Duplicity Theory
• Two independent visual systems
• Photopic system
– day vision (high light) composed primarily of cones
• Scotopic system
– a night vision (low light) composed primarily of rods
Sensation and Perception - duplicity.ppt
© 2001 Dr. Laura Snodgrass, Ph.D.
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8 Differences
• Light sensitivity
• Spectral sensitivity
• Distribution on retina
• Interconnections
• Acuity
• Response to movement
• CFF - critical flicker frequency
• Signaling properties
Sensation and Perception - duplicity.ppt
© 2001 Dr. Laura Snodgrass, Ph.D.
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Light Sensitivity
• Total range of sensitivity is 100,000
• Rods have an absolute threshold 1-10K lower than
cones
• The range of sensitivity is accomplished by dark and
light adaptation
– photopigment “bleaches” (breaks down) in the light
– photopigment regenerates in the dark
– so in the dark, more pigment is available and the eyes
are more sensitive
Sensation and Perception - duplicity.ppt
© 2001 Dr. Laura Snodgrass, Ph.D.
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Dark Adaptation Curve
Sensation and Perception - duplicity.ppt
© 2001 Dr. Laura Snodgrass, Ph.D.
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Dark And Light Adaptation
• Dark adaptation
– as more photopigment regenerates, switch from the
less sensitive cones to the more sensitive rods. End
up using only rods because not enough light
intensity to trigger cone photopigment
• Light adaptation
– use cones because all rod photopigment is broken
down
Sensation and Perception - duplicity.ppt
© 2001 Dr. Laura Snodgrass, Ph.D.
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Spectral Sensitivity
Sensation and Perception - duplicity.ppt
© 2001 Dr. Laura Snodgrass, Ph.D.
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Purkinje Shift
Sensation and Perception - duplicity.ppt
© 2001 Dr. Laura Snodgrass, Ph.D.
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Differences In Spectral Sensitivity
• Purkinje shift
– different colors appear brighter because of diff in
sensitivity, which is brighter depends on overall
illumination
• Scotopic NOT sensitive to deep red light
– use deep red for dark adaptation
• Scotopic NOT process color
– photopic has three cone types needed for color
processing
Sensation and Perception - duplicity.ppt
© 2001 Dr. Laura Snodgrass, Ph.D.
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Three Cone Types
The interaction of
the three cone
types is what
signals color
differences
Sensation and Perception - duplicity.ppt
© 2001 Dr. Laura Snodgrass, Ph.D.
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Distribution On The Retina
• Cones in fovea
• No rods in fovea
• No receptors in blind spot
What does this mean for color
perception, acuity, and motion
detection?
Sensation and Perception - duplicity.ppt
© 2001 Dr. Laura Snodgrass, Ph.D.
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Interconnections
• Photopic system
– one-to-one wiring, direct line to brain
– produces spatial discrimination
• Scotopic system
– one-to-many wiring , convergence
– produces spatial summation
Sensation and Perception - duplicity.ppt
© 2001 Dr. Laura Snodgrass, Ph.D.
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Spatial Discrimination
Sensation and Perception - duplicity.ppt
© 2001 Dr. Laura Snodgrass, Ph.D.
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Spatial Summation
Sensation and Perception - duplicity.ppt
© 2001 Dr. Laura Snodgrass, Ph.D.
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Acuity
• Photopic acuity
– measured by Snellen charts
– degree of spatial discrimination
• Problems:
– Presbyopia - loss of lens elasticity
– astigmatism - corneal imperfections
– cataracts - cloudy lens
Sensation and Perception - duplicity.ppt
© 2001 Dr. Laura Snodgrass, Ph.D.
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Opias
Acuity errors caused
by length of the
eyeball or curvature
of the lens.
Myopia -
Hyperopia -
Sensation and Perception - duplicity.ppt
© 2001 Dr. Laura Snodgrass, Ph.D.
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Response To Movement
• Scotopic system
– wired to detect motion, very sensitive
– very sensitive to motion in the periphery
• Photopic system
– less sensitive to motion
• Night time driving
Sensation and Perception - duplicity.ppt
© 2001 Dr. Laura Snodgrass, Ph.D.
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CFF (critical flicker frequency)
• Frequency at which a flickering light appears fused
– index of temporal discrimination
• Photopic
– light fuses at lower frequency
– temporal summation - add up light over time
• Scotopic
– higher frequency
– temporal discrimination
Sensation and Perception - duplicity.ppt
© 2001 Dr. Laura Snodgrass, Ph.D.
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Signaling Properties
• Photopic
– sustained signal
– continues to fire as long as stimulus is present
• Scotopic
– transient signal
– fires at onset and offset, brief signal
• Contributes to difference in CFF
Sensation and Perception - duplicity.ppt
© 2001 Dr. Laura Snodgrass, Ph.D.
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