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POWERPOINT PRESENTATION
FOR BIOPSYCHOLOGY,
9TH EDITION
BY JOHN P.J. PINEL
P R E PA R E D B Y J E F F R E Y W. G R I M M
WESTERN WASHINGTON UNIVERSITY
COPYRIGHT © 2014 PEARSON EDUCATION, INC.
ALL RIGHTS RESERVED.
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Chapter 6
The Visual System
How We See
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Learning Objectives
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LO1: Describe the human eyes and how their properties influence vision.
LO2: Describe the structure of the human retina, and explain how the
structure of the retina influences vision.
LO3: The retina-geniculate-striate system is retinotopic, with signals from
each retina arriving in both ipsilateral and contralateral cortices. Explain with
diagrams.
LO4: Explain the Mach Band demonstration and the important point that it
makes about vision.
LO5: Describe Hubel and Wiesel’s method, and summarize their major
findings.
LO6: Explain color constancy and its important implication.
LO7: Describe and discuss the dorsal and ventral streams and their relation
to prosopagnosia and akinetopsia.
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What Do We See?
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Somehow a distorted and upside-down 2D retinal image is transformed into the 3-D
world we perceive.
Two types of research are 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
wavelengths 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 (Con’t)
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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 6.4 The human eye, a product of 600
million years of evolution (Lamb, Collin, & Pugh,
2007).
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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: the difference between
the images on the two retinas
Both are greater when objects are close—
together, they provide the brain with a 3-D
image and distance information.
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The Retina and Translation of
Light into Neural Signals
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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
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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 (Con’t)
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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
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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. The
fovea is the indentation at the center of the
retina; it is specialized for high-acuity
vision.
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Cone and Rod Vision
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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
There is more convergence in the 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. There is a low
degree of convergence in cone-fed pathways and a high
degree of convergence in rod-fed pathways.
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FIGURE 6.9 The distribution of cones and rods over the human retina.
The figure illustrates the number of cones and rods per square
millimeter as a function of distance from the center of the fovea.
(Based on Lindsay & Norman, 1977.)
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Spectral Sensitivity
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Lights of the same intensity but different
wavelengths may not all look equally
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. The peak of each curve has been arbitrarily set
at 100%.
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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.
The 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 (Con’t)
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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. When
light bleaches rhodopsin
molecules, the rods’ sodium
channels close; as a result, the
rods become hyperpolarized and
release less glutamate.
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From Retina to Primary
Visual Cortex
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The retinal-geniculate-striate pathways include
about 90 percent 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.
(Based on 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 percent 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 (Con’t)
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The channels project to slightly different
areas in lower layer IV in striate cortex; M
neurons are just above the P neurons.
The channels 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.
(Based on 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
the retinal ganglion, LGN, and lower layer IV
of striate cortex of a cat.
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Receptive Fields: Neurons of
the Retina-Geniculate-Striate
System
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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 (Con’t)
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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 offcenter.
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FIGURE 6.16 The receptive fields of an oncenter cell and an off-center cell.
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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, with larger receptive
fields, and respond best to a particular stimulus
anywhere in their receptive fields
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Receptive Fields: Simple and
Complex Cortical Cells (Con’t)
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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FIGURE 6.18 Examples of
visual fields of simple
cortical cells.
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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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Contextual Influences in
Visual Processing
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Plasticity appears to be a fundamental
property of visual cortex function.
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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
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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 (Con’t)
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Opponent-process theory was proposed by Hering.
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Two different classes of cells encoding color, and another
class encoding brightness
Each encodes two complementary color perceptions.
This theory 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 (Con’t)
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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 6.19 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 6.20 The method of Land’s (1977)
color-vision experiments. Subjects viewed
Mondrians 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
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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:
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Become larger
Respond to more complex and specific stimuli
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FIGURE 6.21 The visual areas of the human
cerebral cortex.
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Damage to Primary Visual
Cortex
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Scotomas
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Areas of blindness in contralateral visual field
due to damage to primary visual cortex
Detected by perimetry test
Completion
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Patients may be unaware of scotoma; missing
details are supplied by “completion.”
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FIGURE 6.23 The completion of a
migraine-induced scotoma as described
by Karl Lashley (1941).
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Damage to Primary Visual
Cortex (Con’t)
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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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Functional Areas of
Secondary and Association
Visual Cortex
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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 have been identified so far.
Retinotopically Organized
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FIGURE 6.24 Some of the visual areas that
have been identified in the human brain.
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Dorsal and Ventral Streams
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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 the 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 the conscious perception of objects
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FIGURE 6.26 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 prosopagnosics’ 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 6.27 The location of the fusiform face
area and the MT area: Damage to the fusiform face
area is associated with prosopagnosia, damage to
the MT area is associated with akinetopsia.
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