Visual Perception: Spatial Vision Contrast Sensitivity

Define Spatial Vision

The ability of visual system to detect and resolve stimuli of various Sizes & Contrasts

Absolute luminance is more or less important in spatial vision than Relative luminance?

Less important, because because luminance levels vary greatly in the environment but relative luminance (contrast) for visual stimuli do not

The visual system is evolved to do what?

Detect and recognize objects from patterns of light

Provide an example of clinical application

VA measurements

What is visual acuity?

The smallest detail that can be resolved & recognized

Define Contrast Sensitivity

psychophysical measure used to assess the sensitivity if the visual system to spatial luminance changes of various spatial frequencies

a more comprehensive test of spatial vision

How is contrast sensitivity measured?

finding the lowest contrast needed to see light/dark grating of varied fineness or spatial frequency

VA is one point of

contrast sensitivity function

Define Luminance and its unit

the physical amount of light emitted by a source or reflected from an illuminated object

Define brightness

the perception of a luminous object by the human visual system

What affects brightness perception

Adaptation, aftereffects, the presence or absence of other objects in the visual field

Define Contrast

the difference in luminance between an object Lmax and its background Lmin

where the object is typically brighter than the background

Define Modulation?

The term for contrast of a repeating pattern, commonly referred to as a grating

where it is not apparent which part of the pattern is the object and which is the background

How is modulation (contrast) calculated?

the ratio of the difference of the luminance to the sum of the luminance

M=(Lmax-Lmin)/(Lmax+Lmin)

How can modulation be alternately expressed?

by the mathematically equivalent ratio of the difference between maximal and average luminance to the average luminance's

M=(Lmax-Lavg)/Lavg

where Lavg=(Lmax+Lmin)/2

Define Contrast Threshold

the least amount of contrast that can be seen in a chart, expressed at a %

(weber contrast)

Define log contrast sensitivity

converts the contrast sensitivity values to a linear scale, more suitable for comparisons between the levels of contrast sensitivity,

The smaller the log CS value=the poorer the pt CS

Define Contrast sensitivity

The reciprocal of contrast threshold

How do you calculate contrast threshold from log contrast sensitivity and vice versa?

CT=1/CS

CS=1/CT

The log contrast sensitivity score of a patient in Pelli Robson Chart is 1.65. What is the contrast sensitivity and contrast threshold?

10^1.65=CS

CT=1/CS

What is spatial frequency?

refers to the number of light/dark cycles per degree

Name and describe the 3 types of contrast

Weber contrast for letter stimuli

Michelson contrast for Gratings

RMS contrast for natural stimuli

What is Weber contrast?

(luminance of background-luminance of target)/luminance of background

commonly used in cases where the small target/features are present on a large uniform background

*preferred for letter stimuli

What is Michelson contrast?

(Lmax-Lmin)/(Lmax+Lmin)

*preferred for gratings

What does RMS (root mean square) contrast not depend on?

Does not depend on the angular frequency content or the spatial distribution of contrast of the image

What is RMS?

the standard deviation of the Pixel intensities

Weber contrast values are about _________ the Michelson values

twice

What is a sine wave?

mathematical curve that describes a smooth repetitive oscillation

characterized by their spatial frequency, contrast and phase

Why do we use sine wave?

They maintain shape even after degradation and only change in contrast and phase

Sine wave gratings are most useful for?

studying thresholds of the visual system than any other waveform because it is composed of a SINGLE spatial frequency

What are the elements used to describe sine-wave grating?

frequency, contrast, phase, orientation

What does phase refer to?

the position of a sine wave grating with respect to another sine wave grating

What is orientation?

describes the angle made by grating with respect to a reference (such as the horizontal, vertical)

Our visual system is believed to act as what?

a Fourier analyzer, the visual system is thought to deconstruct the retinal image into its spatial frequency components

What is Fourier Transformation?

A mathematical process that transforms a signal (function) from the Spatial domain to frequency domain

Image processing, signal detection

Why did we learn about Fourier transformation in this topic?

In Fourier Analysis, Sine waves of the proper frequency, contrast, phase and orientation can be used to construct complex stimuli

Square wave gratings

luminance profiles show abrupt changes between bright and dark bars "step" changes in luminance

How do you construct a square wave?

by adding since waves together

Sine wave that is the same frequency as square wave is referred to as what?

Fundamental

Higher frequency sine waves are referred to as

Harmonics

What does 3rd harmonics mean?

it has 3 times the frequency of the fundamental and 1/3rd of its contrast

What does 5th harmonic mean?

5 times the frequency of the fundamental and 1/5th of its contrast

Addition of the fundamental sine wave and the odd numbered harminonic (3 and 5) produces a....

square wave

Adding all the ODD numbered harmonics to the fundamental will produce

perfect square wave

Fourier analysis can determine

the component sine waves & contrast of complex waveforms

Fourier Synthesis

can construct square wave grating using the component sine wave & contrasts

What is MTF?

The ability of an optical system to produce an accurate image of an object

modulation transfer function

MTF=image contrast/object contrast

Why wont an image never be perfect

due to diffraction, chromatic and monochromatic aberrations, focus, and power errors

How does MTF change for high and low spatial frequency?

for low spatial frequencies the image blur is hardly noticeable

for high spatial frequencies the image is almost fully obscured

Describe Optical system (SMTF=Image quality)

Simple lenses or complex lens systems (telescopes and cameras), the MTF gives the modulation transfer for all possible spatial frequencies

Describe the human visual system

CSF is the MTF of the visual system

Neural and cortical processing and the optics of the eye contribute to the final outcome of the CSF

How can we quantify the precision with which an optical lens (system) transfers information?

grating of specfic spatial frequency and contrast

image contrast is measures

the ratio=image quality (MTF)

repeat procedure ranging from low to high to get SMTF

MTF=0 to 1 (1 is the best image quality!!!!)

How does SMTTF change with 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

How does SMTTF change with Lens in defocus?

reduction in image quality in higher spatial frequencies

How does SMTTF change with translucent lens?

Equal degradation at all spatial frequencies

What is CSF?

Contrast sensitivity function

an observers sensitivity to gratings of widely varied spatial frequency

What is the range and peak of adult CSF?

2-10cpd

peak at 4cpd

In the CSF curve what's on the x-axis and what's on the y-axis?

x-axis: spatial frequency

y-axis: contrast sensitivity

How does the CSF curve for photopic conditions?

largest curve, high frequency cut off 100 cpd

How does the CSF curve change for Mesotopic conditions?

middle curve, high frequency cutoff at 60cpd

How does the CSF curve change for scotopic conditions?

lowest curve, high frequency cutoff 15cpd

Name the two types of CSF?

spatial contrast sensitivity function: sCSF (across space)

Temporal contrast sensitivity function: tCSF (across time)

Name the 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

it's a window of visibility

What does the high spatial frequency cut off correspond to?

best VA

What does the peak CS 4cpd correspond to

average receptive field size of retinal ganglion cells

What is the low SF cut off due to?

lateral inhibition throughout the visual system

The lower and higher spatial frequencies require more or less contrast to be detected?

More contrast

highest spatial frequency can be resolved only at a...

very high contrast and this corresponds to the acuity level

What does the shape of CSF reflect?

underlying spatial frequency channels

What is the CSF high frequency cutoff?

60 cpd

visual systems limited ability to resolve detail when the contrast is 100% (normal VA)

What is the relationship between VA and the CSF?

high spatial frequency cutoff is VA

bottom row (100% contrast) represents VA

*Vertically contrast diminish

Low contrast VA not measured routinely in clinic

Why does the visual system show reduction in sensitivity for high spatial frequencies?

optical limitations-due to aberrations

packing density of photoreceptors

The ______ of the CSF is the same for many species but the ______ occurs at different spatial frequencies

Same shape, different peaks

The CSF peak for Falcons is shifted where?

Toward the higher spatial frequencies

What is important about the Macaque monkey spatial CSF curve?

its nearly identical to that of humans

What happens to the high frequency cut-off if the eye is out of focus, such as uncorrected refractive errors (myopia)?

reduction in high spatial frequency cut off

What factors influence CS?

1. optical defocus

2. retinal illuminance

3. Retinal eccentricity

The spatial CSF changes in what 3 ways as mean retinal luminance of the grating is reduced?

1. Peak contrast sensitivity is decreased and shifts toward lower spatial frequencies

2.High-frequency cutoff spatial frequency (acuity) decreases and shifts toward the lower spatial frequency

3. low spatial frequency rolloff becomes less prominent

How does retinal eccentricity affect CSF?

increasing retinal eccentricity causes the CSF to shift towards lower spatial frequency

What are the reasons for decreases in CSF with retinal eccentricity?

1. size of 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

2. the cortical area devoted to representing the retinal periphery (cortical magnification) which tend to decrease with eccentricity

How does the CS change with aging?

at birth, improves steadily with age

steady in young adults

declines with age

What is the high frequency cut off shift from 8 to 2 cpd due to when illuminance levels are decreased?

transition from photopic, to cone-driven (small) receptive fields to scotopic, rod driven (large) receptive fields

How does CS develop in infants?

sensitivity to higher spatial frequency gratings is very poor at birth, improves steadily with age

Postnatal acuity= infants age in months is cpds

ex: 12months old=12 cpd

How is the contrast sensitivity function of infants different than that of adults?

peak is lower

function is shifted to the left

shape is "low pass" rather than a "band pass" function

When does the overall CS reach adult levels?

7-9 yrs

How can you determine CSF in the clinic?

Charts designed to measure spatial contrast sensitivity functions clinically are based on what's important for recognizing objects

Provide examples of the charts used in the clinic?

Fixed Size, vary contrast

-Pelli-Robinson

-MARS CS (near)

-Rabin

Fixed contrast (low), vary size

-EDTRS

Fixed contrast (high), Vary size

-EDTRS

-Snellen

Variable contrast, vary size

-FACT

VCTS/VISTECH charts

Describe the Pelli Robson Chart

Fixed size, variable contrast

all of the letters in the chart are the same size

consists of 3-letter sets, each triplet has the same contrast

each triplet is 0.15

stop when pt gets 2 or more in a triplet incorrect

What are the normal values for adults and above 60yrs in pelli robson?

Log CS 1.84 (20-39yrs)

Log CS 1.68 (60yrs and up)

the smaller the CS value, the poorer the pts CS

Which chart has grating displayed on a printed chart?

VISTECH

What is a FACT chart? what is the target?

Functional Acuity Contrast Test

Target: Sine wave gratings

Describe MARS CS chart

letter contrast sensitivity test

stop when 2 consecutive mistakes occur

48 different contrast levels, decline gradually 0.04 log unit steps

rapid testing

(the smaller the log CS value=poorer CS)

Why measure CS?

sensitive measure to smaller changes in the visual system

complaints of reduced vision not proportionate to the reduction in high contrast acuity

optical factors

drug usage

monitor disease

What are the factors affecting CS?

refractive error, age, cataract, refractive surgeries, CL induced corneal edema, glaucoma, optic neuropathies, diabetic retinopathy, ARMD, Pituitary Adenoma, Drugs, Toxic chemicals

describe optical corrections of refractive errors?

improves only a limited aspect of a patients VA (no effect on low frequencies)

high spatial frequency resolution is critical (road signs, computers screen, books)

What is the importance of low and middle spatial frequencies for seeing?

Pts with central vision loss whose high frequency spatial resolution is impaired may still be able to perform certain visual task due to retention of low frequencies

(trouble reading but can still walk around)

pts 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

Name a few clinical ocular conditions where CS is affected?

High optical defocus (myopia) -reduces contrast sensitivity primarily at high spatial frequencies

Cataracts- cause reduction is low, moderate and high spatial frequencies

CL induced corneal edema- causes reduced contrast sensitivities across low, moderate and high frequencies

Scarring of the stromal layers-due to trauma or refractive surgery reduces CS at low and moderate levels

Swelling of stroma(Fuchs endothelial dystrophies)-reduces low and moderate frequencies

Corneal epithelial disruptions- low and moderate

Why has the spatial CSF not become a useful diagnostic test for ocular pathology?

the sCSF is sensitive to man ocular disorders but does not have high specificity for an any particular disorder

No unique patterns of spatial CSF loss have emerged that are useful in differential diagnosis of particular condition

Most of these conditions depress the contrast sensitivity at high spatial frequencies so that these conditions are also detected by simple measures of MAR

CS has been no more useful than MAR is detecting ocular disease

When can a dissociation occur between MAR and the spatial CSF?

in conditions that affect the central visual pathway

MS-loss of contrast at low frequencies but remains in high, so MAR is unaffected

Strabismic Amblyopia-loss a middle to high resulting in a reduction in high spatial frequency cut off that is routinely detected in acuity measure

Spatial CSF are more useful in what compared to what?

more useful in assessing the effects of vision problems on the quality of life than used as a first-line diagnostic tool