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Part 1 > Visual Perception > Flashcards

Visual Perception Flashcards

1
Q

Define radiometry.

A

The science of the measurement of electromagnetic radiation

NOTE: both visible and non-visible light

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2
Q

What are the units of radiometry?

A
  1. Radiant flux
  2. Radiant intensity
  3. Radiance
  4. Irradiance
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3
Q

Which unit of radiometry is this describing?

  • A measurement of ALL light waves coming off a light source
  • Units: Watts (joules/sec)
A

Radiant Flux

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4
Q

Which unit of radiometry is this describing?

  • Total measure of total light (possibly from multiple steradians) on a surface*
  • Units Watts/m^2
A

Irradiance

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5
Q

Which unit of radiometry is this describing?

  • Power per solid
  • Power output of a (point) light source in a given direction (solid angle)
  • Units: Watts/steradian
A

Radiant Intensity

NOTE: steradian is a 3D, cone-shaped measurement of light, w a spherical cap coming from a point source (a sphere is 4pi steradians
Solid angle is the angle of light coming from the point source that forms the point of the steradian cone

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6
Q

Which unit of radiometry is this describing?

  • Radiant intensity per unit projected area of an (extended source)
  • Area of light in one steradian incident on a particular area
  • Units: Watts/m^2(steradian)
A

Radiance

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7
Q

What is the science of the measurement of the eye’s response to (or perception of) electromagnetic radiation? I.e. ONLY visible light?

A

Photometry

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8
Q

What are the units of photometry?

A
  1. Luminous flux
  2. Luminous intensity
  3. Luminance
  4. Illuminance
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9
Q

Which unit of Photometry is this describing?

  • Luminous intensity per unit projected area of an (extended source)
  • Area of the visible light in ONE steradian incident on a particular area
  • Unit: cd/m^2 (nit OR ft-lambert)
A

Luminance

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10
Q

Which unit of Photometry is this describing?

  • Power output of a light source
  • Measurement of ALL visible light emitted from a source
  • Unit: Lumens
A

Luminous Flux

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11
Q

Which unit of Photometry is this describing?

  • Power output of a point light source in a given direction (solid angle)
  • A measurement of visible light in ONE steradian
  • Unit: Lumens/steradian (Candela, cd)
A

Luminous intensity

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12
Q

Which unit of Photometry is this describing?

  • Luminous flux incident on an object’s surface
  • Total measure of total visible light (possibly from multiple steradians) on a surface
  • Unit: Lumens/m^2 (lux)
  • English: Foot-candle (ft-cd)
A

Illuminance

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13
Q

Lumens is the unit for what?

A

Luminous flux

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14
Q

Candela is the unit for what?

A

Luminous intensity

“a Candle is INTENSE”

NOTE: Luminous intensity is also Lumens/steradian

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15
Q

Candela/m^2 OR nit OR Foot-Lambert (fL) are the units for what?

A

Luminance

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16
Q

Lux or Lumens/m^2 or Foot-candle are the units for what?

A

Illuminance

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17
Q

At 555nm how many lumens are there for both scotopic and photopic systems?

A

683 lumens

On plot: 1 lm/W @ 555 nm

Find the relative luminance efficiency for the given wavelengths on the plot & multiply by 683 lm/w

Multiply by given watts for each wavelength

Add together

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18
Q

What is the equation to convert illuminance to luminous intensity?

A

Intensity (I) = Distance (D)^2 x Illuminance (E)

I=D^2 x E

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19
Q

Describe Retinal illuminance and the equation

A

Its the amount of light getting into the eye

Unit:
Trolands (td)

Equation:
T = L x A
Trolands = Luminence (cd/m^2) x Pupil Area (mm^2)

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20
Q

What’s the equation Inverse square law.

A

E = I/d^2

E = Illuminance (lux; lumens/m^2; ft-cd)
I = Intensity (cd)
d = distance (m)
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21
Q

Describe the concept of the inverse square law

A

As the (d) INCREASES, the same amount of light is dispersed over a proportionally larger area …. therefore, DEC intensity of light!

NOTE: smaller point light sources make this law work BETTER

NOTE: only works if there is NO additional reflected light

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22
Q

Describe the Cosine Law of Illuminance

A

Illuminance DECREASES with the cosine of the angle from normal

I.e. as the surface is TILTED AWAY from being perpendiculat to the light source, the illuminance of the surface DECREASES!!! (basically most illuminance is 90 degrees from surface)

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23
Q

Equation for Cosine Law of Illuminance?

A

E = (I/d^2) x cos(theta)

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24
Q

What is a special kind of surface whose luminance does NOT vary with the angle at which it is viewed (i.e. cosine law of illuminance does NOT apply)

A

Lambertian diffuser

EXAMPLES: matte paper, unfinished wood

With perfect reflectance (r=1.0) all light that falls on a surface is reflected

Illuminance at the surface would equal the measurement of luminance equals the measurement of the luminance from the surface (i.e. same amount of light is incident on the surface and reflected off the surface)

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25
Q

Equation for Lambertian Diffusers?

A 
L = E x r
L = luminance (cd/m^2)
E = illuminance
r = reflectance
* only works for english system
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26
Q

How does increase distance affect a light source?

A

With increased distance from light source to object the projected area decreases

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27
Q

How is Illuminance affected by increased distance?

A

It DECREASES, inverse square law

E=I/d^2

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28
Q

How is Luminous Intensity affected by increased distance?

A

Remains CONSTANT

Smaller proportion is actually hitting the surface

Constant/proportional ratio of candelas (intensity) to projected surface area

Recall:
Luminous Intensity = Lumens/steradian

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29
Q

True or False. Luminance remains constant with increase in distance because the power of the light source does NOT change.

A

True

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30
Q

What is the definition of threshold?

A

The minimum value of a stimulus required to elicit a perceptual response or an altered perceptual response

NOTE: the stimulus level (or value) above which a response is elicited and below which a response is NOT elicited

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31
Q

What is absolute threshold?

A

The lowest level of stimulus for which a response can be elicited (can you see it or not?)

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32
Q

What is the difference/increment threshold?

A

The minimum difference in stimuls levels btwn 2 stimuli that can be detected

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33
Q

What is the most efficient way to measure threshold?

A
  1. Fewest # of stimulus presentations
  2. Task is easy to understand
  3. Few opportunities for cheating or checking for correctness
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34
Q

Define detection.

A

Do you see the stimulus or not?

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35
Q

Define discrimination/resolution.

A

Can you tell the difference? Are there one or two lines?

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36
Q

Define recognition.

A

Identification/making a cognitive choice; what shape or letter do you see?

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37
Q

What are some reasons why threshold may vary?

A
  1. Stimulus
  2. Neural activity
  3. Attention
  4. Psychological bias: older pts don’t want to be wrong, children want to prove themselves
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38
Q

What is the best psychological method?

a. Method of constant stimuli
b. Method of limits
c. Staircase method
d. Bias guessing

A

a. Method of constant stimuli

This is when you choose several stimuli levels to present in random order

Pt does not know when/what they will see, so they need to pay attention

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39
Q

Does the method of constant stimuli get rid of bias?

A

NO

Typically gives a sigmoid psychometric function

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40
Q

What are catch trials?

A

They check for guessing, bias, compliance, etc

The pt is shown something they should never see or something they should definitely see and check their response

Alternative/interval forced choice tasks/methods aid in this effort (2AFC, 3AFC, etc)

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41
Q

How do we determine threshold?

A

Guessing rate + 50% of the chance of guessing incorrectly

Ex. threshold for a 4AFC is 62.5% bc if guessing, the subject will be correct 1/4 or 25% of the time, leaving a 3/4 or 75% chance that they will guess incorrectly; half of 75% is 37.5% and this is added to the guessing rate is 62.5%

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42
Q

What is formed from plots of frequency of “noise” responses and frequency of
noise + signal” responses with increasing stimulus?

A

ROC curve

the area of overlap can indicate region of confusion

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43
Q

What is on x and y axis of ROC curves

A

x-axis: Hit Rate (TP)

y-axis: False Alarm Rate (FP)

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44
Q

What responses act as catch trials in order to determine the reliability of the test?

A

Fixation errors

False negatives

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45
Q

What is magnitude of sensations?

A

a form of response where the pt must rate the test stimulus given a reference stimulus

Have a magnitude, but no obvious scale or units

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46
Q

Describe ratio production.

A

Show a light, tell subject to make/set a second light that is 2/ or 1/2x for example as bright

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47
Q

What is is ratio estimate?

A

Show a light

Ask the subject to tell how much brighter or dimmer a second test-light is

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48
Q

Sensory and stimulus magnitude are proportional is describe by which law?

A

Steven’s Power Law

Brightness: sensation perception is limited at high levels; adaptation

Length: fairly accurate

Shock/pain: increases rapidly after initial shock; fear

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49
Q

What does a shutter in an optical system used for?

A

To control FLASH duration

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50
Q

A long wavelenged dim red light is used for what in the dark adaptation test?

A

Excites cones at fixation and does NOT affect rod performation

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51
Q

L x A = C
m = 1
This describes which law?

A

Ricco’s Law

52
Q

L x A^1/2 = C
m = 1/2
This describes which law?

A

Piper’s Law

“PIPE down”

NOTE: beyond critical area; partial summation (occurs at slower rate)

53
Q

As background intensity DEC/contrast INC, how much luminance is needed for a given flash area (deg^2)?

A

LESS luminance is needed

54
Q

L x t = C

Describes which law?

A

Bloch’s Law

Critical duration = 100 msec

NOTE: threshold amount of quant (light units) must be presented within the critical duration to be perceived

55
Q

What is threshold magnitude related to and how?

A

Threshold mag is Inversely proportional to sensitivity

Meaning higher threshold magnitude means lower sensitivity. Lower threshold magnitude means high sensitivity!

56
Q

How many quanta get to the rods?

A

4% absorbed by the cornea –> 96%
50% –> retina
20% –> absorbed by rods!

So, 20%…

57
Q

One rod can be stimulated by one quantum, but you need at least how many stimulated rods to perceive a signal/light?

A

10 rods

58
Q

One cone can be stimulated by how many quant? At least how many cones need to be stimulated to perceive a signal/light?

A

5 quanta

2 cones

59
Q

What is absolute threshold for light defined as?

A

The LEAST amount of light needed for light perception

60
Q

What is the absolute threshold for humans?

A

~10 photons minimum on the retina is needed to detect light

61
Q

Where is the greatest density of rods in the retina?

A

~20 degrees eccentrically from the fovea at all major meridians

62
Q

What was the fixation wavelength for the HSP experiment?

A

Long wavelength or DIM red light

NOTE: any stray photons entering eye could mess up threshold, therefore fixation must be done by cones & NOT rods. Any photons from dim red light (~650 nm) that might reach the rods would have 0% relative luminous efficiency & would NOT affect threshold reading

63
Q

In low light, is there a receptive-field surround?

A

NO

Rather, there is a LARGE receptive field center

NOTE: spatial summation occurs over a receptive-field center

64
Q

In the dark, is there a purpose to have a receptive field surround?

A

NO

NOTE: spatial summation occurs over a receptive-field center

65
Q

What happens to spatial summation are you INC retinal eccentricity?

A

Spatial Summation INCREASES!

More rods

66
Q

HSP chose what size stimulus?

A

10 min arc

RECALL: 4-7 degrees eccentric = 20-30 min arc which is too large and would stimulate too many receptive fields and alter threshold for light detection

67
Q

What is ricco’s law and its equation?

A

Describes reciprocal relationship btwn THRESHOLD LUMINANCE & STIMULUS AREA

L x A = C
L = luminance
A= area
C = # quanta at threshold

m= -1.0 (1:1)

68
Q

Ricco’s law only holds for stimulus areas ____ than the critical area.

A

LESS than critical area

NOTE: size of “critical area” varies w retinal location

69
Q

What is the critical size for ricco’s law? What happens when you are outside of the critical size?

A

21’

You need to use PIPER’s LAW when you are outside

70
Q

What is the equation for Piper’s Law and what is it?

A

L x A^1/2 = C
m = -0.5 (1:2) –> 1 log unit INC in L offset by 2 log units DEC in A

Piper’s law is a PARTIAL SUMMATION function & applies outside the critical area of ricco’s law

This happens bc the retina can no longer sum light perfectly

Spactial summation still occurs, but at slower rates (i.e. smaller slop)

71
Q

True or False. Once you hit a flash size BEYOND piper’s critical point, you no longer summate

A

TRUE

72
Q

What is temporal summation

A

The adding together of events that occur at slightly diff time

NOTE: in the eye, temporal summation occurs w/in a limited time period of approx 10-100 ms

73
Q

True or false. Any combination of quanta w/in the temporal summation interval (10-100 ms) will result in a subject reporting ONLY a single flash

A

TRUE

74
Q

Bloch’s law equation and definition?

A

L x t = C

Describes a reciprocal relationship btwn threshold luminance & stimulus duration

Change in L can be offset solely by a change in t (or vice versa)

m= -1.9

75
Q

True or False. Bloch’s law is a total summation function.

A

True

76
Q

The critical point of 100 ms for Bloch’s law applies only for which retinal cells?

A

RODS

The critical point for cones is much shorter

77
Q

What is the square root law?

A

an unnamed square root law is a PARTIAL SUMMATION function & applies outside the crical area of Bloch’s law

After upper limit of square root law is passed –> straight line (vertical!)

78
Q

96% of quanta get past the cornea and only 50% reach the retina…. what happened to the rest of the quanta?

A
  1. Internal reflection
  2. Scattering
  3. Absorption
79
Q

Of the 50% of the quanta that reach the retina how many get absorbed by the rods?

A

20%

Loss is due to:

  1. Absorption by cones & RPE cells
  2. Passing through outer segment w/o isomerizing rhodopsin
80
Q

How much worse are cones than rods at catching quanta?

A

~2X worse

Need ~200 photons at the cornea to reach threshold of light perception

NOTE: 5 quanta need to excite a single cone but summation over 2 cones are required to reach threshold

81
Q

When we dark adapt is sensitivity increased or decreased?

A

INCREASED

NOTE: inc sense is DEC threshold

82
Q

What is a cone branch

A

Time period in which the test flash is detected by the cones

83
Q

What is a rod-cone break?

A

Point during dark adaptation at which rods become more sensitive than cones

NOTE: divides the portion of the dark adaptation curve where the subject detected the test flash w cones from the portion where rods were used to detect the test flash

84
Q

What is the rod branch?

A

Time period in which the test flash is detected by the rods

85
Q

What is the Dowling-Rushton Equation?

A

log (I threshold/I dark adapted threshold intensity) =P (fraction of photopigment bleached)

86
Q

What 4 factors allow our eyes to operate over such a wide range of light levels?

A
  1. Photoreceptors
  2. Pupil Size (~1 log unit)
  3. Changes in Photopigment Concentration (9 log units)
  4. Neural reponsiveness (1-2 log units)
87
Q

What is the Duplicity theory under photoreceptors describe?

A

Rods are used in low-luminance (scotopic) conditions & cones are used in high-luminance (photopic) conditions

88
Q

How much can our pupil account for light levels at the absolute extreme?

A

25X difference (~1.4 log units)

The PR/neural adaptation accounts for the other 10-11 log units

89
Q

What is the equation for pupil size area?

A

A = pi * (r)^2

1 mm –> 5mm is a 25X difference

If the eye is exposed to 2 log unit INC in luminance, pupil constriction CANNOT fully compensate to keep the retinal illuminance constant

90
Q

What happens when all photopigments are isomerized?

A

You are at THRESHOLD & can’t perceive any more light

91
Q

How long does it take to regenerate rods?

A

5 minutes, slower than Cones

92
Q

How long does it take for cones to regenerate?

A

2 minutes

93
Q

When does the rod cone break occur?

A

When 10% of the rods still bleached on a standard dark adaptation curve

NOTE: 10% bleach —> 2 log units above rod threshold
50% bleach —> 8 log units above rod threshold

94
Q

Does it how much you are bleached with light in order to reach threshold?

A

NO final threshold will always be the same ~1 log unit

95
Q

As you dark adapt how will blue light appear vs red light?

A

Blue=BRIGHTER

Red is the same…

96
Q

What is a photochromatic interval?

A

Difference brown the final rod threshold and cone final threshold

97
Q

When the target luminance is > background luminance, what kind of contrast is this ?

A

Positive contrast

OR

Negative polarity

NOTE: target luminance is what the target letter is

98
Q

When the target luminance is < background luminance, what kind of contrast is this ?

A

Negative contrast

OR

Positive polarity

NOTE: this is what the standard snellen chart is

99
Q

What is the Weber’s contrast equation?

A

Cw = [(L target - L background)]/L background

NOTE: 
Neg contrast (darker letter) —> range: -1.0 to 0
Pos contrast (darker background) —> range: 0 to infinity
100
Q

What’s the Michaelson Contrast equation?

A

C m = (Lmax - Lmin)/(Lmax + Lmin)

NOTE:
Negative Contrast range = 0 to 1
Positive Contrast range = 0 to 1

101
Q

What is deVris Rose Law?

A

Theoretical minimum for deltaL based on quantum fluctuations alone

m = 0.5
For every 0.5 log increase in background, the deltaL increases by 1 log unit

102
Q

When does the deVries-Rose law hold true for?

A

Rod mediated vision under scotopic conditions

NOTE: this law is also known as the square root law

103
Q

Describe the Weber’s law.

A

Constant proportional relationship bran the increment threshold deltaL

m = 1
For every 1 log unit increase in background, the deltaL also increases by 1 log unit

104
Q

When does the Weber’s law hold true?

A

Higher light levels (~3.18 cd/m^2)

Holds true for BOTH rod-mediated and cone-mediated vision

105
Q

A line/circle connecting retinal regions with similar increment thresholds (or sensitivity) is what?

A

Isopter

Inside –> above threshold

Outside –> test light not seen (below threshold)

106
Q

An optic nerve or coloboma will show what kind of scotoma on VF testing?

A

Absolute

Visual sensitivity = 0 and the increment threshold is infinity!

107
Q

A 10 dB change in sensitivity indicates how many log unit change in sensitivity?

A

1 log unit OR 10X change in sensitivity to a given stimulus

108
Q

What is veiling luminance/glare? How does it affect contrast?

A

Scattered light causing a veiling luminance over the retinal image that is proportional to the square of the angle of displacement of a glare source from a directly viewed light

NOTE: extra light in the eye can undergo SCATTER (more w more media opacities), causing a reduced contrast in the retinal image (spreading of the point spread function)

109
Q

How does veiling luminance affect Weber and Michelson contrast?

A

It REDUCES both bc it adds together with the background luminance

110
Q

Threshold acuity depends primarily on what?

A

CONTRAST/LUMINANCE

NOT on the angle of subtends…… for example a star subtends 0.018 degrees & is primarily detected bc of the contrast/luminance (how bright the star is) & not the small angle

111
Q

Which type of acuity is the angular size of the smallest visible target (i.e. diameter of the smallest spot of light, width of the thinnest line)?

A

Detection acuity

112
Q

Which type of acuity is the smallest spatial offset or difference in location btwn targets that can be disriminated?

A

Localization acuity

AKA hyper-acuity, spatial-interval acuity, and vernier acuity (5 years old adult like)

113
Q

What type of acuity is the smallest spatial separation btwn 2 nearby points or lines that can be discriminate?

A

Resolution acuity

AKA MAR, clinically implemented as GRATING acuity

  • Pediatrics (teller cards)
  • Disease (aka inferometry)
114
Q

What type of acuity is an acuity that measures the subject’s MAR by determining their ability to identify a letter, number, or character?

A

Recognition acuity

AKA VA

115
Q

True or false. Threshold acuity is dependent on PR spacing

A

FALSE

It is NOT dependent on PR spacing - it is ONLY dependent on LIGHT

NOTE: a just-detectable spot is wider than the just-detectable line bc the visual system has less opportunity to spatially summate contrast info for a spot, as it can along the length of a line

116
Q

What is the threshold for a single black line?

A

0.5”

117
Q

What is the threshold for a single black dot?

A

15”

118
Q

What is the threshold for spatial interval acuity?

A

2-6”

NOTE: this is the same for vernier as well

119
Q

Where are the neural mechanisms for localization acuity occur at?

A

Visual Cortex!

NOT THE RETINA

120
Q

How is resolution set?

A

From the trough to one single peak, which is formed btwn 2 photoreceptors (aka center-to center distance of 2 PR)

121
Q

What is it called when the numerator is 1 min of arc times the test distance and the denominator is the tested observer’s MAR times the test distance?

A

Snelle fracion

122
Q

What is the critical detail always?

A

1/5 of the 5x5 grid OR 1’ (1/5 of total letter height)

123
Q

To maintain MAR, what must happen to the dimensions of the letter as you move it FORWARD and BACKWARDS?

A

Forward: decrease

Backwards: increase

124
Q

What is the equation for VAR?

A

VAR = 100 - (50 x logMAR)

125
Q

Each letter is worth how many logMARs?

A

0.02 logMAR

therefore, 5 letters per line

126
Q

How much is a one line improvement change a 1.0 logMAR acuity?

A

0.9 log MAR now

NOTE: every line is a 0.10 logMAR change

In this case 20/200 -> 20/160. 20/20 is 0.0 logMAR

Part 1 (31 decks)

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