Inhibition
Light
B
A
Increase the intensity of B
Conclusion the stimulation of nearby receptors
can inhibit another receptors response
Electrode to record neural response of A
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Neural Circuit for Mach Bands
Using the knowledge of lateral inhibition we can
model the neural circuit for Mach Bands Without
lateral inhibition, A, B, and C will have equal
responses (light intensity 10) and D, E, and F
will have equal responses (light intensity
5) Determine the amount of inhibition form
neighboring cells by assuming that each cell
sends an inhibitory signal 20 of initial
output 10 X .20 2 5 X .20 1
A
B
C
D
E
F
-2
-2
-2
-2
-2
-2
-1
-1
-1
-1
-1
-1
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Hermann Grid
__________ is due to the lateral connections of
the horizontal and amacrine cells These lateral
connections form the receptive fields of the
ganglion cells We can model the Hermann Grid
illusion using ganglion cell receptive fields
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Neural Model for the Hermann Grid
Model the response of ganglion cells using an
excitatory center and inhibitory surround
-
-
-
-
-


-
-
Foveal region what happens to the illusion?
-
-


-
-
-
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The Visual Pathways
Left Visual Field
Right Visual Field
X

• Notice the left visual field is represented in
  the right hemisphere of the visual cortex
• and the right visual field is represented in the
  left hemisphere of the visual cortex.
• Two visual pathways are represented here
• _____________________
• _____________________

Pulvinar Nuclei Superior Colliculi
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Tectopulvinar Pathway

• Will not spend much time on this pathway
• Projections from the optic tract go to the
  superior colliculi, then the pulvinar nuclei
• Mostly magnocellular and koniocellular (?)
• Receptive fields are sensitive to motion and
  location
• Also thought that sensory modalities may be
  integrated here
• Projections to the extrastriate regions of the
  visual cortex (V2)

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Geniculostriate Pathway

• The first place the axons of the ganglion cells
  synapse is in the LGN (lateral geniculate
  nucleus) of the thalamus

There are six discrete cell layers that receive
projections from either magnocellualr or
parvocelular cells. They also receive
information from either the contralateral
(opposite side) or the ipsilateral (same side)
eye.
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Lateral Geniculate Nucleus
I Ipsilateral projections (same side) C
Contraleteral projections (opposite side) Layers
1 2 receive magnocellular projections -cells
respond best to movement Layers 3, 4, 5, 6
receive parvocellular projections -cells respond
best to form and color
C
6
I
5
C
4
I
3
I
2
C
1
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Geniculostriate Pathway

• From the LGN cells project to the visual cortex
  in the occipital lobe

The majority of projections go to area V1 the
primary visual cortex. Also known as the striate
cortex.
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The Striate Cortex
There are 6 layers to the striate cortex
(V1) Axons from the LGN project mainly to layer
IV C The magnocellular and parvocellular systems
remain separate -__________ projects to IV C
a -__________ projects to IV C b
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Receptive Fields in the Striate Cortex

• Hubel and Weisel (1959) identified 3 categories
  of receptive fields through serendipity
• Simple Cells
• Complex Cells
• Hypercomplex

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Simple Cells

• Found in layers 4 a b, and layer 5 of the
  striate cortex
• Characteristics
• Spatially antagonistic (on-off areas)
• Elongated receptive field
• Respond best to narrow bar
• Orientation specific

Thought to be formed by cells with
center-surround receptive fields.
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Orientation Tuning Curve
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Complex Cells

• Found mainly in layers 2 3 of the striate
  cortex
• Not found in layer 4
• Characteristics
• Orientation specific
• Respond best to a moving bar or edge (do not
  respond well to stationary stimuli)
• Some complex cells are sensitive to the direction
  of movement

-
-
-
Assumed that simple cells make up complex cells
(convergence)
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Hypercomplex / End-stopped Cells

• Found in layers 1, 2, 3 of the striate cortex,
  not found in layer 4
• Characteristics
• Respond best to lines or edges of a specific
  length
• Respond best to corners or angles (diffuse light
  is ineffective)
• Orientation specific
• Some respond to motion in a certain direction

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Receptive Fields in the Striate Cortex

• As we move through the receptive fields they
  become more specific as to what features they
  will respond
• They are called feature detectors because of how
  specific they are to features they will respond

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Organization of the Primary Visual Cortex

• Projections from the LGN are organized as to
  where they project in the striate cortex
• The striate cortex has a columnar organization
  (hypercolumns)
• Information is organized in terms of
• ___________________
• ___________________

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Organization of the Primary Visual Cortex
Adjacent columns are similar in orientation
preference
(opposite side)
(same side)
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Organization of the Primary Visual Cortex
Layer 1 contains hypercomplex cells
Layers II III contain the blobs and
interblobs Contain complex and hypercomplex cells
Layers IV A B contain simple cells
IV B only receives magnocellular projections
Receives input from the LGN IV C a
magnocellular IV C b - parvocellular
The receptive fields are large and sensitive to
motion
The receptive fields are long, narrow, and
directionally sensitive
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Higher-level Visual Coding Extrastriate Cortex

• Visual Processing Streams
• Peter Schiller (1990) lesioned magnocellular and
  parvocellular layers of the LGN
• ____________ movement perception affected
• ____________ color, form, and depth perception
  affected

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Visual Processing Streams

• We know magnocellular and parvocellular synapse
  in different locations in the striate cortex
• Continue in separate streams into the
  extrastriate cortex
• Two streams that transmit signals from the
  striate cortex to the extrastriate cortex

92
What and Where Streams

• Dorsal Pathway where pathway
• Ventral Pathway what pathway
• Milner Goodale (1995) believed instead of what
  and where streams it should be what and how
  streams

M ganglion cell magno LGN V1 V2 V3 MT Parietal
P ganglion cell parvo LGN V1 V2 V4 IT Temporal
Pathway
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What and How Streams

• Milner Goodale (1995) believed instead of what
  and where streams it should be what and how
  streams
• Ventral perceiving objects what
• Dorsal taking action toward objects how

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Blindsight

• Blindsight is vision without awareness
• Damage to V1 causes holes (scotomas)