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PowerPoint Presentation for
Biopsychology, 8th Edition
by John P.J. Pinel
Prepared by Jeffrey W. Grimm
Western Washington University
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Inc. All rights reserved.
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Chapter 6: The Visual System
How We See
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What Do We See?
Somehow a distorted and upside-down
2-D retinal image is transformed into the
3-D world we perceive
 Two types of research needed to study
vision


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Research probing the components of the
visual system
Research assessing what we see
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Light Enters the Eye and
Reaches the Retina
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No species can see in the dark, but some are
capable of seeing when there is little light
Light can be thought of as

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Particles of energy (photons)
Waves of electromagnetic radiation
Humans see light between 380-760
nanometers
Wavelength – perception of color
Intensity – perception of brightness
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FIGURE 6.2 The electromagnetic spectrum
and the colors associated with the
wavelengths that are visible to humans.
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The Pupil and the Lens

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Light enters the eye through the pupil, whose
size changes in response to changes in
illumination
Sensitivity – the ability to see when light is
dim
Acuity – the ability to see details
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The Pupil and the Lens
Continued

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Lens – focuses light on the retina
Ciliary muscles alter the shape of the lens as
needed
Accommodation – the process of adjusting
the lens to bring images into focus
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FIGURE 6.3 The human eye.
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Eye Position and Binocular
Disparity

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Convergence – eyes must turn slightly
inward when objects are close
Binocular disparity – difference between
the images on the two retinas
Both are greater when objects are close –
provides brain with a 3-D image and distance
information
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The Retina and Translation of
Light into Neural Signals

The retina is in a sense “inside-out”
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Light passes through several cell layers before
reaching its receptors
Vertical pathway – receptors > bipolar cells >
retinal ganglion cells
Lateral communication
 Horizontal cells
 Amacrine cells
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FIGURE 6.5 The cellular structure of the
mammalian retina.
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The Retina and Translation of
Light into Neural Signals
Continued

Blind spot: no receptors where information
exits the eye
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The visual system uses information from cells
around the blind spot for “completion,” filling in the
blind spot
Fovea: high acuity area at center of retina

Thinning of the ganglion cell layer reduces
distortion due to cells between the pupil and the
retina
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FIGURE 6.6 A section of the retina.
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Cone and Rod Vision

Duplexity theory of vision – cones and rod
mediate different kinds of vision
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Cones – photopic (daytime) vision
 High-acuity color information in good lighting
Rods – scotopic (nighttime) vision
 High-sensitivity, allowing for low-acuity vision in dim
light, but lacks detail and color information
More convergence in rod system, increasing
sensitivity while decreasing acuity
Only cones are found at the fovea
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FIGURE 6.8 A schematic representation of
the convergence of cones and rods on
retinal ganglion cells.
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FIGURE 6.9 The distribution of cones and
rods over the human retina. (Adapted from
Lindsay & Norman, 1977.)
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Spectral Sensitivity
Lights of the same intensity but different
wavelengths may not all look as bright
 A spectral sensitivity curve shows the
relationship between wavelength and
brightness
 There are different spectral sensitivity
curves for photopic (cone) vision and
scotopic (rod) vision

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FIGURE 6.10 Human photopic (cone) and
scotopic (rod) spectral sensitivity curves.
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Eye Movement
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We continually scan the world with small and
quick eye movements – saccades
These bits of information are then integrated
Stabilize retinal image – see nothing
Visual system responds to change
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Visual Transduction: The
Conversion of Light to Neural
Signals
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Transduction – conversion of one form of
energy to another
Visual transduction – conversion of light to
neural signals by visual receptors
Pigments absorb light
Absorption spectrum describes spectral
sensitivity
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FIGURE 6.11 The adsorption spectrum of
rhodopsin compared with the human
scotopic spectral sensitivity curve.
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Visual Transduction: The
Conversion of Light to Neural
Signals Continued
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Rhodopsin is the pigment found in rods
A G protein-linked receptor that responds to
light rather than to neurotransmitters
In the dark
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Na+ channels remain partially open (partial
depolarization), releasing glutamate
When light strikes
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Na+ channels close
Rods hyperpolarize, inhibiting glutamate release
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FIGURE 6.12 The inhibitory
response of rods to light.
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From Retina to Primary Visual
Cortex
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The retinal-geniculate-striate pathways are about
90% of axons of retinal ganglion cells
The left hemiretina of each eye (right visual field)
connects to the right lateral geniculate nucleus
(LGN); the right hemiretina (left visual field)
connects to the left LGN
Most LGN neurons that project to primary visual
cortex (V1, striate cortex) terminate in the lower part
of cortical layer IV
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FIGURE 6.13 The retina-geniculatestriate system: the neural projections
from the retinas through the lateral
geniculate nuclei to the left and right
primary visual cortex (striate cortex).
The colors indicate the flow of
information from various parts of the
receptive fields of each eye to various
parts of the visual system. (Adapted
from Netter, 1962.)
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Retinotopic Organization
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Information received at adjacent portions
of the retina remains adjacent in the
striate cortex (retinotopic)
More cortex is devoted to areas of high
acuity – like the disproportionate
representation of sensitive body parts in
somatosensory cortex
About 25% of primary visual cortex is
dedicated to input from the fovea
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The M and P Channels
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Magnocellular layers (M layers)
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Big cell bodies, bottom two layers of LGN
Particularly responsive to movement
Input primarily from rods
Parvocellular layers (P layers)
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Small cell bodies, top four layers of LGN
Color, detail, and still or slow objects
Input primarily from cones
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The M and P Channels
Continued


Project to slightly different areas in lower
layer IV in striate cortex, M neurons just
above the P neurons
Project to different parts of visual cortex
beyond V1
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Seeing Edges
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Contrast Enhancement
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Mach bands: nonexistent stripes the visual
system creates for contrast enhancement
Makes edges easier to see
A consequence of lateral inhibition
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FIGURE 6.14 The illusory
bands visible in this figure are
often called Mach bands,
although Mach used a different
figure to generate them in his
studies (see Eagleman, 2001).
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FIGURE 6.15 How lateral inhibition
produces contrast enhancement.
(Adapted from Ratliff, 1972.)
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Receptive Fields of Visual
Neurons
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The area of the visual field within which it is
possible for a visual stimulus to influence the
firing of a given neuron
Hubel and Wiesel looked at receptive fields in
cat retinal ganglion, LGN, and lower layer IV
of striate cortex
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Receptive Fields: Neurons of
the Retina-Geniculate-Striate
System

Similarities seen at all three levels:
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Receptive fields of foveal areas are smaller than
those in the periphery
Neurons’ receptive fields are circular in shape
Neurons are monocular
Many neurons at each level had receptive fields
with excitatory and inhibitory area
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Receptive Fields: Neurons of
the Retina-Geniculate-Striate
System Continued
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
Many cells have receptive fields with a
center-surround organization: excitatory and
inhibitory regions separated by a circular
boundary
Some cells are “on-center” and some are
“off-center”
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FIGURE 6.17 The responses of an oncenter cell to contrast.
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Receptive Fields: Simple and
Complex Cortical Cells
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In lower layer IV of the striate cortex, neurons
with circular receptive fields (as in retinal
ganglion cells and LGN) are rare
Most neurons in V1 are either
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Simple – receptive fields are rectangular with “on”
and “off” regions, or
Complex – also rectangular, larger receptive fields,
respond best to a particular stimulus anywhere in
its receptive field
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Receptive Fields: Simple and
Complex Cortical Cells
Continued
SIMPLE
 Rectangular
 “On” and “off” regions,
like cells in layer IV
 Orientation and location
sensitive
 All are monocular
COMPLEX
 Rectangular
 Larger receptive fields
 Do not have static “on” and
“off” regions
 Not location sensitive
 Motion sensitive
 Many are binocular
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Columnar Organization of
Primary Visual Cortex
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Cells with simpler receptive fields send
information on to cells with more complex
receptive fields
Functional vertical columns exist such
that all cells in a column have the same
receptive field and ocular dominance
Ocular dominance columns – as you
move horizontally, the dominance of the
columns changes
Retinotopic organization is maintained
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FIGURE 6.19 The
organization of the primary
visual cortex: the receptivefield properties of cells
encountered along typical
vertical and horizontal
electrode tracks in the
primary visual cortex.
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Plasticity of Receptive Fields of
Neurons in the Visual Cortex

Plasticity appears to be a fundamental
property of visual cortex function

e.g. receptive field properties depend on the
scene in which the stimuli to its field are
embedded
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Seeing Color: Component and
Opponent Processing

Component theory (trichromatic theory)
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Proposed by Young, refined by Helmholtz
Three types of receptors, each with a different
spectral sensitivity
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Seeing Color: Component and
Opponent Processing
Continued
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Opponent-process Theory proposed by Hering:
Two different classes of cells encoding color, and
another class encoding brightness
Each encodes two complementary color perceptions
Accounts for color afterimages and colors that
cannot appear together (reddish green or bluish
yellow)
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Seeing Color: Component and
Opponent Processing
Continued

Both theories are correct: coding of color
by cones seems to operate on a purely
component basis, opponent processing of
color is seen at all subsequent levels
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FIGURE 6.21 The absorption spectra of the
three classes of cones.
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Color Constancy and the
Retinex Theory
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Color constancy – color perception is not altered by
varying reflected wavelengths
Retinex theory (Land)– color is determined by the
proportion of light of different wavelengths that a
surface reflects
Relative wavelengths are constant, so perception is
constant
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Dual-opponent color cells are sensitive to color contrast
Found in cortical “blobs”
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FIGURE 6.22 The method of Land’s (1977)
color-vision experiments. Subjects viewed
Mondrians that were illuminated by various
proportions of three different wavelengths:
a short wavelength, a middle wavelength,
and a long wavelength.
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Cortical Mechanisms of Vision
and Conscious Awareness

Flow of visual information:
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Thalamic relay neurons, to
1˚ visual cortex (striate), to
2˚ visual cortex (prestriate), to
Visual association cortex
As visual information flows through hierarchy,
receptive fields


become larger
respond to more complex and specific stimuli
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FIGURE 6.24 The visual areas of the human
cerebral cortex.
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Damage to Primary Visual
Cortex

Scotomas
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Areas of blindness in contralateral visual field
due to damage to primary visual cortex
Detected by perimetry test
Completion

Patients may be unaware of scotoma –
missing details supplied by “completion”
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FIGURE 6.26 The completion of a migraineinduced scotoma as described by Karl
Lashley (1941).
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Damage to Primary Visual
Cortex Continued
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Blindsight
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Response to visual stimuli outside conscious
awareness of “seeing”
Possible explanations of blindsight
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Islands of functional cells within scotoma
Direct connections between subcortical structures and
secondary visual cortex, not available to conscious
awareness
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
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Functional Areas of
Secondary and Association
Visual Cortex
Neurons in each area respond to different
visual cues, such as color, movement, or
shape
Lesions of each area results in specific
deficits
Anatomically distinct (about 12 functionally
distinct areas identified so far)
Retinotopically organized
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FIGURE 6.27 Some of the visual areas that
have been identified in the human brain.
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Dorsal and Ventral Streams

Dorsal stream: pathway from primary visual cortex
to dorsal prestriate cortex to posterior parietal
cortex
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The “where” pathway (location and movement), or
Pathway for control of behavior (e.g. reaching)
Ventral stream: pathway from primary visual
cortex to ventral prestriate cortex to inferotemporal
cortex
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
The “what” pathway (color and shape), or
Pathway for conscious perception of objects
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FIGURE 6.29 The “where” versus “what”
and the “control of behavior” versus
“conscious perception” theories make
different predictions.
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Prosopagnosia
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Inability to distinguish among faces
Most prosopagnosic’s recognition deficits are
not limited to faces
Prosopagnosia is associated with damage to
the ventral stream between the occipital and
temporal lobes
Prosopagnosics may be able to recognize
faces in the absence of conscious awareness
Prosopagnosics have different skin conductance
responses to familiar faces compared to unfamiliar
faces, even though they reported not recognizing
any of the faces
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Akinetopsia
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Deficiency in the ability to see movement
progress in a normal smooth fashion
Can be induced by a high dose of certain
antidepressants
Associated with damage to the middle
temporal (MT) area of the cortex
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FIGURE 6.30 The location of MT: Damage
to this middle temporal area of the human
brain is associated with akinetopsia.
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