- concerned with variations in luminance across space - of fundamental importance in routine eye care - visual systems ability to detect and resolve luminance-defined stimuli

Spatial Vision Flashcards by Sarah Heath

Spatial vision is

concerned with variations in luminance across space
of fundamental importance in routine eye care
visual systems ability to detect and resolve luminance-defined stimuli

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The simplest spatial stimulus

Sine-wave gratings

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Why are sine wave gratings the simplest spatial stimulus

Serve as building blocks to construct more complex stimuli

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What do the sine wave gratings consist of

Alternating bright and dark bars

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What does the peak of the luminance profile correspond to

A bright bar of the grating

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What does the trough of the luminance profile correspond to

A dark bar

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What kind of transition is it from the bright to dark bars in a sine wave grating

Gradual (sinusoidal), not an abrupt transition

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What is spatial frequency of a grating specified by

the number of cycles/degree of visual angle (frequency)

- the number of cycles per unit of space (contrast)

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Two characteristics of a sine-wave grating are _____ and _______

Frequency and contrast

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What does the amplitude of the sine-wave grating mean

Intensity or brightness

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Is the amp higher or lower when the contrast is higher

Higher

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Difference between dark and bright

Contrast

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Formula for contrast

(Delta I)/Iave

Delta I=the difference between the peak and average luminance

Iave= the average luminance of the grating (the average of the light peaks and the dark troughs)

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Michaelson equation for contrast

(Imax-Imin)/(Imax +Imin)

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What does contrast range between

0 and 100%

Contrast cannot be lower than 0% and higher than 100%

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The intensity distribution of a lens forming an image of a sine-waving grating

In the image plane, it will also form a sine wave

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The spatial frequcny of the lens forming an image of a sine-wave grating

Depends on the magnification of the lens

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What does the modulation depth of a lens forming an image of a sine-wave grating depend on

The quality of the lens

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What does high magnification do to the image of a sine-wave grating

Low mag=high frequency

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For a real lens, as the spatial frequency of the object (and therefore in image) because greater and greater, the amplitude of modulation of the image distribution becomes

Smaller and smaller

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What happens to contrast info when spatial frequency increases

Decrease

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What is important to understand optical transfer function (OTF)

Resolution and contrast

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An imaging systems ability to distinguish object detail

Resolution

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How faithfully the minimum and maximum intensity values are transferred from object plane to image plane

Contrast or modulation

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MTf of a perfect lens

1, not possible

What does the OTF include

Is a complex quantity that includes both the modulation transfer function (MTF) and the phase transfer function (PTF) OTF=MTF+PTF

The amplitude of A' of the image divided but the amplitude A of the object and it is a function of spatial frequency

Modulation transfer function (MTF) MTF=A'/A=1

What is MTF normalized to

This is a function of spatial resolution, which refers to the smallest line-pair the system can resolve

MTF

Measurement of a lens ability to transfer contrast at a particular resolution from the object to the image

MTF

MTF is a way to incorporate what into a single specification

Resolution and contrast

As line spacing decreases (freq increasing) on the test target, it becomes increasingly difficult for the lens to efficiently transfer this decrease in contrast, as a result:

MTF decreases

Those aberrations that produce a transverse shift produce an effective phase shift in the image. The phase shift across a range of spatial frequencies is the

Phase transfer function

When is there not phase transfer function

In an on-axis in a rotationally symmetric optical system, there is no change in phase with spatial frequency. OTF=MTF in these situations

Lens transfer information (contrast)=

Image contrast/object contrast

MTF _______ when there is a high spatial frequency

decreases

Aberrations effect on optical image quality and different spatial frequencies

Have comparatively little effect on optical image quality at low and moderate spatial frequencies as compared to high frequencies

Optics that overceom aberrations are called

Diffraction limited optics

Low spatial frequency: image transfer fidelity

High

Low spatial frequency: image degradation

Little

Med spatial frequency: image transfer fidelity

High-med

Med spatial frequency: image degradation

Little

High spatial frequeny: image transfer fidelity

Low

High spatial frequency: image degradation

More (due to aberrations)

What is the human SMTF often referred to

Contrast sensitivity function (CSF) because sensitivity, not image contrast, is measured

A point at which the grating first appears to be seen by the patient in the SMTF (CSF)

Contrast threshold

The reciprocal of the contrast threshold is the ________ for the grating

Contrast sensitivity

How is sensitivity determined for the SMTF (CSF)

For a large number of diffferent spatial frequencies, resulting in a graph that shows contrast sensitivity as a function of spatial frequency, a CSF

What is maximum sensitivity (SMTF, CSF)

4 cycles per degree

As the spatial frequency of a 100% contrast grating is increases, a point is reached where the grating can no longer be resolved. This point is represented on the CSF as the

High frequency cut off

The high frequency cut off for humans

60 cylces/degree

Peak contrast at spatial freq of ______ and freq cut off of ______

4 cyc/degree | 60 cyc/degree

The larger the letter on the smelled chart

Lower spatial frequency

The smaller the letter on the snellen chart, the _______ the spatial frequency

Higher

What is the VA for 60 cyc/degree

20/10

What does the typical clinical acuity measurement test

Only a very limited aspect of the patients spatial vision, the high spatial frequency cutoff

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

Optical limitations | The packing density of retinal photoreceptors

How to optical limitations affect the sensitivity at higher spatial frequencies?

Any optical system, including the eye, manifests a high frequency limitation because of optical aberrations, this is the case even when the eye is in focus

Why is the packing density of retinal photoreceptors a factor that reduced sensitivity at higher spatial frequencies

- the coarse matrix could not resolve grating - the finer matrix could, however, resolve this grating because the photoreceptors are packed sufficiently densely to allow bright bars to fall on alternate rows of photoreceptors (nyquist)

What happens to the high frequency cutoff if the eye is out of focus, such as in uncorrected myopia

There is a reduction in the high frequency cut off, which manifests itself as decreases VA

A typical ganglion cell receptive field

Consists of a center region that responds to illumination with either excitation or inhibition and a surround region that responds with the opposite sign. The result is spatial antagonism, which is also called lateral inhibition

For the receptive field, light falling on the center

Causes excitation, while light falling on the surround causes inhibition

A lower spatial frequency where the bright bar falls on both the receptive fields center and surround, causes what

Lateral inhibition, results in a smaller response

______ may be the basis for the CSFs low-frequency drop off

Lateral antagonism

VA determined with optptyprs is equivalent to

CSF H-F cutoff

The angle that just resolvable bars (or gaps) make with the eye is called the

Minimum angle of resolution (MAR)

What is the detail of the letter E (MAR)

The combination of the bar and a gap, which is equivalent to one complete cycle of grating

Is twice the MAR E has 2.5

The numerator in snellen fraction

Distance at which the measurement is taken

The denominator in the snellen fraction

Is the foot-size of the smallest optotype the patient can resolve

What is the expected Hugh-frequency cutoff of a patient with a 20/40 acuity

(1 cycle/4arc)(60arc/1 degree)=15 cycles/degree Snellen 20/40 is equivalent to 15 cycles/degree

H-F cutoff VA calculation

(0.5 degrees/60)(60arc/degree)=0.5arc 20/10

Test-retest variability (TRV)

- a pts VA may vary upon repetition even when there has been no change in the pts viual status - standard VA optotypes are constituted of both low and high spatial frequencies - low spatial freq content is an important contributor to TRV - a new test designed to correct this potential source of variability removes low spatial freq from the optotypes, creating what are called high pass optotypes, and places them on a background of the same average luminance Vanishing optotypes