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Chapter 6: The Visual System
How We See
Copyright © 2009 Allyn & Bacon
What Do We See?
• Somehow a distorted and upside-down 2-D
retinal image is transformed into the 3-D world
we perceive
Copyright © 2009 Allyn & Bacon
Light Enters the Eye
• No species can see in the dark, but some are
capable of seeing when there is little light
• Light can be thought of as
– Particles of energy (photons)
– Waves of electromagnetic radiation
• Humans see light between 380-760
nanometers
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The electromagnetic spectrum: colors and wavelengths visible to humans
Copyright © 2009 Allyn & Bacon
Figure 6.2
Light Enters the Eye (continued)
• Wavelength – perception of color
• Intensity – perception of brightness
• 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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Light Enters the Eye (continued)
• 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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A diagram of the human eye
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Eye Position and Binocular
Disparity
• 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
Signals
The retina
is in ainto
senseNeural
“inside-out”
– Light passes through several cell layers before
reaching its receptors
• Vertical pathway – receptors > bipolar cells >
retinal ganglion cells
• Lateral communication
– Horizontal cells
– Amacrine cells
Copyright © 2009 Allyn & Bacon
Copyright © 2009 Allyn & Bacon
The Retina
• Blind spot: no receptors where information exits the
eye
– 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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Cone and Rod Vision
• Duplexity theory of vision – cones and rod
mediate different kinds of vision
– 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
Copyright © 2009 Allyn & Bacon
Copyright © 2009 Allyn & Bacon
Cone and Rod Vision (continued)
Distribution of rods and cones
• More
convergence in
rod system,
increasing
sensitivity while
decreasing acuity
• Only cones are
found at the fovea
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Human
photopic
and
scotopic
spectral
sensitivity
curves
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•
Visual Transduction: The
Conversion of Light to Neural
Transduction – conversion of one form of
Signals
energy to another
• Visual transduction – conversion of light to
neural signals by visual receptors
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From Retina to Primary Visual
Cortex
The Retinal-Geniculate-Striate Pathways
• ~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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From Retina to
Primary Visual
Cortex
The retinageniculatestriate
system
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Retinotopic Organization
• Information received at adjacent portions of the
retina remains adjacent in the striate cortex
• 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
Copyright © 2009 Allyn & Bacon
The M and P Channels
• Magnocellular layers (M layers)
– Big cell bodies, bottom two layers of LGN
– Particularly responsive to movement
– Input primarily from rods
• Parvocellular layers (P layers)
– 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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Receptive Fields of Visual Neurons
• 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:
– 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
• Many cells have
receptive fields with a
center-surround
organization:
excitatory and
inhibitory regions
separated by a circular
boundary
• Some cells are “oncenter” and some are
“off-center”
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Receptive Fields in Striate Cortex
• 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
– 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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Columnar Organization of V1
• 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
• Retinotopic organization-like map of retina
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Cortical Mechanisms of Vision
and Conscious Awareness
Visual areas of the human cerebral cortex
• Flow of visual information:
–
–
–
–
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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Damage to Primary Visual Cortex
• Scotomas
– 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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Damage to Primary Visual Cortex
(continued)
• Blindsight
– Response to visual stimuli without conscious
awareness of “seeing”
– Possible explanations of blindsight
• 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 Second 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
• Retinotopically organized
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Dorsal and Ventral Streams
• Dorsal stream: pathway from primary visual cortex to
dorsal prestriate cortex to posterior parietal cortex
– 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
– The “what” pathway (color and shape), or
– Pathway for conscious perception of objects
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Prosopagnosia
• Inability to distinguish among faces
• Most prosopagnosic’s recognition deficits are not
limited to faces
• Often associated with damage to the ventral
stream
• 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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Retinal Diseases
• Macular Degeneration-destruction of
photoreceptors
– Wet (blood vessels) and Dry (drusen)
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Retinitis Pigmentosa
• Progressive degeneration of photoreceptors
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Prostethic Retina
• Bioelectronic implant
• Images collected by camera hidden in
glassesdata sent to the unharmed retinal
cells then onto optic nerve
• 60 pixels (distinguish btwn light and dark)
• Artifical Retina Project Video
Copyright © 2009 Allyn & Bacon