why cover “acuity” in a course called “central visual mechanisms”? …because we are now...
Post on 03-Jan-2016
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CHAPTER 5. SPATIAL ACUITY
Harold Bedell, College of Optometry, University of Houston
Why cover “acuity” in a course called “Central Visual Mechanisms”?
…because we are now talking about how the whole visual system works and how we can measure vision.
Acuity has a neural basis, but it is typically measured in a “whole organism” (person or animal), though it is also possible to measure the acuity of single neurons.
Acuity Task Typical Stimulus Minimum Threshold
Detection Single black spot 15” – 20”
Single black line 0.5” – 1.0”
Localization Spatial interval 2” – 4”
Vernier lines 3” – 6”
Resolution Two black lines/spots 30” – 40”
Grating 30” – 40”
Identification Letters or numerals 30” – 40”
Debate: are resolution and identification acuity the same?
Comparison of Spatial Acuity Tasks and Thresholds
Detection acuity is the angular size of the smallest visible target
It is an intensity discrimination task
You need to be able to explain the reason that detection acuity is an intensity discrimination
The retinal image
Photoreceptor sampling
Convergence (receptive field center size) = “neural defocus”
All types of visual acuity are determined
largely by optical and “neural” defocus
For all types of acuity, need to consider these three things:
Angular Distance (min)
-4 -2 0 2 4
RelativeRetinal Illuminance
1.5 mm
2.4 mm
6.6 mm
Dashed line = theoretical point-spread function based on pupil alone (larger pupil give narrowest point-spread)
Solid line = actual point-spread function based on all factors (intermediate pupil is best)
How images spread out on the retina & interaction with pupil size
For wide objects the eye’s optics only affect the edges of the shadow’s image on the retina
At cornea
On retina
dark
light
As the object gets thinner, the shadow gets thinner.
BUT, when the object is smaller than 3 arc seconds (3”)
the width of the shadow stays the same
Shadow on cornea
Shadows on retina
The line against the sky
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Shadow on cornea
Shadows on retina
The line against the sky
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Shadow on cornea
Shadows on retina
To detect the line, the hyperpolarization of the cone at 0 must be different enough from that of adjacent cones so that the ganglion cell activity sends a strong enough signal to cortex to be detected.
In the fovea, each cone connects, through the bipolar cell, to a ganglion cell. One cone = RF center. This forms a direct line to LGN and cortex.
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Shadow on cornea
Shadows on retina
To detect the line, the hyperpolarization of the cone at 0 must be different enough from that of adjacent cones so that the ganglion cell activity sends a strong enough signal to cortex to be detected.
Actually, a series of cones in the center of the shadow would be less hyperpolarized than the ones on either side. These cones would signal, through a bipolar cell and a ganglion cell, the presence of a line.
Detection acuity is the angular size of the smallest visible target
It is an intensity discrimination task
When the shadow is so pale that the row of cones under the shadow is not hyperpolarized enough, relative to the rows of cones on each side, to cause less firing in on-center ganglion cells and more firing in off-center ganglion cells than is produce in the ganglion cells fed by adjacent rows of cones.
Visual acuity is determined largely by optical and "neural" defocus
The retinal image
Photoreceptor sampling
Convergence (receptive field center size) = “neural defocus”
A line needs to be thicker in the periphery to be detected because of convergence; several photoreceptors connect to a bipolar cell and several bipolar cells connect to a ganglion cell.
Localization acuity (also called “hyperacuity”) is the
smallest spatial offset or difference in location between
targets that can be discriminated
Acuity Task Typical Stimulus Minimum Threshold
Detection Single black spot 15” – 20” Single black line 0.5” – 1.0” Localization Spatial interval 2” – 4” Vernier lines 3” – 6” Resolution Two black lines/spots 30” – 40” Grating 30” – 40” Identification Letters or numerals 30” – 40”
Comparison of Spatial Acuity Tasks and Thresholds
Visual acuity is determined largely by optical and "neural" defocus
The retinal image
Photoreceptor sampling
Convergence (receptive field center size) = “neural defocus”
The threshold for detecting the mis-alignment of lines is less than the width of a cone, so the retina cannot detect this itself. Rather, this discrimination is achieved at the cortical level (somewhere).
Localization (hyperacuity) tasks seem to involve neural mechanisms beyond
the retina (presumably in visual cortex)
Resolution acuity is the smallest spatial separation between two
nearby points or lines that can be discriminated
The Minimum Angle of Resolution (MAR)
This is what is generally called “visual acuity” and is the most common measure of visual function made by eye-care practitioners
Uses of Spatial Acuity Measures
• Assess if refractive error is present
• Decide when to change glasses Rx
• Assess visual function (best corrected refractive error)
• Assess job eligibility (pilots, police, etc.)
• Follow disease and treatment
• Decide whether a person should drive
• Decide whether a person qualifies for disability
• Low vision assessment
• Prediction of improvement with vision aids
Table 5-1: Comparison of Spatial Acuity Tasks & Thresholds
Acuity Task Typical Stimulus Minimum Threshold
Detection Single black spot 15 - 20 sec of arc Single black line 0.5 - 1.0 sec of arc Localization Spatial interval 2 - 4 sec of arc Vernier lines 3 - 6 sec Resolution Two black lines/spots 30 - 40 sec of arc Grating 30 - 40 sec of arc Identification Letters or numerals 30 - 40 sec of arc
Chapter 1: most of the measures of vision people make are threshold measures.
All acuity measures (all three types) are threshold measures
Detection acuity: we measure the threshold line width (or spot size)
Localization acuity: we measure the threshold offset
Resolution acuity: we measure the threshold separation
Visual acuity is determined largely by optical and "neural" defocus
The retinal image
Photoreceptor sampling
Convergence (receptive field center size) = “neural defocus”
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Retinal Position (min)
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More realistic depiction of the point-spread function (line-spread function here, viewed in cross-section)
The closer together the points or lines, the less of a “dip” in intensity in between the retinal images
At the fovea, there is a match between photoreceptor size and spacing, and MAR
Center-to-center spacing of 20” – 40” in the fovea
Resolution 30” – 40” (0.5’) – one row of cones in between
What happens when the retinal image is defocused?
Why is it that the MAR gets larger (poorer acuity) when
images are out of focus?
(slides from Dr. Fullard)
Use Blur Ratio
tan
AyRatioBlur
pupil diameter (in meters)
ametropia
tan (visual angle of VA chart letter)
The closer together the points or lines, the less of a “dip” in intensity in between the retinal images
The closer together the points or lines, the less of a “dip” in intensity in between the retinal images
Convergence (receptive field center size) = “neural defocus”
The larger the receptive field, the poorer the resolution acuity (lines must be spaced farther apart)
V1-1 V1-2 V1-3
LGN-1 LGN-2 LGN-3
G-1 G-2 G-3
B-1 B-2 B-3
1 2 3
At Fovea
A
1
G
LGN
B
V1
B B
G G
LGN LGN
Outside of FoveaB
The result of larger receptive fields is that the stimuli need to be farther apart for the central “dip” in intensity to be detected at the cortex
V1-1 V1-2 V1-3
LGN-1 LGN-2 LGN-3
G-1 G-2 G-3
B-1 B-2 B-3
1 2 3
At Fovea
A
1
G
LGN
B
V1
B B
G G
LGN LGN
Outside of FoveaB
Resolution acuity is the smallest spatial separation between two
nearby points or lines that can be discriminated
The Minimum Angle of Resolution (MAR)
This is what is generally called “visual acuity” and is the most common measure of visual function
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Resolution acuity can be measured using multiple lines (gratings)
How do you measure resolution acuity?
1 degree
40 light-dark pairs = 40 cycles per degree
1 light-dark cycle = 1.50 minarc
1 line = 0.75 minarc
If there are 60 lines per degree, each line is 1’ and each pair (cycle) is 2’ (30 c/deg)
“Standard normal” VA (resolution visual acuity; MAR) is 1 min of arc (1’ arc)
On a log scale, log(1) = 0, so standard normal VA is 0 on a logMAR chart; 10’ arc = 1 on chart
“better” acuity means able to resolve smaller angles
“worse” or “poorer” acuity means larger angles are needed
Grating acuity measures are relatively insensitive to optical defocus
An example of “How you measure vision determines the result”
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