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 imagePhotoreceptor samplingConvergence (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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Photoreceptor mosaic at the fovea (2 people)

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.

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 imagePhotoreceptor samplingConvergence (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 betweentargets 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

s

s s

Offset

Spatial Interval Vernier lines

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 imagePhotoreceptor samplingConvergence (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 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 offsetResolution acuity: we measure the threshold separation

Visual acuity is determined largely by optical and "neural" defocus

The retinal imagePhotoreceptor samplingConvergence (receptive field center size) = “neural defocus”

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PhotoreceptorArray

Retinal Position (min)-1 0 +1

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

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

tanAyRatioBlur

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

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At FoveaA

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LGN

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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 FoveaA

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G

LGN

B

V1

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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”

Blur (diopters)

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LogMAR

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1.8Gratings Letters

Worse

(poor acuity)

Better

(good acuity)