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Spatial Vision – 1 Stiles-Crawford Effect (SCE) Hyperacuity Visual Acuity Acuity Lab PowerPoint PPT Presentation


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Spatial Vision – 1 Stiles-Crawford Effect (SCE) Hyperacuity Visual Acuity Acuity Lab (hyperacuity, logMAR/defocus, grating/spurious resolution) Reading: Schwartz Chapter 7, Norton Chapter 5 (handout)

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Spatial Vision – 1 Stiles-Crawford Effect (SCE) Hyperacuity Visual Acuity Acuity Lab

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Spatial vision 1 stiles crawford effect sce hyperacuity visual acuity acuity lab

Spatial Vision – 1

Stiles-Crawford Effect (SCE)

Hyperacuity

Visual Acuity

Acuity Lab

(hyperacuity, logMAR/defocus, grating/spurious resolution)

Reading: Schwartz Chapter 7, Norton Chapter 5 (handout)

Approach: functional retinal anatomy, physiological optics, cortical processing and clinical application.

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Spatial vision 1 stiles crawford effect sce hyperacuity visual acuity acuity lab

Stiles-Crawford Effect (SCE-I, SCE-II)

SCE of the First Kind (SCE-I, 1933)

The brightness of a beam of light (ray)

incident on a cone depends on its entry

point in the pupil.

Describes luminous efficiency.

SCE of the Second Kind (SCE-II, 1937)

The hue & saturation of a beam of light

incident on a cone depends on its entry

point in the pupil.

Describes color effects.

Example: when centrally fixating,

monochromatic light presented at two

locations in object space is perceived

as having unequal hue and saturation.

cone

ray to pinhole

Ref-4

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Spatial vision 1 stiles crawford effect sce hyperacuity visual acuity acuity lab

Study of SCE-I >> SCE-II.SCE-II typically not on NBEO.SCE-IA ray striking a cone perpendicularto its surface produces more bleachingof photopigment; psychophysically theperception of an increase in brightness.

ray to pinhole

cone

Ref-4

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Spatial vision 1 stiles crawford effect sce hyperacuity visual acuity acuity lab

SCE-I

The entry point resulting in a ray striking a cone perpendicular to its surface represents the SCE-I peak.

The SCE-I peak typically does not correspond to the pupil center. Average empirical findings:

~ 0.4 mm nasal, ~ 0.2 mm superior

Ref-5

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Spatial vision 1 stiles crawford effect sce hyperacuity visual acuity acuity lab

SCE-I peak typically does not

correspond to the pupil center.

Average empirical findings:

~ 0.4 mm nasal

~ 0.2 mm superior.

Very similar to angle kappa ().

Recall that your clinical measure

is ‘angle ’ but you are actually

measuring angle lambda ().

Ref-5

Visual Optics Review

Optical Axis: best fit line through centers of curvatures of optical surfaces.

Pupillary axis = achromatic axis = chief ray: ray  K ---> center E ---> retina.

Visual axis = neural axis: fixation point ---> N ---> N’ ---> fovea.

Line of sight: fixation point ---> E ---> E’ ---> fovea.

Angle : pupillary axis to line of sight.

Angle  : pupillary axis to visual axis.

Angle : optical axis to visual axis .

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Spatial vision 1 stiles crawford effect sce hyperacuity visual acuity acuity lab

Full Effect

Reduced Effect

Minimal Effect

(much loss from

exit through wall)

SCE-I is attributed to the waveguide properties of cones

(analogy: the total internal reflection of fiber optics systems) and the

similar dimensions of the cone outer segment and a quantum of light.

Minimal in scotopic conditions, that is, luminous efficiency by rods do not require that rays strike perpendicular to their surface.

Logical since the intent of scotopic vision is the capture of all light.

Incident Ray Foveal Cone Inner-Outer Segment

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Spatial vision 1 stiles crawford effect sce hyperacuity visual acuity acuity lab

Empirical studies

find variable 

for function form

and peak.

SCE-I reduces the effective (actual) retinal illuminance or trolands; equivalent to having a smaller effective pupil size.

Enhances retinal image quality by reducing the effects of light scatter, defocus and ocular aberrations.

Overall acts as a variable density optical filter placed at the pupil:

Bradley & Thibos

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Spatial vision 1 stiles crawford effect sce hyperacuity visual acuity acuity lab

SCE-I is believed to reflect the overall alignment of the cones.

This alignment shifts under conditions of natural, pathological or contact lens-induced displaced pupil.

Thus the SCE-I represents a phototropic effect: foveal cones align themselves to the incident light.

Ref-4

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Spatial vision 1 stiles crawford effect sce hyperacuity visual acuity acuity lab

SCE-I Methods

Subjective methods: flicker photometry (reduce until not seen).

Objective methods: fundus reflectometry, electroretinogram.

Not measured clinically. Does not normally change with age.

Clinically useful in understanding functional retinal plasticity.

Bradley & Thibos

Ref-4

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Spatial vision 1 stiles crawford effect sce hyperacuity visual acuity acuity lab

SCE-I Contemporary method: adaptive optics.

Image cones (1 nasal to fovea) and reflected light; 7 pupil entries.

Ref-6

Ref-6

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Cones located close to each other share similar pointing directions

Cones located close to each other share similar pointing directions.

Ref-6

Ref-6

1 mm displacement magnitude (pupil)

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Spatial vision 1 stiles crawford effect sce hyperacuity visual acuity acuity lab

Not all cones are pointed nasal and superior to the pupil center.

Central

2 mm pupil.

Cones

pointing

direction.

Pupillary

axis.

95% CI for

each cone

direction.

Ref-6

Ref-6

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Spatial Acuity: Definitions

Generally the smallest spatial detail that can be detected, discriminated or identified.

Detection Acuity: angular size of the smallest visible target; typically a dot or line target.

Resolution Acuity: smallest spatial separation between two targets that can be discriminated.

Localization Acuity: smallest spatial offset between targets that can be discriminated.

Also called hyperacuity or vernier acuity.

Identification Acuity: smallest detail of the letters or numbers that can be identified.

Typically named for the optotype used for

acuity, e.g. Snellen, logMAR, Sloane, HOTV.

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Typical stimuli and thresholds arc seconds for normal subjects

Ref-1

Typical stimuli and thresholds (arc seconds) for normal subjects.

Note: 30–40 arc seconds correlates to the width of a

cone inner segment in the foveola of normal subjects.

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Spot Target Example:

Target: point source (like a star).

Pupil size: 1.5, 2.4, 6.6 mm.

Solid lines: retinal image spread.

Dashed lines: pupil affect alone.

Sharpest image: with 2.4 mm pupil.

Star angular size 0.018” at the eye.

Not visible in day due to light from

the surrounding sky, despite its

size increasing ≥ 200x due to the

eye’s aberrations. Easily visible at night due to enhanced contrast.

Note that target size (threshold)

is often not equal to image size,

especially for very small targets.

diffraction

limited

image

optimal

image

maximum

aberration

affect on

image

Detection acuity is the angular size of the smallest visible target. Threshold is limited more by the luminance or contrast of the retinal image of the target and less by its angular size (which is altered by the eye’s aberrations).

Ref-1

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Spatial vision 1 stiles crawford effect sce hyperacuity visual acuity acuity lab

Detection acuity is limited more by the luminance or contrast

of the retinal image of the target and less by its angular size.

Line Target Example:

Black line angular size 1” or 3”

(thickness) at the eye.

Visible due to the decrement

in luminance from the surround,

more than from increase in size

(similar size (~2’) as the star)

due to the eye’s aberrations.

Note that target size (threshold)

is often not equal to image size,

especially for very small targets.

Ref-1

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Spatial vision 1 stiles crawford effect sce hyperacuity visual acuity acuity lab

Ref-2

Optics. Point source image =

Point Spread Function (PSF).

Airy Disk (diffraction) pattern:

a = angular radius = 1.22/d,

where d = pupil diameter and

 = wavelength of light.

Resolution acuity is the smallest separation between two targets that can be discriminated. It is limited by the quality target images (eye’s optics) and the sampling grain (retina).

Ref-9

Sampling. Smallest separation =

width of the the target PSF, i.e. first

peak in first adjacent trough. Allows

one cone per change in luminance.

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Spatial vision 1 stiles crawford effect sce hyperacuity visual acuity acuity lab

Resolution acuity for lines is like that for points of light.

Line Spread Function (LSF) for different pupil sizes:

dashed lines = pupil effect alone.

Ref-2

Ref-2

Optimal pupil size (least aberrations) is 2.4 mm. Keep in mind for clinical tests.

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Resolution acuity for lines is like that for points of light.

Aberration effect on a line and filament for different pupil sizes:

Aberrations change with pupil size:

large pupil ---> smaller pupil ---> smallest pupil

Ref-2

Ref-2

Optimal pupil size (least aberrations) is 2.4 mm. Keep in mind for clinic tests.

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Spatial vision 1 stiles crawford effect sce hyperacuity visual acuity acuity lab

(1” line, 3 mm pupil)

Ref-2

Resolution Acuity.Line separation must enable one cone to detect the luminance decrement, i.e. hyperpolarize less than the adjacent cones.

With optimal eye optics

& cone spacing (avg 30”):

Resolution acuity for two

points or lines is 30–40” .

Called the Minimum Angle

of Resolution (MAR).

Ref-1

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Spatial vision 1 stiles crawford effect sce hyperacuity visual acuity acuity lab

Ref-3

Resolution Acuity. Everyone cannot achieve a MAR of 30” – 40”

due to differences in cone size, cone spacing and ocular aberrations.

Ref-1

Ref-2

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Localization Acuity, or Hyperacuity, is the smallest spatial offset or difference in location between targets that can be distinguished.

Examples: spatial-interval acuity and vernier acuity.

Ref-1

Ref-4

Ref-3

Due to the LSF, targets that are close result in worse acuity.

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Spatial vision 1 stiles crawford effect sce hyperacuity visual acuity acuity lab

Localization acuity develops late (age 9–11) – implicating visual cortex.

Stereoacuity

Localization (vernier)

acuity starts late and

reaches near adult

levels late (age 9–11).

Acuity

Contrast

Fusion

OKN

VEP

Ref-7

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Spatial vision 1 stiles crawford effect sce hyperacuity visual acuity acuity lab

Estimates of retinal light

distribution demonstrate

the substantial difference

difference between target

separation and shift.

Separation is analogous,

regardless of spatial scale,

to both resolution and

identification acuity.

How does the retina and

visual system encode the

shift information?

Localization Acuity (Hyperacuity)

Under optimal conditions, spatial offsets of 2–4” can be distinguished.

This is much smaller than the 30” subtense of foveola cones.

How does the visual system achieve this ‘hyperacuity’?

Ref-2

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Spatial vision 1 stiles crawford effect sce hyperacuity visual acuity acuity lab

The Line Spread Function enables target shifts much smaller than

the 30” subtense of foveola cones to influence adjacent cones.

With LSF,

x can be small

to stimulate the

adjacent cones.

Without LSF,

x must be large

to stimulate the

adjacent cones.

Ref-3

Aberrations thus enable this fine discrimination ability.

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Spatial vision 1 stiles crawford effect sce hyperacuity visual acuity acuity lab

The retinal mosaic generally matches the eye’s aberrations.For example, poor retinal sampling in the periphery is matched to the very aberrated peripheral retinal images.Localization acuity is just one example of how aberrations are beneficial, and could be viewed as designed, for optimal visual performance (perception and efficiency) in the central retina.

Ref-3

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Spatial vision 1 stiles crawford effect sce hyperacuity visual acuity acuity lab

Lab

used this

example

Triangles = reference line

Circles = shift line (12”)

Inset plots the

absorption ratio:

reference line / shift line

Shows a 50% difference

in absorption rate due

to only a 12” shift.

This information could

signal the spatial offset.

Many cones encode absorption rates for each line spread function; the difference across space is the likely mechanism for hyperacuity.

Ref-2

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Spatial vision 1 stiles crawford effect sce hyperacuity visual acuity acuity lab

Localization acuity appears to involve specific retinal encoding and

corresponding cortical processing. Evidence for the latter comes from

developmental amblyopes (onset age 3–5) who show a disproportionate

loss of vernier acuity relative to letter acuity. Vernier loss = cortical loss.

Ref-2

Ref-7

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Nice lsf summary schematic to help visualize mechanisms for all acuities

2 microns =

foveola cone

inner segment

width

We use 30”

for a cone in

the foveola.

Nice LSF summary schematic to help visualize mechanisms for all acuities.

Ref-9

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Spatial vision 1 stiles crawford effect sce hyperacuity visual acuity acuity lab

References

Norton, T., Corliss, D., & Bailey, J. (2002). The psycholphysical measurement of visual function. Butterworth–Heinmann.

Wandell, B. (1995). Foundations of vision. Sinauer.

Regan, D. 2000). Human perception of objects: early visual processing of spatial form, defined by luminance, color, texture, motion, and binocular disparity. Sinauer.

Schwartz, S. (2004). Visual perception: a clinical orientation. 3rd Ed. McGraw-Hill.

Atchison, D., & Smith, G. (2000). Optics of the human eye. Butterworth–Heinmann.

Roorda, A., & Williams, D. (2002). Optical fiber properties of individual human cones. Journal of Vision, 2, 404–412.

Daw, N. (2006). Visual Development. Springer Science, NY.

Moses, R. (Ed.) (1981). Adler’s physiology of the eye. 7th ed. Mosby.

Michaels, D. (1980). Visual optics and refraction. 2nd. Ed. Mosby.

Kaufman, P, & Alm, A. (Eds.). (2002). Adler’s physiology of the eye, 10th ed. Mosby.

Chalupa, L, & Werner, J. (Eds). (2004). The visual neurosciences. Sinauer.

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