1
Q
Define Spatial Vision. Provide an example of the clinical application of spatial vision
A
- Defined: the ability of visual system to detect (or discriminate) and resolve (luminance-defined) stimuli of various size & contrasts
- Clinical assessment of spatial vision is part of routine eye care
- Example: VA measurement – determination of spatial resolution a highly sensitive measure of visual function
- Concerned with variations in luminance across space
- Measured with sine-wave gratings of varying spatial frequency and contrast
- Clinically assessed using VA & Contrast sensitivity tests
- The visual system is evolved to detect & recognize objects from patterns of light and dark on the retina
2
Q
Is absolute luminance or relative luminance more important in spatial vision?
A
- Absolute luminance is less important in spatial vision than relative luminance (contrast) levels because luminance levels vary greatly in the environment, but relative luminance’s (contrast) for visual stimuli do not
- The visual system responds to the luminance differences (contrast) as the boundaries between objects and their background, which is the basis for brightness constancy
3
Q
Define Visual Acuity.
A
- Measures the smallest detail that can be resolved & recognized
- It measures our ability to see objects/targets of different size at high contact (i.e., of different spatial frequency)
- One point on the CSF
4
Q
Define Contrast Sensitivity.
A
- A psychophysical measure used to assess the sensitivity of the visual system to spatial luminance changes of various spatial frequencies
- Provides a more comprehensive test of spatial vision compared to VA
- CS is measured by finding the lowest contrast needed to see light/dark gratings of varied fineness or spatial frequency
5
Q
Define luminance (L) and its unit.
A
- The physical amount of light emitted by a source of reflected from an illuminated object
- Unit of measure: candela per square meter, cd/m2
6
Q
Define brightness. What affects brightness perception?
A
- Aka. Intensity
- The perception of a luminous object by the human visual system
- Perception of brightness can be affected by:
- Adaptation
- Aftereffects
- The presence or absence of other objects in the visual field
7
Q
Define Contrast.
A
- Aka. Contrast Threshold
- Refers to the difference in luminance between an object (Lmax) and its background (Lmin) where the object is typically brighter than the background
- Contrast can be defined by: ∆I / Laverage
- ∆I = difference between peak and average luminances (Lmax – Lavg)
- Laverage = the average luminance of the grating (the average of light peaks and dark troughs)
- Though useful, not practical for the measurement of contrast
8
Q
Define modulation (M). How is it calculated?
A
- The contrast of a repeating pattern, commonly referred to as grating, where it is not apparent which part of the pattern is the “object” and which is the “background”
- Example: a series of light (Lmax) and dark (Lmin) stripes, as in the zebra
- Calculated as the ratio of the difference of the luminances to the sum of the luminances
- M = (Lmax – Lmin) / (Lmax + Lmin)
- Where the luminance is measured across the spatial extent of each component part of the pattern
- M = (Lmax – Lmin) / (Lmax + Lmin)
- Alternatively, modulation can be expressed by the mathematically equivalent ratio of the difference between maximal and average luminances to the average luminance (Lavg), i.e. modulation (contrast) = ∆I / Lavg
- M = (Lmax – Lmin) / Lavg, where Lavg = (Lmax + Lmin) / 2
9
Q
Describe CS Test designs.
A
- Most manufacturers of CS tests refer to their charts and instruments as presenting stimuli at certain “contrast” levels, also mention modulation
- Correct usage: modulation values
- Chart 5016 designated for Low Vision: uses the upper 12 contrast levels ranging from 0 – 1.65 log CS (100% to 2.2% Weber)
- Chart 5017 designated for Peak Contrast Sensitivity: uses the lower 12 contrast levels ranging from 0.6 – 2.25 log CS (25% - 0.56% Weber)
10
Q
What is a logarithm? Describe the use of Logarithm in CSF.
A
- The logarithm of a number represents that value as a power of 10. Mathematically, it is much easier to characterize the range
- For CSF, because of the potentially large ranges of responses for both spatial frequency and sensitivity, the data is commonly plotted on logarithmic scales on both axes
- This results in the characteristic inverted-U shape of the function
11
Q
Contrast Threshold (CT)
A
- The least amount of contrast that can be seen (or resolved by the patient) in a chart (expressed in %) = Weber contrast in letter chart
- The maximum contrast is 100% contrast (Snellen compared to 10% Bailey-Lovie chart)
- The optotypes of the visual acuity charts are close to the maximum contrast
12
Q
Contrast Sensitivity (CS)
A
- The reciprocal of contrast threshold (CS = no units)
- CS = 1/CT
13
Q
Log Contrast Sensitivity (Log CS)
A
- “Log of CS” converts the CS values to a linear scale
- Suitable for comparisons between levels of contrast sensitivity
- Smaller the log CS values = poorer the patients CS
14
Q
Spatial Frequency
A
- Refers to the number of light/dark cycles per degree (c/deg) of visual angle (i.e., components of a repeating pattern, or grating, occur within a given area or space) on the retina
15
Q
Gratings
A
- Usually defined by gradual sine-wave (i.e., sinusoidal) variations in luminance
16
Q
Contrast Threshold (CT) to Contrast Sensitivity (CS)
A
CT = 1/CS
17
Q
Contrast Sensitivity (CS) to Contrast Threshold (CT)
A
CS = 1/CT
18
Q
Contrast Sensitivity (CS) to Log Contrast Sensitivity (logCS)
A
LogCS = log of CS
19
Q
Log Contrast Sensitivity (logCS) to Contrast Sensitivity (CS)
A
CS = 10logCS
20
Q
Name and describe the 3 types of contrast.
A
- Weber Contrast is preferred for Letter Stimuli
- Michelson Contrast is preferred for Gratings
- RMS Contrast is preferred for Natural Stimuli
21
Q
Weber Contrast
A
- Weber Contrast = (Lb – Lt) / Lb
- Lb = luminance of background
- Lt = luminance of target
- Commonly used in cases where small target/features are present on a large uniform background
- Weber contrast is preferred for letter stimuli
- the measure is also referred to as Weber fraction
22
Q
Michelson Contrast
A
- Michelson Contrast = (Lmax – Lmin) / (Lmax + Lmin) = ∆L / Lavg
- Preferred for gratings
- Incorporates the maximum and minimum luminance
23
Q
Describe the Scales of Weber vs. Michelson Contrast.
A
- On both scales:
- 0% indicates the absence of contrast
- 100% indicates the theoretical maximal contrast
- For values between the two scales differ
- In the range from 1% to 10%, which is the range most used for clinical measurements, the Weber contrast values are about 2x the Michelson values
- The log(CS) values differ by 0.3
- In the range from 1% to 10%, which is the range most used for clinical measurements, the Weber contrast values are about 2x the Michelson values
24
Q
What is RMS Contrast?
A
- Root Mean Square (RMS) Contrast – does not depend on the angular frequency content or the spatial distribution of contrast in the image
- Defined as the standard deviation of the Pixel Intensities
- Not used clinically, but for image processing/analysis
25
How do we study the complex processes of spatial vision?
* The sensitivity of the visual system to spatial luminance changes is measured with sine-wave gratings of varying spatial frequency and contrast
26
What is Sine wave? Why do we use sine waves?
* Sine wave (or sinusoid) – a mathematical curve that describes a smooth repetitive oscillation
* Sine waves are characterized by their spatial frequency (dark/light stripes in a given distance @ cycles/deg), contrast, and their phase
* Sine wave grating – alternating bright and dark bars
* The transition from bright to dark is gradual
27
Why do we use sine waves?
* When passed through an optical system, they maintain shape even after degradation (defocus, aberration, diffraction) and only change in contrast (amplitude) and phase (luminance remains sinusoidal)
* Sine wave gratings constitute the building blocks of complex periodic waveforms
* A sine wave grating is more useful for studying the thresholds of the visual system than any other waveform because it is composed of a SINGLE spatial frequency
28
What are the elements used to describe sine-wave gratings?
* Frequency
* Wave cycles:
* Low spatial frequency, high spatial frequency, SAME CONTRAST
* The spatial frequency of a grating can be specified by giving the number of cycles/degree of visual angle
* Contrast
* Same spatial frequency – but vary in low and high contrast gratings
* Average luminance (Lave) is the same for both the gratings
* Higher contrast is indicated by a larger difference between its peak and the average luminance
* Phase
* Refers to the position of a sine wave grating with respect to another sine wave grating
* In phase: the peak and troughs of the luminance profiles are in alignment
* 180˚ out of phase: the peaks of one luminance profile aligns with troughs of other profile
* Orientation
* Describes the angle made by a grating with respect to a reference (such as the horizontal, vertical)
29
Describe Fourier Transformation.
* Mathematical process that involves transforming a signal (function\_ from the spatial domain to frequency domain
* Image processing, signal detection
* Sine waves of the proper frequency, contrast, phase, and orientation can be used to contrast complex stimuli
* Square wave grating – luminance profiles shows abrupt changes between bright and dark bars “step” changes in luminance
30
How do you construct a square wave in Fourier Analysis?
* Square wave can be constructed by adding together appropriate sine waves
* Sine wave that is of the same frequency as square wave is referred as fundamental
* Addition of the fundamental sine wave and the odd numbered harmonics (3rd and 5th) produces a square wave
* Adding all the ODD numbered harmonics (up to infinity) to the fundamental will produce perfect square wave
31
What are Harmonics?
* Higher frequency sine waves
* 3rd harmonics has 3x the frequency of the fundamental and 1/3rd of its contrast
* 5th harmonics has 5x the frequency of the fundamental and 1/5th of its contrast
32
What is Fourier Analysis?
* Can determine the component sine waves and contrasts of complex waveforms
33
What is Fourier Synthesis?
* Can construct SQUARE WAVE GRATING using the component sine waves and contrasts
34
Why did we learn about Fourier transformation in this topic?
* Our visual system is believed to act as a Fourier analyzer: the visual system is thought to deconstruct the retinal image into its spatial frequency components
35
What is MTF? How does it change for High and Low spatial frequency?
* MTF is the ability of an optical system to produce an accurate image of an object
* MTF = Image modulation (contrast) / Object modulation (contrast)
* The ratio of the image modulation to the object modulation for a given spatial frequency
* For low spatial frequencies, the image blur is hardly noticeable
* For high spatial frequencies, the image is almost fully obscured
36
Why will an image never be a perfect representation of an object?
* Diffraction
* Chromatic and monochromatic aberrations
* Focus and power errors
37
Define SMTF in the optical system.
* SMTF = image quality
* Simple lenses (e.g., spectacles) or complex lens systems (e.g., telescopes and cameras), the modulation transfer function (MTF) gives the modulation transfer for all possible spatial frequencies
38
What is the MTF of the human visual system?
* CSF
* Neural and cortical processing and the optics of the eye contribute to the final outcome of the CSF
39
How can we quantify the precision with which an optical lens (system) transfers information?
* Grating of specific spatial frequency and contrast (as object)
* Image contrast is measured
* The ratio = image quality (MTF)
* This procedure is repeated for a spectrum of spatial frequencies ranging from low to high to get SMTF
40
What MTF provides the best image quality?
* MTF = 0 to 1, where 1 = best image quality
41
How does SMTF change with lens in focus, lens in defocus, and translucent lens?
* For a Lens in Focus:
* Low and moderate frequencies are unaffected, the image is transferred with good fidelity as aberrations (in the optical system) have very little or no effect in lower spatial frequencies and typically affect higher spatial frequencies
* For a Lens in Defocus:
* Reduction in image quality in higher spatial frequencies
* For a Translucent Lens:
* Equal degradation at all spatial frequencies
42
What is CSF? What is the range and peak of adult CSF?
* Depicts an observer’s sensitivity (i.e., 1/CT) to sinusoidal bar gratings of widely varied spatial frequency
* The human SMTF is referred as contrast sensitivity function (CSF) because sensitivity (and not image contrast) is measured
43
How do you measure CSF?
* Spatial grating is presented at varying levels of contrast (low to high) starting below threshold until its seen (values are plotted)
* The reciprocal of threshold contrast is the contrast sensitivity of the grating
* CSF Determination – the sensitivity (of the visual system) to spatial luminance changes is measured with gratings of varying spatial frequency and contrast
44
What is adult contrast sensitivity?
* Adults contrast sensitivity is greatest to intermediate spatial frequencies (about 2-10 cycles/degree)
* Peak sensitivity at 4 cycles pr degree
45
In the CSF curve what’s on x-axis and what’s on y-axis?
* X-axis – cycles/degree (log)
* Y-axis – contrast sensitivity (log)
46
What factors cause the CSF curve shape to vary?
* Luminance levels from photopic, mesopic, and scotopic conditions
* In the real world, brightness and contrast vary
* Visual disorders may affect patients at lower contrast levels
* May score normally but still complain of blurry vision
47
How does the CSF curve change for photopic, mesopic, and scotopic conditions?
* _Spatial Contrast Sensitivity Function: sCSF_
* Ability to differentiate static stimuli (target) that vary in luminance across space
* Example: most contrast sensitivity eye charts/tests
* _Temporal Contrast Sensitivity Function: tCSF_
* Ability to differentiate stimuli (target) that vary in luminance across time
* Temporal (time-varying, flickering)
48
Where is Contrast Sensitivity highest?
* Mid-spatial frequencies
49
Name the key landmarks of CSF curve and their limitations.
* Provides an evaluation of real-world vision
* Everything under the CSF is visible
* Everything above the CSF is invisible
* The inverted U-shaped function is a window of visibility
50
What does the shape of CSF reflect? What is CSF high frequency cutoff?
* Reflects a window of visibility
* High SF cut off (x-axis) – corresponds to best VA
* Limited by optics (aberrations) and paking of photoreceptors
* Peak CS 4 cpd (2-5 cpd) may – correspond to average receptive field size of retinal ganglion cells
* Low SF cut off – due to lateral inhibition throughout the visual system
* If a single spatial frequency channel is affected, notches may appear in the CSF
* Lower and higher spatial frequencies require more contrast to be detected, resulting in an inverted-U function
51
What is the relationship between VA and the CSF?
* The highest spatial frequency can be resolved only at very high contrast and corresponds to the observer’s acuity level
* CSF high frequency cutoff – refers to the visual system’s limited ability to resolve detail when the contrast is 100% (normal VA)
* As the spatial frequency of 100% contrast is increased, a point is reached where the grating is no longer resolved
* Bottom row (100% contrast) represent VA chart optotypes
* Even at 100% contrast, point is reached where the details cannot be resolved
* VA is the high-frequency cutoff determined with optotypes rather than gratings
* Optotypes are very high contrast
* Low contrast VA is not routinely measured
52
Describe the Snellen Fraction.
* Numerator – distance at which the measurement is taken
* Denominator – foot-size of the smallest optotype patient can resolve
* Defined: distance at which an optotype subtends 5-min of arc
* The optotype detail subtends 1/5th (1-min of arc)
53
What is the advantage of Snellen?
* Easy calculation of MAR
54
What is the high-frequency cut off for healthy adults?
* 60 cycles/degree
55
Why does the visual system show reduction in sensitivity for high spatial frequencies?
* Optical limitations
* High spatial frequency limitations due to aberrations
* Packing density of photoreceptors
* Limits high spatial frequency
* Consider low vs. high densities of photoreceptors
* If each receptor sums up the light that falls on it
* High density can resolve
* Foveal cones 0.5 min arc
* If each cycle subtends 1 min arc, then spatial frequency = 60cpd
* This corresponds to high spatial frequency cut off for the CSF
* 60cpd = Snellen acuity 20/10
56
How does the Ganglion Cell respond when light falls on its receptive field?
* Center, surround
* Excitation
* Increase in the frequency of action potentials
* Inhibition
* Decrease in the frequency of action potentials
* High spatial frequency
* Strongly activated
* Low spatial frequency
* Smaller response
57
How does the shape of the SCSF differ for species other than humans? Give an example.
* The spatial CSF for many species shows band pass filtering similar to that of humans, but the peak sensitivity occurs at different spatial frequencies
* There is a correlation between spatial frequency ranges that can be easily detected by various species and the spatial dimensions of the visual objects that they encounter in their daily life
* Examples:
* Falcons – shifted toward higher spatial frequencies
* Macaque monkeys – nearly identical to that of humans
58
What happens to the high frequency cut-off if the eye is out of focus, such as uncorrected refractive errors (myopia)?
* Out of focus = results in reduction in high-spatial frequency cut-off
59
What are the 3 ways the spatial CSF varies with retinal illuminance?
* As mean luminance of the grating decreases:
* Peak contrast sensitivity decreases and shifts toward lower spatial frequencies
* The high-frequency cutoff spatial frequency (acuity) decreases and shifts to lower spatial frequency
* A shift in peak spatial frequency from 8 to 2 cpd and a drop in the cutoff is explained by the transition from photopic, cone-driven (small) receptive fields to scotopic, rod-driven (large) receptive fields
* Low spatial frequency rolloff becomes less prominent until it completely disappears
* This is related to the inactivation of the receptive-field surround under conditions of very low luminance
60
What effect does retinal eccentricity have on spatial CSF?
* The spatial SCF shifts toward lower spatial frequencies with retinal eccentricity
* Receptive fields become larger with distance from the fovea
* The size of receptive fields dictates the peak spatial frequency and the cutoff high spatial frequency
61
What are the reasons for decrease in CSF with retinal eccentricity?
* The size of the receptive field depends on the number of photoreceptors that converge on a bipolar cell and the number of bipolar cells that converge on a ganglion cell
* Both types of convergence increase with eccentricity
* The amount of cortical area devoted to representing the retinal periphery (the cortical magnification factor) decreases with eccentricity
62
How does CS change with aging? How does CS develop in infants?
* At birth, infants’ sensitivity to fine, high-spatial frequency gratings, like their acuity, is very poor but improves steadily with age
* Newborns: cannot resolve targets above 2-3 cpd
* 3 months & 6 months: improves
* Adult CSF (reached at 7-9 years): 4 cpd
* CSF of an infant differs in 3 ways from an adult:
* Peak is lower
* Function is shifted to the left
* Shape is a “low pass” rather than a “band pass” function
* With increasing age, the infant is increasingly able to see fine-detail, low-contrast elements in their environment
* Spatial contrast sensitivity is stable in young adults and declines with age at intermediate and high spatial frequencies
63
Describe Low-pass and Band-pass functions in relation to CSF.
* Low-pass – refers to signals of a low frequency range
* Band-pass – refers to signals within a certain band or spread of frequencies without distorting the input signal
64
What is the postnatal acuity formula to determine CSF in infants?
* Postnatal acuity (formula):
* Infants age in months = grating acuity in cpd
65
What is Test-retest Variability (TRV)?
* The fact that a patient’s measured VA may vary upon repetition (even when there has been no change in the patient’s visual status)
* The clinician may have difficulty asserting if the patient’s VA has worsened or if there is variability in the technique used as a measurement
* Low spatial frequency content is an important contributor to TRV
* A new test has been designed to correct this potential source of variability
* Removes low spatial frequencies form the optotypes, creating what are called high-pass optotypes, and places them on a background of the same average luminance
* Vanishing optotypes – they become invisible at about the same time they cannot be resolved
* Compared to standard charts, there is a reduction in TRV
66
What is the Snellen to Spatial Frequency (CPD) Conversion?
* 20-ft Snellen demand, use 600 = (spatial frequency) x (Snellen denominator)
* 6-m Snellen demand, use 180 = (spatial frequency) x (Snellen denominator)
67
How can you determine CSF in the clinic? Provide examples of charts used in the clinic.
* Charts designed to measure spatial contrast sensitivity functions clinically are based on what is important for recognizing object
* Printed Chart vs. Projected Snellen Chart
* Printed chart – room illumination has little or no effect on contrast (except in very low light levels)
* Projected chart – contrast is affected by change in room illumination
* 5 commonly used CSF charts in the clinic
* Low contrast Bailey-Lovie Acuity Chart
* Pelli-Robson Contrast Sensitivity Chart
* On printed chart (VISTECH)
* FACT: Functional acuity contrast test
* MARS: letter contrast sensitivity test
68
Fixed size, variable contrast charts:
* Pelli-Robson
* MARS CS (near)
* Rabin
69
Fixed contrast (low), variable size charts:
* ETDRS
70
Fixed contrast (high), variable size charts:
* ETDRS
* Snellen
* Can use the latter two types to get a slope of high frequency end
71
Variable contrast, variable size (gratings) charts:
* FACT
* VCTS/VISTECH charts
72
What are the most accurate clinical method display for CSF?
* Display gratings on a video monitor that is driven by a microprocessor
* Such instruments are typically available only in large eye clinics
73
Describe the Pelli Robson chart. What are the normal values for adults and above 60 years?
* Constant optotypes, variable contrast
* All of the letters in the chart are the same size, subtending 0.5˚ in height at 3 m
* The chart consists of 3-letter sets (or triplets), drawn from 10 equally visible letters
* Each triplet has the same contrast
* As one moves to the right and down the chart, the contrast, but not the size of the letters, decreases, such that each set of 3 letters on the right half of the chart has a slightly lower contrast on a logarithmic scale than the triplet on the left
* Testing distance: 3 m normally, 1 m for LOW VISION
* Scoring:
* Each triplet is 0.15 (each letter is 0.05)
* STOP – when the patient gets 2 or more letters in a triplet wrong
* The score for the test is the number of the triplet in which at least 2 of the 3 letters were identified correctly
* Normal (mean) log CS values for Pelli-Robson:
* Log CS: 1.68 (60+), 1.84 (20-39 years)
* Smaller the CS value, poorer the patient’s CS
74
Describe FACT.
* Target: sine wave gratings
* 5 spatial frequencies (row A to E)
* 9 levels of contrast
* Contrast decreases left to right uniformly from 1 through 9 in steps of 0.15 log units
* Spatial frequencies vary top to bottom
* Test distance: 10 ft, 3m
* Last seen is taken as patient’s CS
75
Describe MARS CS chart.
* Test distance: 40cm
* STOP: when 2 consecutive mistakes occur
* Handheld and designed for convenient near-vision testing
* Each chart is printed with 48 different contrast levels, declining gradually in 0.04 lot unit steps – the finest steps available in ANY printed contrast level
* Testing is rapid – generally under a minute per eye for testing and scoring
* 3 charts: OD, OS, OU
* Available in both letter and numeral versions
* Smaller the log CS value = poorer the CS of the patient
76
Why measure CS? What are the factors affecting CS?
* Sensitive measure to smaller changes in the visual system
* Complaints of reduced vision not proportionate to the reduction in high contrast acuity needed investigation
* Optical factors: corneal changes (refractive surgery, corneal edema, scarring of stromal layers, stromal edema), lens changes (cataract)
* Affect high spatial frequencies
* Drug usage
* Measure effect of progress of disease or monitor treatment effect
77
What is the importance of low and middle spatial frequencies for seeing?
* Low spatial frequencies
* Allows patients to ambulate around their visual world
* Patient with central (foveal) vision loss whose high frequency spatial resolution is impaired may still be able to perform certain visual tasks if they retain low frequencies
* May have difficulty with high spatial frequency tasks: reading, driving, inserting key into keyhole
* Low and middle Spatial frequencies are critically important for detecting and recognizing objects
* The natural visual environment is abundantly rich in midrange spatial frequencies that are well matched to the peak contrast sensitivity of the human sCSF and cortical neurons
* Spatial frequencies in the intermediate range of 1-15 cycles per degree are critically important for much of what is called “seeing objects in everyday vision
* These spatial frequencies are much lower than the cutoff high spatial frequency
* Low and medium spatial frequencies – spatial vision is possible
* Some instances, high spatial frequencies mask low spatial frequency images
* Less degraded by image quality defocus
* But people with loss of CS at these ranges can have a great deal of difficulty seeing, even if their MAR is unaffected
78
What spatial frequency is necessary for Face Recognition?
* Low spatial frequencies are sufficient
* Midrange spatial frequencies are also important, with neither range being absolutely essential for face recognition
79
What is the significance of high facial frequencies?
* Necessary for recognizing details
80
What are the implications of filtering out High spatial frequencies? What about Low spatial frequencies?
* Filtering all high spatial frequencies
* Faces are blurry but may be recognizable
* Important point: the image is clearly recognizable as a human face
* Filtering out all low and medium spatial frequencies
* Difficult to see what the figure contains
* If the image were to be optically defocused (removing all high frequencies) all information would vanish
* A low pass filtering that has low and intermediate frequencies but is masked by spurious high-frequency information at sharp boundaries between squares
* If image is defocused (high frequencies removed), it would just show medium and low frequencies
* Distance vs. near
* It is easier to identify an object in an image with this kind of filtering at a distance, rather than close up, because viewing at a distance diminishes the effect of the high spatial frequencies
81
What is the relationship between VA and VS contribution to visual guided activities?
* Reduction in VA and CS contribute independently
* Pt who has deficits at low and moderate spatial frequencies with only minimal reduction at high spatial frequencies may present with “COMPLAINTS” of significant visual impairment
* But when you perform a standard visual acuity test, there may be normal or near-normal VA
82
What is the importance of high spatial frequencies?
* High spatial frequency resolution is critical
* Allows reading road signs, blackboards, computer screens, and books
* Reduction in acuity secondary to refractive error is often well tolerated by patients whose acuity demands are not high
* It can be frustrating to clinicians when a patient whose acuity can be substantially improved prefers his/her old correction
* Demonstrates there is more to spatial vision than high-frequency resolution
83
Name a few clinical ocular conditions where CS is affected.
* Early stages of eye diseases (decrease in higher spatial frequencies), Later stages of eye disease (decrease in low and high spatial frequencies)
* Cataract (all spatial frequencies
* Macular degeneration
* Loss at low spatial frequencies:
* Optic neuritis
* Multiple sclerosis
* Glaucoma – POAG
* Papilledema
* Visual pathway lesions
* Diabetic retinopathy
* Parkinson’s disease
* Alzheimer’s disease
* i.e., diseases that affect all or part of the visual pathway
* Others:
* Refractive errors
* Age
* Refractive surgeries
* Contact lens-induced corneal edema (all spatial frequencies)
* Optic neuropathies
* Pituitary adenoma
* Drugs
* Toxic chemicals
* Scarring of the stromal layers (low and moderate)
* Stromal swelling (low and moderate)
84
How does optical correction of refractive errors improve CSF?
* Improves only a limited aspect of patient vision (VA improves, but no effect on low frequencies)
85
How does Optical Defocus affect CSF?
* HIGH spatial frequencies affected
* Can be caused by myopia
* Even small amounts of defocus may hinder driving performance, particularly under nighttime conditions
86
How do Cataracts affect CSF?
* ALL spatial frequency levels affected
* Light scatter occurs
* Early cataracts – often report headlights of oncoming traffic greatly reduce their vision
* Minimal reduction at high contrast VA, but complains of diabling loss due to cataract acting as diffuser reducing image contrast across all frequencies (high, moderate, low)
87
How can the effect of Cataracts on CSF be tested?
* BAT – Brightness Acuity Tester
* Veiling glare
* Substantial acuity decrease – surgery warranted
88
Does Cataract & Resultant CS Reduction impair drivers’ vision?
* Yes, slower in detecting traffic hazards
* Patients who have had cataract surgery are in significantly fewer motor vehicle accidents
89
How does Corneal Edema affect CSF?
* ALL spatial frequency levels affected
* Increased light scatter causes similar effects as cataracts
* Pt’s may have normal or near normal acuity but still complain of substantial blur
90
How does Stromal scarring or edema affect CSF?
* Low and Moderate spatial frequencies affected
* Scarring can cause light scattering and can be caused by trauma or refractive surgery
* Stromal swelling – Fuchs endothelial dystrophy
91
How do Corneal Epithelial disruptions affect CSF?
* Low and Moderate spatial frequencies affected
* Occur in dry eye syndromes
92
What do conditions that affect the Central Visual Pathway affect? Give examples of conditions in which the central visual pathway is implicated.
* Cause a dissociation between the MAR and the Spatial CSF
* MS:
* Patients with multiple sclerosis often have a loss of contrast sensitivity at low spatial frequencies but retain contrast sensitivity at high spatial frequencies, so the MAR is unaffected
* Strabismic Amblyopia:
* Patients with strabismic amblyopia typically have a reduction in the spatial CSF at middle to high spatial frequencies, resulting in a reduction in the cutoff of high spatial frequency (typically checked with VA measures)
93
Why has the spatial CSF not become a useful diagnostic test for ocular pathology?
* CSF does not have high specificity for any disorder
* Reductions can routinely be detected with an acuity measure
* Losses in contrast sensitivity have not proven to be generally useful for diagnosing pathology
* Hence, measures of spatial CSF are not generally used to explain vision losses experienced by patients and not as a first-line diagnostic tool
* They are more useful in assessing the effects of vision problems on quality of life
94
How is reading speed impacted by spatial CSF losses?
* Normals:
* Reading speed in normal observers is relatively tolerant of substantial reductions in contrast had to be reduced to a value of 0.06 to decrease reading speed to ½ its maximum value (i.e., when normal observers read text composed of letters approximately 2.5x the size of ordinary newsprint)
* Low Vision Patients:
* Reading speed reduction is related to patient’s spatial CSF losses
* An overall reduction in a patient’s spatial VSF has a greater effect on reading performance than do small depression in sensitivity at particular spatial frequencies
95
How is CSF measurement useful in low vision?
* Low-vison observers have contrast attenuation due to optical factors, such as intraocular scatter in eyes with cloudy media, or to a reduction in effective contrast in eyes with visual field losses
* Two people with equal vision acuity could have different spatial CSFs, and the person with poorer spatial CSF experiences a greater visual disability
* In low-vision patients with unequal impairment in the two eyes, it has been found that spatial CSF, rather than visual acuity, determines which eye is preferred in situations in which only one eye can be used
* This information can be helpful in deciding which eye to fit with low-vision aids
* The visual complains of low-vision patients include difficult recognizing faces
* The purpose is to raise the contrast of the spatial frequencies that are critical for face recognition to a level such that the low-vision patients can detect the important facial features
* Possible new treatments: selective contrast enhancement at certain spatial frequencies to boost face recognition
96
Why is the visual system considered as Fourier Analyzer?
* The visual system is thought to deconstruct the retinal image into its spatial frequency components
* Several narrow CSF channels (4-6)
* Prior adaptation to a specific frequency causes discrete reduction in sensitivity to that specific frequency
* Example: prior adaptation 6cpd spatial frequency will cause reduction in sensitivity to that spatial frequency
97
Does the Visual System act as Fourier analyzer?
* Although there are evidences, it does not prove to be correct
* Nonetheless, the CSF is useful to understand how the visual system processes spatial information
98
99
What demonstrates the existence of multiple spatial frequency channels in the visual system?
* Contrast adaptation
100
What is MACH band?
* Relative brightness enhancement of high spatial frequencies created by the visual system that demonstrate the accentuation of luminance changes at surface boundaries (illusion of brightness)
* The non-existent perceptual bands caused by the relative enhancement of high spatial frequencies – resulting in perception of enhanced boundaries
* The visual system is insensitive to low spatial frequencies
* The responses of retinal neurons underlie both the dependence of brightness on contrast and the brightness enhancement of Mach bands
* Named after Ernst Mach
101
What is the MACH band effect due to?
* Due to the spatial high-boost filtering performed by the human visual system on the luminance channel of the image captured by the retina
* Exaggerates the contrast between edges of slightly differing shades of grey when they contact another, triggering edge-detection
* Filtering is largely performed in the retina by lateral inhibition among neurons
102
What explains the dependence of brightness on contrast and the brightness enhancement of Mach bands?
* Retinal receptive-field center-surround interactions
103
Why does MACH band occur? Which part of the visual system is responsible for that?
* The visual system does not perceive a transition between light and dark regions as gradual
* Instead, observers report bright and dark bands at the junctions of bright and dark regions
* The non-existent perceptual bands caused by the relative enhancement of high spatial frequencies – resulting in perception of enhanced boundaries
104
Apply the Spatial CSF to Spatial Vision.
* The visual scene contains many spatial frequencies
* All spatial luminance patterns are comprised of sine-wave gratings of particular spatial frequencies and contrasts
* We look at these with our spatial CSF – filter visual scene
* We do not see/detect spatial frequencies above ~60 c/deg
* Low spatial frequencies are attenuated (reduced/weakened) by our visual system, emphasizing that we are more sensitive to mid-range spatial frequencies than to low ones
105
Do we have to have high spatial frequencies (near the acuity limit) to “see objects,” or can we “see objects” with just low and intermediate spatial frequencies?
* Yes… we can see objects with just low and intermediate spatial frequencies
* They are critically important for detecting and recognizing objects
106
State some facts about the visual system
* The visual system exhibits brightness constancy, simultaneous contrast, and assimilation
* In all three instances, the visual system judges brightness based primarily on the local contrast at luminance boundaries
* The visual system recognized objects from patterns of light and dark
* The visual sytem detects objects by determining the location of edges of them (ex: MACH Bands)
* Absolute luminance is less important in spatial vision than relative luminance (contrast) levels
* The reflectance from real objects does not vary much, so contrast remains fairly constant under different lighting conditions
* The visual system responds to luminance differences (e.g., contrast change) more than to luminance (visual system is a poor light meter)
107
Define brightness constancy.
* The brightness of objects is relatively invariant even though the absolute luminance varies widely
* Brightness is determined largely by relative local contrast
* Example: objects do not look much brighter or darker when the room lights are of high or low intensity
108
What is simultaneous contrast?
* The brightness of an object is not always predicted by its luminance, but by the local contrast with surrounding objects
* An example of how the perception of brightness of an object depends on CONTRAST more than absolute luminance
* An illusion of brightness
* Example: two center squares of equal luminance appear to have different brightness induced by their (white or black) background
* Positive contrast (increases brightness)
* Negative contrast (decreases brightness)
109
What is assimilation?
* Complex version of simultaneous contrast
* The brightness of a stimulus covaries with the brightness of a surrounding stimulus
* Assimilation is believed to originate in the visual cortex (as it is absent in the retina)
* It has been found that the amount of assimilation depends on the amount of simultaneous contrast – assimilation is ½ as strong as simultaneous contrast
* Example: 2 center squares of equal luminance, located on gray backgrounds (also having equal luminance), appear to have different brightness’s induced by the other backgrounds
110
Which spatial frequency reduction prompts most complaint from patients?
Reduction in high spatial frequencies trigger most complaints
111
What do boundaries (edges) provide critical information for?
* Form perception
112
How do clinical measurements of the VA/MAR and the sCSF complement each other?
* Generally they complement each other, but it should not be surprising that a patient can have BCVA 20/20 (MAR of 1’) but still have visual complaints
* This is because MAR measures the cutoff high spatial frequency, which depends only on the neurons with the smallest receptive fields in the fovea
* It does not access the functioning of most of the neurons in the retina or the rest of the visual system
* Conversely, a patient may have poor acuity but be relatively unimpaired on other visual tasks (that do not involve reading or detecting other fine details) in the visual environment
* Such a discrepancy between VA and the ability to function in the visual environment is relatively rare because the most prevalent ocular pathologies affect the spatial CSF in the range from intermediate to high spatial frequencies
* Patients with visual impairments such as, cataract, ARGMD, and glaucoma, the spatial CSF can be a useful predictor of daily visual functioning over and above measures of resolution acuity
113
Apply spatial CSF to ARMD.
* Pelli-Robson CS chart performance is a good predictor of time for patients with ARMD to complete an obstacle course and number of errors (collisions)
* Detection of low and intermediate spatial frequencies correlated well with recognition of real world targets, such as faces and road signs
114
What are some other applications of the spatial CSF?
* Predicting visual performance when changes are made to the dioptric power of the human eye (e.g., IOL, radial keratotomy, CLs)
* Lakshminarayanan et al. (1995) have described a method to predict relative changes in clinical performance for a given change in the optics of the eye by using a simple model of the paitent’s spatial CSF and the in vitro MTF of the eye’s optics
* This method has been applied successfully to predicting visual performance in patients who were implanted with multifocal IOLs
Visual Perception
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