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
