Exam 1 Flashcards
Variables required to determine the visual angle a stimulus subtends on the retina?
1) Dimensions of the object
2) Distance between observer and object
equation used to calculate the visual angle an object subtends on the retina
tan(theta)=opposite/adjacent
where opposite is the object size and adjacent is the distance from the observer
Subject is instructed to look at fixation cross. Test flash (of diameter=15cm) is presented 60 cm to the right of the fixation cross. Assume subject is 2 m from the stimulus display. Test flash is projected to retinal region ______ degrees from the fovea.
Ɵ= tan-1 (60/200) = 16.70°
On a monitor, 2 stimuli are 25 cm apart. The subject is 1 m from the monitor. How far apart are the stimuli?
Ɵ= tan-1(25/100) =14.04°
What is a sinewave grating pattern
•A grating pattern is a series of light and dark bars.
-a pattern in which bars have fuzzy edges. Their luminance changes very gradually from dark to light, then from light back to dark.
What is a square-wave grating pattern?
a pattern in which bars have crisp sharp edges. Their luminance changes abruptly from dark to light and from light to dark.
What is luminance profile?
•Luminance profile is a graph showing how luminance/ intensity changes over space.
◦Y-axis= luminance and color appearance
◦X-axis= position
Luminance profile for the square-wave pattern
looks like blocks
if in the dark bar the luminance is lower on the graph
If in the light bar the luminance is higher on the graph
if right on the border the line is vertical
Luminance profile for the sinewave pattern
the graph is a sine wave
the middle of the bars are the darkest/lightest points
What are the four characteristics that are needed to fully describe/define a sinewave/squarewave pattern?
a. Spatial frequency
b. Contrast
c. Orientation
d. Phase
Spatial frequency
a. width of bars (bars are of equal width in a grating pattern)
i. High when narrow bars; Low when wide bars
ii. Recorded as cycles per unit area (i.e. cpd)
Contrast
a. difference in lightness between the dark and light bars.
i. High when the difference in brightness is great
ii. Recorded as percentage calculated using Michelson Contrast Equation
orientation
a. tilt of the grating pattern
i. Horizontal, vertical or oblique
Phase
a. position relative to some landmark; alignment of the grating pattern bars
i. In phase= light bars line up with light bars and dark bars with dark bars
ii. Out of phase= bars do not line up nicely
What is a cycle?
a pair of bars
one dark and one light
Bar width = 2 cm. Observer’s distance = 40 cm
Calculate the cycles per degree of visual angle a grating pattern subtends
Ɵ= tan -1 (2/40)=2.86° per bar and 5.71° per cycle so 0.17 cpd
•Remember that if single bar subtends 15’ than 1 cycle subtends 30’and so 2 cycles per degree because 60’ per degree
Michelson contrast equation
Contrast= (Luminance max - Luminance min)/(Luminance max + Luminance min)
Lmax=50cd/m2 and Lmin=20 cd/m2
calculate Michelson contrast
Contrast = (50-20)/(50+20)=30/70= 42.86
Psychophysics
scientific study of the relationship between physical stimuli and their resulting sensations.
Detection task
measurement of minimum amount of energy needed to just detect the presence of a stimulus. Absolute threshold= intensity which produced detection of the stimulus 50% of the time.
Discrimination task
measurement of minimum amount of a stimulus that must be added or taken away from one stimulus so that it is just noticeably different from a second stimulus. Difference threshold is the difference between two stimuli which is detected 50 % of the time.
Recognition Task
measurement of minimum amount of stimulus needed to be present to correctly name or categorize the visual stimulus.
Example of detection threshold
AKA absolute threshold
- Is stimulus detectable?
- at what luminance can target just be detected?
Example of discrimination threshold
AKA difference threshold
Is one suprathreshold stimulus different from another?
-how bright must light be to be just noticeably different from another?
-how far apart must 2 point sources be in order to appear separate?
-when are 2 lines just noticeably different in length?
-select visual match for target from group of objects.
-which object is different?
Example of Recognition threshold
Identify (name, classify) suprathreshold stimulus.
-Call out letters in V/A chart.
Suprathreshold
stimulus which is above threshold
Relationship between threshold and sensitivity
- Threshold and sensitivity are inversely related
* Sensitivity=1/threshold
Stimulus a subject is most/least sensitive to
- If Log-relative sensitivity=2.00,
- Relative sensitivity=10^(2.00)= 100,
- Threshold =1/100=0.01
- Log-Threshold=Log(0.01)= -2.00
Method of Adjustment
ONLY psychophysical method that allows the subject to have control over the stimulus
Absolute threshold via the method of adjustment
Stimulus intensity is set very far above or very far below actual threshold and subject adjusts stimulus until it is either just noticeable or just disappears
Mean=absolute threshold
Difference threshold via the method of adjustment
Subject adjusts a comparison stimulus until it seems equal to a standard stimulus. Sometimes subject will underestimate, sometimes overestimate.
To calculate: Average of readings= point of subjective equality Standard deviation (SD)= variability in the results SD= difference threshold
Method of limits
most frequently used psychophysical method
Absolute threshold via the method of limits
Experimenter presents stimulus well above or below threshold. The stimulus intensity is changed just slightly until subject estimates that stimulus just appears or just disappears.
To calculate:
Transition point for each trial is obtained.
Average of transition points=absolute threshold
Difference threshold using the method of limits
Standard and comparison stimuli are presented. Intensity of variable target is increased or decreased and subject is to indicate whether the comparison is greater/ equal/ lesser as compared to standard.
To calculate:
Upper transition points (UTP) and lower transition points (LTP) are determined.
Interval of uncertainty
(IU) = (mean UTP –mean LTP)
Point of subjective equality=
( mean UTP + mean LTP) / 2
Difference Threshold =IU / 2
Method of Constant Stimuli
VERY timing consuming
Absolute threshold using method of constant stimuli
Large # of trials is presented with subject responding either yes they detect target or no they don’t detect target.
To calculate:
% yes responses is calculated and plotted in psychometric function.
50% point in graph= Absolute threshold
Difference threshold using method of constant stimuli
Subject experiences pairs of stimuli and judges whether or not the variable is greater or less than standard.
To calculate
50% on graph= point of subjective equality= complete lack of discrimination between the 2 stimuli
25th and 75th % = ½ way point between 0 and 100% discrimination= difference thresholds
Three reasons thresholds can differ between observers
1.Variations in neural noise. Thresholds must be detected against background neural activity.
- Change in subject’s attention.
a. Conservative patients say “Yes, I see the stimulus” only when they are VERY SURE they see it.
b. Relaxed, loose, lax subjects say “Yes, I see the stimulus” if they know they see it as well as when think they see.
3.Fluctuations in Stimulus Energy or quantal fluctuations refers to the not steady emission of photons from a light source.
what does noise mean when used in reference to visual thresholds?
Ongoing, spontaneous activity of neurons. Even in absence of any stimulus, ganglion cells in the retina generate potentials.
What two errors do we try to avoid with the method of limits and how do we do so?
•Error of habituation: subject gets locked into repeating same response over and over.
◦To fix: shorten each trial
•Error of anticipation: subject gets the hang of the technique and anticipates expected change in response.
◦To fix: vary starting point
Why is method of constant stimuli rarely used?
Requires many trials; It is very time consuming.
Which psychophysical technique REQUIRES a graph to determine both the absolute and difference threshold?
Method of constant stimuli
What does the 50th percentile tell you when the absolute threshold data are plotted for the method of constant stimuli?
Absolute threshold; that stimulus intensity is detected 50% of the time
What does the 50th percentile tell you when difference threshold data are plotted for the method of constant stimuli?
Lack of discrimination between the 2 stimuli. They look the same. They are considered smaller 50% of the time and larger 50% of the time. This is the point of subjective equality.
Why are the 25th and 75th percentiles used to define the difference thresholds with the method of constant stimuli?
- These are the difference thresholds as they indicate how much above or below the PSE one must go to get reliable just noticeable difference.
- These points are the point halfway between lack of discrimination and complete discrimination.
Can you compare the sensitivities of different subjects using the method of constant stimuli (difference threshold data)? How?
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Which method typically allows the subject to adjust the stimulus intensity themselves?
Method of adjustment
How can you compare the relative sensitivities between subjects using the method of adjustment (difference threshold data)?
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Who is affected by the Charles Bonnet Syndrome? (i.e. what kinds of patients are most susceptible to the CBS?).
- Those who are older and have lost vision later in life
- Children, especially if their vision loss was sudden
- Patients who lost vision due to Macular Degeneration
- Remember: CBS is condition in which people with vision loss begin seeing hallucinations!
What is the range of visible light?
- 380nm-750nm
* Shorter wavelengths= bluish light; Longer wavelengths= yellow/ orange reddish light
What is the relationship between wavelength and frequency?
- They are inversely proportional.
* High frequency= Low wavelength
What is the relationship between frequency and energy/photon?
Frequency and photon energy increase simultaneously as wavelength decreases (in ROYGBIV order)
What is the amplitude of electromagnetic energy correlated with?
Brightness of light
UV light
340-400nm
i. UVA (most abundant in sublight, least harmful to humans)
ii. UVB (responsible for melanoma, tanning, synthesis of Vitamin D)
iii. UVC (most harmful to humans but mainly absorbed by earth’s ozone layer)
Infrared light
Infrared (750+ nm) Portion of infrared radiation is given off as heat
1.What is black light and why do some objects glow when illuminated by them?
Light source that produces wavelenghts below 400nm (mainly UV-A). The UV light excites phosphors when it encounters them.
Incandescent light
Emission of light by material because it is heated. Light is produced when electrons become excited. The external excitation is heat. The process of incandescence is the conversion of thermal energy to photon energy.
Examples of Incandescent light
- typical GE incandescent bulb
- tungsten bulb
- halogen lamp
- halogen-xenon lamp
Luminescent light
2 step process. Step 1 produces only UV light. In stage 2, UV light is used to make photons within the visible spectrum.
Type of phosphor determins wavelengths of light produced. Warm white fluorescent produces more long-wavelength and cool white produces more short wavelength
Examples of luminescent light
- fluorescnet lamp
- black light
What is the energy source that is converted into electromagnetic energy for incandescent light?
heat
Blackbody radiator
Black body radiator is any source of light that converts heat into electromagnetic energy.
What happens to the spectral emission function (SEF) of blackbody radiation as the blackbody temperature is increased
- More light is emitted (intensity increases) (greater area under curve)
- Wavelength decreases (shift to left)
- As temperature increases, color changes from infra-red to red to yellow to blue white
What is color temperature?
Temperature to which black body radiator must be heated to produce same SEF as light source being tested
What is correlated color temperature (CCT)?
Temperature to which black body radiator must be heated to produce same light color appearance as light source being tested
What does it mean if an illuminant has a color temperature (CT)?
It has
•Color appearance
•Spectral emmission function
•Incandescent
What does it mean if an illuminant has a CCT?
- Color appearance
* Luminescent
How does the shape of the SEF affect the color appearance of an object?
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As CT or CCT increases, how does color appearance of the illuminant change?
- 5000°K is cool white
- Short wave length- cool-white
- Long wave length- warm-white
What does The color rendering index (CRI) compare?
i. How color appears under gold standard
ii. How color appears under specific illuminant
How does one interpret the CRI scores?
- High score (i.e. 100%) if the color appears the same under condition 1 and 2.
- Low score (i.e. 0% if color does not appear similar under the 2 conditions. (greater difference= lower score)
What does the term efficiency mean? Why is efficiency rated in lumens/watt?
Efficiency= measure of how much visible light (lumens) is produced per unit of input (power)
What is meant by the term Standard Illuminant A? How about B and C?
> Standard Illuminant represents an incandescent
Standard Illuminants B,C, D represent luminescent and so daylight
*C is used in color vision testing (b/c balanced short, medium and long wavelengths)
Which Standard Illuminant has a CT and not a CCT?
Standart Illuminant A
What is the difference between radiometry and photometry?
- Radiometry: measure of pure energy
* Photometry: measure of energy, corrected for the human eye
What, exactly, does photometry “take into account” with respect to the human eye?
Relative sensitivity of human eye across wavelength across visible spectrum
What is the formula for converting from radiometric watts to photometric lumens?
•# of lumens= constant X power in watts X luminoscity coefficient
•# of lumens= (683)(Pλ)(Vλ)
◦Vλ max=1.00 for 555 nm for photopic system
V’λ max=1.00 for 507 nm for scotopic system
Will 50 watts of 555 nm light look as bright as 50 watts of 600 nm light?
No 50 Watts of 555 nm light will be brighter because V555 is greater than V600.
Will 50 lumens of 400 nm light appear as bright as 50 lumens of 600 nm light?
•Yes because lumen is a lumen is a lumen (Lumens determine the brightness)
4 types of photometric measurements
Luminous Power= Luminous Flux Luminous Intensity Illuminance Luminance Retinal Illuminance
Retinal Illuminance
measured in Troland
luminous power/luminous flux
total light power produced or emitted by a light source in every direction (Lumens)
Luminous Intensity
light power produced (in a specific direction) in a solid angle by a point source
( Lumens/Steradian or Candela)
Illuminance
amount of light incident upon a receiving surface
Lux or lumens/ unit area) (usually Lux
Luminance
amount of light reflected off a surface
Watts/ steridian/ unit area) (usually Candela/ m2 also written Cd/m2
4 types of radiometric measurements
Radiant power (Watts)
radiant intensity (Watts/ Steridian)
irradiance (Watts/ Steridian)
radiance (Watts/ steridian/ unit area)
Define the 5th photometric measurement (which doesn’t have a radiometric counterpart) and its unit.
- Retinal illuminance refers to the eye. It is measured in trolands.
- It measures amount of luminance reaching the patient’s eye
- Troland= (pupil diameter) (illuminance falling on the pupil)= (pupil area)(luminance refracted off a surface)
What are the two other terms used to describe the photopic luminosity function?
- Vλ
* Standard Observer
How were the luminosity coefficients determined for the photopic and scotopic systems?
•Heterochromatic Flicker Photometry
What is the scotopic luminosity function?
•Luminosity function as detected by rods
What is the other name for the scotopic luminosity function?
V’λ
Describe how the measuring distance affects illuminance readings.
•Inverse square law •If d=1 and E=1 •D=2, E=1/4 •D=3, E=1/9 because… •E1d12=E2d22 •Remember that Luminance also changes when illuminance changes ◦L=Er
Suppose the illuminance falling on your desk is 75 lux when the light source is 1m away. What will the illuminance be when the light source is 5 m away from your desk?
•E1d12=E2d22
•(75 lux) (1m)2 = (E2)(5 m)2
E2=3 lux
Describe how the distance between a reflecting surface and a photometer affects luminance measurements.
Changing the distance between reflecting surface and a photometer does not change luminance
The fact that light spreads out as it travels through space affects which photometric measurement?
illuminance
What is the formula that relates illuminance and luminance?
•Luminance= (illuminance) (radiance)
What is the inverse square law and what photometric measurement is affected by it?
- Indicates that
- when distance between light source and surface is 2X original, illuminance is ¼,
- when distance is 3X, E=1/9 etx.
What is a cosine-corrected surface?
- Aka Lamber surface
- Reflects light equally in all directions
- Could be matte- flat surface
- Not mirror or glossy-finish
Is a mirror considered to be a cosine corrected surface? If not, then give an example of one.
No but TV screen, projector screen are examples
What is an arc min?
Arc minute
1 degree= 60 min
To calculate Snellen Acuity
Denominator=20 x arc minutes resolvable
- Can you compare the sensitivities of different subjects using the method of constant stimuli (difference threshold data)? How?
Compare their difference thresholds (25th and 75th percentiles)
What does step size mean?
a. The amount that the stimulus is increased or decreased.
Do all staircases have the same size?
a. No, some do the same size all the time, some go halfway between the last step.
Does the forced-choice procedure eliminate differences in observer criteria?
Yes
What is the difference between strict criterion and lax criterion?
Strict criterion means they will only say yes when the stimulus is clearly visible. Lax criterion means they will say yes even if they are not sure.
How do criterion differences affect sensory thresholds?
a. A strict criterion leads to a higher threshold (FC will bring it down), a lax criterion leads to a low threshold (doesn’t change too much with FC, might bring it up a little bit)
- What is the task of the subject in a forced-choice experiment?
Choose where the stimulus is
- How does the task of a FC experiment differ from the task of a subject when tested with classical psychophysical thresholds?
a. Not saying if they see it, saying where it is. Must choose.
- Can you describe how the MCS procedure paired with forced choice differs from the traditional MCS?
a. Instead of asking if they see it, they are asked if it is to the left or right of the standard (the contrast will vary with the 5-9 stimuli)
PL as it is now practiced is ALWAYS paired with another psychophysical technique. But when it was first invented could it stand alone?
a. Yes, it could. Now it is paired with FC.
- Can you interpret the results of a FCPL experiment if paired with the MCS? For example, can you interpret the graph on page 19 in the class notes for Modern Psychophysics?
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- If given the highest spatial frequency that an infant or adult can resolve can you convert from spatial frequency in cpd to Snellen acuity? (Step 1: determine the width of the smallest bar a patient can resolve in min or arc. Step 2: multiply this value times 20 to determine the Snellen denominator). Do not use the equation for Leah paddles in Lab V.
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- For example: if a patient can resolve 30 cpd then they can detect a bar 1’ in width. This means they have 20/20 Snellen acuity. What will the acuity be if the highest spatial frequency they can resolve is 15 cpd?
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- If one paired MCS with FC to measure a contrast threhsold how would that experiment differ from one that measures grating acuity?
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Risk factors for Charles Bonet Syndrome? (Melissa’s answers)
a. Visual impairment
b. Above 70
c. Low extroversion
d. Isolation
e. Loss of energy
f. Stress
g. Low illumination
Explain how phosphors glow
a. UV light strikes the phosphorous coating lining the inside of the tube, photons are emitted in the visible spectrum.
- Be familiar with the energy source that is converted into electromagnetic energy for incandescent light and some examples of luminescent light. For example, what energy is converted to the electromagnetic energy that phosphors give off?
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- How do the SEFs differ between warm and cool fluorescent light?
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- What determines the color of an object?
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- Can the spectral emission function of the illuminant affect the perceived color of an object?
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- Be able to predict the color of objects given different illuminants.
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- How does the shape of the SEF affect the color appearance of an object?
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- Which Standard Illuminant should be used when testing color vision?
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- What does it mean if an illuminant is called a “warm white”?
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- What does it mean if an illuminant is referred to as “cool white”?
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