Why cover acuity in a course called Central Visual Mechanisms

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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.
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Comparison of Spatial Acuity Tasks and Thresholds
Debate are resolution and identification acuity
the same?
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It is an intensity discrimination task
You need to be able to explain the reason that
detection acuity is an intensity discrimination
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All types of visual acuity are determined
largely by optical and neural defocus For
all types of acuity, need to consider these three
things
The retinal image Photoreceptor
sampling Convergence (receptive field center
size) neural defocus
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How images spread out on the retina interaction
with pupil size
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)
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For wide objects the eyes optics only affect the
edges of the shadows image on the retina
light
At cornea
dark
On retina
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
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The line against the sky
Shadow on cornea
Shadows on retina
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Photoreceptor mosaic at the fovea (2 people)
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Shadow at cornea
Spread out image on retina
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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.
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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.
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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.
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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.
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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.
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Outside the fovea, convergence increases
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Comparison of Spatial Acuity Tasks and Thresholds
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Spatial Interval
Vernier lines
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Various forms of localization tasks
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March 2007 Vision Research Vernier Acuity in the
Barn Owl
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Localization acuity is poorer if the ends of the
lines are distant, but the acuity is less
affected by defocus
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The retinal image Photoreceptor
sampling Convergence (receptive field center
size) neural defocus
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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).
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Also called minimum separable acuity
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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
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• 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

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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 with (or spot size) Localization
acuity we measure the threshold
offset Resolution acuity we measure the
threshold separation
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The retinal image Photoreceptor
sampling Convergence (receptive field center
size) neural defocus
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More realistic depiction of the point-spread
function (line-spread function here, viewed in
cross-section)
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The closer together the points or lines, the less
of a dip in intensity in between the retinal
images
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Photoreceptor sampling
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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
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• 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)

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Use Blur Ratio
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The closer together the points or lines, the less
of a dip in intensity in between the retinal
images
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The closer together the points or lines, the less
of a dip in intensity in between the retinal
images
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Convergence (receptive field center size)
neural defocus
The larger the receptive field, the poorer the
resolution acuity (lines must be spaced farther
apart)
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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
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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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How do you measure resolution acuity?
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If there are 60 lines per degree, each line is 1
and each pair (cycle) is 2 (30 c/deg)
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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
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An example of How you measure vision determines
the result
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Worse (poor acuity)
Better (good acuity)