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.
Comparison of Spatial Acuity Tasks and Thresholds
Acuity Task
Typical Stimulus
Minimum Threshold
Detection
Single black spot
15” – 20”
Single black line
0.5” – 1.0”
Spatial interval
2” – 4”
Vernier lines
3” – 6”
Two black lines/spots
30” – 40”
Grating
30” – 40”
Letters or numerals
30” – 40”
Localization
Debate: are
resolution and
identification
acuity the
same?
Resolution
Identification
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
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”
How images spread out on the retina & interaction with pupil size
1.5 mm
Relative
Retinal
Illuminance
2.4 mm
6.6 mm
-4
-2
0
2
Angular Distance (min)
4
Dashed line = theoretical
point-spread function based
on pupil alone (larger pupil
give narrowest point-spread)
Solid line = actual pointspread function based on all
factors (intermediate pupil is
best)
For wide objects the eye’s optics only affect the edges
of the shadow’s 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
Target
The line against the sky
Target
Luminance
1.0
Shadow on cornea
0.8
0.6
0.4
0.2
0.0
Retinal
Illuminance
1.0
Shadows on retina
0.8
0.6
3 SEC ARC
0.4
0.2
0.0
Photoreceptor
Array
-1
0
Retinal Position (min)
+1
Target
The line against the sky
Target
Luminance
1.0
Shadow on cornea
0.8
0.6
0.4
0.2
0.0
Retinal
Illuminance
1 SEC ARC
1.0
Shadows on retina
0.8
0.6
0.4
0.2
0.0
Photoreceptor
Array
-1
0
Retinal Position (min)
+1
Photoreceptor mosaic at the fovea (2 people)
Shadow at cornea
Spread out image on retina
Target
Target
Luminance
1.0
Shadow on cornea
0.8
0.6
0.4
0.2
0.0
Retinal
Illuminance
1 SEC ARC
1.0
Shadows on retina
0.8
0.6
3 SEC ARC
0.4
0.2
0.0
Photoreceptor
Array
-1
0
Retinal Position (min)
+1
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.
Target
Target
Luminance
1.0
Shadow on cornea
0.8
0.6
0.4
0.2
0.0
Retinal
Illuminance
1 SEC ARC
1.0
Shadows on retina
0.8
0.6
3 SEC ARC
0.4
0.2
0.0
Photoreceptor
Array
-1
0
Retinal Position (min)
+1
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.
Outside the fovea, convergence increases
Localization acuity (also called “hyperacuity”) is the
smallest spatial offset or difference in location between
targets that can be discriminated
Comparison of Spatial Acuity Tasks and Thresholds
Acuity Task
Typical Stimulus
Minimum Threshold
Detection
Single black spot
Single black line
15” – 20”
0.5” – 1.0”
Localization
Spatial interval
Vernier lines
2” – 4”
3” – 6”
Resolution
Two black lines/spots
Grating
30” – 40”
30” – 40”
Identification
Letters or numerals
30” – 40”
Vernier lines
Spatial Interval
s
Offset
s
Ds
Various forms of localization tasks
March 2007 Vision Research
Vernier Acuity in the Barn Owl
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
Also called “minimum separable acuity”
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
Single black line
15 - 20 sec of arc
0.5 - 1.0 sec of arc
Localization
Spatial interval
Vernier lines
2 - 4 sec of arc
3 - 6 sec
Resolution
Two black lines/spots
Grating
30 - 40 sec of arc
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”
Target
More realistic
depiction of the Target
point-spread Luminance
1.0
function (line0.8
spread function 0.6
here, viewed in 0.4
0.2
cross-section)
0.0
Retinal
Illuminance
1.0
0.8
0.6
0.4
0.2
0.0
Photoreceptor
Array
-1
0
Retinal Position (min)
+1
The closer together the points or lines, the less of
a “dip” in intensity in between the retinal images
Photoreceptor sampling
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
pupil diameter
(in meters)
ametropia
yA
Blur Ratio 
tan 
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)
A
B
Outside of Fovea
At Fovea
1
2
1
3
B-1
B-2
B-3
B
B
B
G-1
G-2
G-3
G
G
G
LGN-1
LGN-2
LGN-3
LGN
LGN
LGN
V1-1
V1-2
V1-3
V1
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
A
B
Outside of Fovea
At Fovea
1
2
1
3
B-1
B-2
B-3
B
B
B
G-1
G-2
G-3
G
G
G
LGN-1
LGN-2
LGN-3
LGN
LGN
LGN
V1-1
V1-2
V1-3
V1
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
Target
Target
Luminance
1.0
0.8
0.6
0.4
0.2
0.0
Retinal
Illuminance
1.0
0.8
0.6
0.4
0.2
0.0
Photoreceptor
Array
-1
0
Retinal Position (min)
+1
How do you measure resolution acuity?
Resolution acuity can be measured using multiple lines (gratings)
If there are 60 lines per degree, each
line is 1’ and each pair (cycle) is 2’
(30 c/deg)
1 light-dark cycle = 1.50 min
arc
1 line = 0.75 min
arc
1 degree
40 light-dark pairs = 40 cycles per degree
“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”
Worse
(poor
acuity)
LogMAR
1.8
Gratings
Letters
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
Better
(good
acuity)
0.0
0
2
4
6
Blur (diopters)
8
10
12