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Perception Block 9 > Spatial Vision > Flashcards

Spatial Vision Flashcards

1
Q

Spatial vision is

A
  • 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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2
Q

The simplest spatial stimulus

A

Sine-wave gratings

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3
Q

Why are sine wave gratings the simplest spatial stimulus

A

Serve as building blocks to construct more complex stimuli

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4
Q

What do the sine wave gratings consist of

A

Alternating bright and dark bars

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5
Q

What does the peak of the luminance profile correspond to

A

A bright bar of the grating

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6
Q

What does the trough of the luminance profile correspond to

A

A dark bar

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7
Q

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

A

Gradual (sinusoidal), not an abrupt transition

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8
Q

What is spatial frequency of a grating specified by

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

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

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9
Q

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

A

Frequency and contrast

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10
Q

What does the amplitude of the sine-wave grating mean

A

Intensity or brightness

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11
Q

Is the amp higher or lower when the contrast is higher

A

Higher

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12
Q

Difference between dark and bright

A

Contrast

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13
Q

Formula for contrast

A

(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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14
Q

Michaelson equation for contrast

A

(Imax-Imin)/(Imax +Imin)

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15
Q

What does contrast range between

A

0 and 100%

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

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16
Q

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

A

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

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17
Q

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

A

Depends on the magnification of the lens

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18
Q

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

A

The quality of the lens

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19
Q

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

A

Low mag=high frequency

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20
Q

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

A

Smaller and smaller

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21
Q

What happens to contrast info when spatial frequency increases

A

Decrease

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22
Q

What is important to understand optical transfer function (OTF)

A

Resolution and contrast

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23
Q

An imaging systems ability to distinguish object detail

A

Resolution

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24
Q

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

A

Contrast or modulation

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25
Q

MTf of a perfect lens

A

1, not possible

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26
Q

What does the OTF include

A

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

OTF=MTF+PTF

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27
Q

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

A

Modulation transfer function (MTF)

MTF=A’/A=1

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28
Q

What is MTF normalized to

A

1

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29
Q

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

A

MTF

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30
Q

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

A

MTF

31
Q

MTF is a way to incorporate what into a single specification

A

Resolution and contrast

32
Q

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:

A

MTF decreases

33
Q

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

A

Phase transfer function

34
Q

When is there not phase transfer function

A

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

OTF=MTF in these situations

35
Q

Lens transfer information (contrast)=

A

Image contrast/object contrast

36
Q

MTF _______ when there is a high spatial frequency

A

decreases

37
Q

Aberrations effect on optical image quality and different spatial frequencies

A

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

38
Q

Optics that overceom aberrations are called

A

Diffraction limited optics

39
Q

Low spatial frequency: image transfer fidelity

A

High

40
Q

Low spatial frequency: image degradation

A

Little

41
Q

Med spatial frequency: image transfer fidelity

A

High-med

42
Q

Med spatial frequency: image degradation

A

Little

43
Q

High spatial frequeny: image transfer fidelity

A

Low

44
Q

High spatial frequency: image degradation

A

More (due to aberrations)

45
Q

What is the human SMTF often referred to

A

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

46
Q

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

A

Contrast threshold

47
Q

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

A

Contrast sensitivity

48
Q

How is sensitivity determined for the SMTF (CSF)

A

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

49
Q

What is maximum sensitivity (SMTF, CSF)

A

4 cycles per degree

50
Q

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

A

High frequency cut off

51
Q

The high frequency cut off for humans

A

60 cylces/degree

52
Q

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

A

4 cyc/degree

60 cyc/degree

53
Q

The larger the letter on the smelled chart

A

Lower spatial frequency

54
Q

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

A

Higher

55
Q

What is the VA for 60 cyc/degree

A

20/10

56
Q

What does the typical clinical acuity measurement test

A

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

57
Q

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

A

Optical limitations

The packing density of retinal photoreceptors

58
Q

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

A

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

59
Q

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

A
  • 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)
60
Q

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

A

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

61
Q

A typical ganglion cell receptive field

A

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

62
Q

For the receptive field, light falling on the center

A

Causes excitation, while light falling on the surround causes inhibition

63
Q

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

A

Lateral inhibition, results in a smaller response

64
Q

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

A

Lateral antagonism

65
Q

VA determined with optptyprs is equivalent to

A

CSF H-F cutoff

66
Q

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

A

Minimum angle of resolution (MAR)

67
Q

What is the detail of the letter E (MAR)

A

5

68
Q

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

A

Is twice the MAR

E has 2.5

69
Q

The numerator in snellen fraction

A

Distance at which the measurement is taken

70
Q

The denominator in the snellen fraction

A

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

71
Q

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

A

(1 cycle/4arc)(60arc/1 degree)=15 cycles/degree

Snellen 20/40 is equivalent to 15 cycles/degree

72
Q

H-F cutoff VA calculation

A

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

20/10

73
Q

Test-retest variability (TRV)

A
  • 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

Perception Block 9 (5 decks)

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