why isn’t vision perfect? an exercise in psychoanatomy
TRANSCRIPT
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Why isn’t vision perfect?An exercise in psychoanatomy
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Why isn’t vision perfect?An exercise in psychoanatomy
Tracing the flow of information through the nervous system
using functional experiments
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V1
LGN
Parietal (action)
temporal (perception)
If neural representation fails at any stage, perception will fail
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The most basic aspect of vision: spatial resolution
Resolution has a limit:Coarse patterns are seen, fine detail is not
How and where does resolution fail?
We start at the beginning with the photons themselves
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Cause of imperfect resolution:
A spreading of light, or of the effects of light
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Campbell & Robson (1968)
Spatial frequency (cycles/degree)
Sensitivity
Contrast-Sensitivity Function (CSF)
Resolution limit: 50cpd
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Factors that might limit visual resolution:
Optics of the image (including diffraction)Sampling by the retinal mosaic Light collection by the cone aperturesNeural convergence:
intra-retinal or retino-thalamicthalamo-corticalintra-cortical
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Factors limiting visual resolution:Optics of the image (including diffraction)
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One test for the role of optics:
Do perfect optics make vision perfect?
Two possible approaches:
• Test resolution using interference fringe targets, bypassing the optics; or…
• Use Adaptive Optics to compensate for individual optical aberrations
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DaveWilliams
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Laser interferometrybypassses optical losses:target stripes are generated directly on the retina
by intereference of two uniform laser beams
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Bypassing optics improves vision, but only from 45 cpd to about 60 cpd.Vision is still not perfect; neural losses are at least as important as optics.
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AdaptiveOptics
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Adaptive Optics: supernormal, yet still imperfect vision
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Factors limiting visual resolution:Optics of the image (including diffraction)…important, but not sufficient
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Two ways the retinal mosaic might limit resolution:
• Filtering
• Sampling
… Effective size of cones
… Spacing of cones
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Sampling limits on resolution?
• Foveal photoreceptor mosaic frequency: 110 cpd
• Nyquist sampling limit for 1 row of cones: 55cpd
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30 cpd
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60 cpd
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120 cpd
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120 cpd processed by a single row of cones: aliasing
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Roorda and Williams
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Sampling limits on resolution?
• Foveal photoreceptor mosaic frequency: 110 cpd
• Nyquist sampling limit for 1 row of cones: 55cpd
• Nyquist limit for 8 rows of cones: 440 cpd ??
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Factors limiting visual resolution:Optics of the image (including diffraction)...important, but less important than neural losses Sampling by the photoreceptor mosaic…unimportantLight collection by the cone apertures ?
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Unresolvable high-contrast patterns appear desaturated
Sherif Shady and Don MacLeod(Nature Neurosci 2002)
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10 30 50 70 90 110
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Factors limiting visual resolution:Optics of the image and photoreceptor sampling…Light collection by the cone apertures… not severely limiting (>100cpd)Neural Losses:
intra-retinal or retino-thalamicthalamo-corticalintra-cortical
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LGN
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Peter Lennie…Matt McMahon,
…Dave Williams, and Martin Lankheet
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McMahon et al.J.Neurosci.1999
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Factors limiting visual resolution:Optics of the image (diffraction, etc.)Light collection by the cone aperturesnot severely limiting (>100cpd)Neural convergence:
intra-retinal or retino-thalamic still not limiting (>80cpd)thalamo-corticalintra-cortical ?
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LGNV1
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Seeing spatial pattern
Physicalstimuli
Perceptualexperience
Localized neural activity
correlation correlation
What does V1 see?
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Orientation-Selective Adaptation
Adapt(30 sec)
Test(200 msec)
Same OrientationHard to see!
Need high contrast
Orthogonal OrientationEasy to see!
Need low contrast
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V1 is the first stage in the visual system where orientation information is extracted:
orientation-selective adaptation is only possible at or after V1.
If we find evidence for orientation-selective adaptation, then it implies that the orientation information has at least reached V1.
Adaptation: Psychophysicists’ microelectrode
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a.
adapt
test
5000ms
fig. 1
250 ms
250 ms
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? ?
respond/adaptor
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unresolvable
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orientationdiscrimination
orientation-specificadaptation
testtest
30 40 50 60 7035 45 55 65
(adapting) spatial frequency (cpd)(adapting) spatial frequency (cpd)
30 40 50 60 7035 45 55 65
b.
log sensitivityfor orientationdiscrimination
andlog threshold
elevation fromorientation-
specificadaptation
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Invisibleverticalgrating
Adapt(30 sec)
Test(200 msec)
Same OrientationNeed higher contrast
Orthogonal OrientationNeed lower contrast
Result
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a.
adapt
test
5000ms
fig. 1
250 ms
250 ms
250 ms
? ?
respond/adaptor
250 ms
unresolvable
0.8
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orientationdiscrimination
orientation-specificadaptation
orientationdiscrimination
orientation-specificadaptation
testtest
30 40 50 60 7035 45 55 65
(adapting) spatial frequency (cpd)(adapting) spatial frequency (cpd)
30 40 50 60 7035 45 55 65
b.
log sensitivityfor orientationdiscrimination
andlog threshold
elevation fromorientation-
specificadaptation
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adapt test (250 ms)
250 ms
250 ms
perception (subjective horizontal) (5 sec)
a.
b.
36 48 66
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test
SH
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SH DM
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Tilt Aftereffect in degrees
(mean s.e.m.)
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adapt test (250 ms)
250 ms
250 ms
perception (subjective horizontal) (5 sec)
a.
b.
36 48 66
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Tilt Aftereffect in degrees
(mean s.e.m.)
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Orientation-selective adaptation from an invisible spatial pattern at 60 to 70 cpd:
invisible to us but visible at V1.
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Factors that might limit visual resolution for interference targets:
Optics of the image (including diffraction): not applicableLight collection by the cone apertures: >100 cpdNeural convergence:
intra-retinal or retino-thalamic: 80 cpdthalamo-cortical…lumped with intracortical…intra-cortical: 70 cpd Perception: <= 60 cpd
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Conclusions
• Neural losses slightly exceed optical losses at the limit.• Sampling and light collection in the photoreceptor mosaic are not limiting.• Neural losses are distributed through the system; some are intracortical, since cortex can respond to patterns too fine for conscious perception.
Optional Bonus Conclusion about Consciousness
• Primary visual cortex is not directly represented in conscious experience (Crick and Koch).
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Paper topics
1) When a computer monitor that flickers too fast for the flicker to be perceived, can the unseen flicker nevertheless activate the visual cortex of your brain? Design an experiment to investigate this.
2) Is it possible to explain He and MacLeod’s results without accepting Crick and Koch’s conclusion that primary visual cortex has no immediate representation in conscious experience?
3) How might brain imaging experiments follow up on He and MacLeod’s observations?
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Temporal Resolution: When Vision is Grossly
Imperfect
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120 S 3 S 1 S
AdaptPre-Adapt Test
Time
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3.5
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Scaled adapting modulation
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50403020100
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Frequency (Hz)
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Perception Adaptation
Shady_Fig3
Invisible Invisible
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2.0
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Frequency (Hz)
Perception Adaptation
invisible invisible
Shady_Fig4
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Conclusions
• Oblique adapting and masking gratings are not less powerful than horizontal ones
• We conclude that the Oblique Effect arises after the site of pattern adaptation and masking
• Unexpectedly, oblique adapters are slightly more powerful than horizontal ones
• Additional experiments and modeling will allow us to quantitatively test the possible models we have presented
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Data for subject TL
-3
-2.5
-2
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-1
0 1 2 3 4
Data for subject MW
-2.5
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Modulation Frequency (cpd)
RF Width: Results
=
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Modulation Frequency (cpd)
Spatial integration is anisotropic
Data for subject TL
-3
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8´
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Does the extent of spatial integration vary
with carrier contrast?
50% Contrast5% Contrast
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-3
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0 1 2 3 4Modulation Frequency (cpd)
low contrastlow contrast
high contrast
high contrast
Results: High Contrast
=
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0 1 2 3 4
Data for subject TL Data for subject MW
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High Contrast Properties
• Loss in sensitivity at lower modulation frequencies
• Spatial integration minimal both along and across contours
Modulation Frequency (cpd)
At high contrast:
-3
-2.5
-2
-1.5
-1
0 1 2 3 4
low contrast
high contrast
Th
resh
old
(lo
g C
)
Data for subject TL
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LOW frequency: HIGH frequency:
Contrast Gain Control
Contrast gain locally compensates for low frequency modulation
Loss in sensitivity at lower modulation frequencies
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Receptive Field ShrinksLOW contrast: HIGH contrast:
• Receptive field shrinks at high contrast.- Supported by recent physiological evidence
• Leads to similar behavior for integration along and across image contours.
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Contrast
Constant
PerceivedBrightness
MeanLuminance
time
flicker
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Orientation-selective adaptation from an invisible spatial pattern:
invisible to us but visible at V1.
There is intra-cortical loss of spatial information.
V1 does not directly support conscious vision (Crick and Koch)
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With 8.5 cpd grating: Modulation threshold doubled at 2.5 cpdImplied RF height: 6 min, or 12 cone rows
With 40 cpd grating: Modulation threshold doubled near 4 cpdImplied RF height: 4 min, or 8 cone rows
Would integration over 8 rows prevent aliasing?
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120 cpd
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