Psychophysical Methods Flashcards
The study of the relationship between the physical stimuli and perceptual responses
Psychophysics
The minimum quantity of a stimulus that can be detected
Threshold
-the observer is required to detect a light (stimulus) flashed a surrounding background, similar to a visual field test. The test is repeated for a range of stimulus intensities, from dim to intense
The percentage of stimuli detected is plotted as a function of stimulus intensity to produce a
Frequency of seeing (FOS) curve, also referred to as a psychometric function
Manifests an unambiguous threshold, below the threshold intensity stimulus is never seen, and above this intensity is always seen
An ideal observer
As the intensity increases of the stimulus is increased, the probability of seeing the stimulus increases, there is no clearly defined intensity below which the stimulus is never seen and above which it is always seen
A real observer
There are no perfect observers, therefore threshold is based on
Theoretical considerations
What is threshold defined as
The intensity that results in detection of the stimulus on 50% of the presentations
A diseased individual is _____ than a healthier one, making the FOS curve less steep (more flat)
Noisier
From several methods to measure a threshold, the most suitable method is determined by the ____ of the experiment or clinical procedure
Nature
What are the methods of threshold determination
Method of ascending limits Method os descending limits Staircase method Method of constant stimuli Method of adjustment Forced choice method
The stimulus is initially not visible, the stimulus intensity is increases systematically until it is visible, several trials may be performed, and the results are averaged to obtain a threshold
Method of ascending limits
Advantages of method of ascending limits
Dark adaptometry-becuse the state of retinal adaptation is affected minimally by the stimulus
Disadvantages of method ascending limits
Observer anticipation
Mitigation of potential bias-begin each trial at a different intentisty
Reverse of method of ascending limits, the stimulus is clearly visible at the beginning, the stimulus intensity is decreases systematically until it is no longer seen. Several trials may be performed, and the results are averaged to obtain a threshold
Method of descending limits
Advantages of the method of descending limits
Determination fo VA: s Ellen eye chart optotypes become progressively smaller from top to bottom. Threshold MAR is determined when the optotypes are too small to be resolved
Disadvantages of method descending limits
Observer anticipation
Mitigation of potential bias-begin each trial at a different intensity
Combination of ascending and descending limits. Initially the stimulus is below threshold, and increases systematically until visibility. At this point, the shape is reversed. The stimulus intensity is decreased systematically until it is no longer seen. Either on particular reversal or average of few reversals are used to determine threshold
Staircase method of determination of threshold
Advantages of staircase method of determination go threshold
Quick and reliable
Psychophysical experimentation
Automated VF testing
Stimulus visibility is varied randomaly from presentation to presentation
Method of constant stimuli
FOS curve is plotted based on the subjects repsonses
Method of constant stimuli
‘Yes-no’ procedure: observer is asked whether the stimulus is seen or not
Method of constant stimuli
Blank trials-when no stimulus is presented, even though observer is asked
Method of constant stimuli
When no stimulus is presented, even though observer is asked
Blank trials
The number of times the observer reports seeing the stimulus during a blank trials
False positive (false alarm)
What method uses false positives
Method of constant stimulus
When the stimulus is clearly above threshold and the subject reports that such a stimulus is not seen, this response is a
False negative
What method uses a false negative
Method of constant stimuli
Advantages of method of constant stimuli
Maintains oberservers expectations at the same level
Disadvantages of method of constant stimuli
Time consuming
Not prescribed from clinical applications
False-positive and false negative affect FOC. FOC curves has to be compensated or adjusted for false -(+/-)ve rates
-mitigation: forced choice method
The subject adjusts the stimulus until it is barely visible (or invisible)
Method of adjustment
Advantages of method of adjustment
Allows for a relatively quick threshold determination
Disadvantages of method of adjustment
Repeatability is not as high as the other methods discussed
Observer is asked whether the stimulus is seen or not
Yes no procedure
When no stimulus is prestend, even though observer is asked
Blank trial
Observers criterion of forced choice method
- Not all observers use the same criteria when deciding whether to not they see a stimulus
- The threshold criteria may vary trial to trial for the same observer
- An observer may use one set of criteria for one type of stimulus and another set for a different stimulus
Strict threshold criterion in forced choice
Do not report seeing a stimulus until they are absolutely certain they see it.
What does strict threshold criterion result in
Relatively high threshold (low sensitivity)
Lax threshold criterion in forced choice
Do report seeing a stimulus even though they may have a great deal of uncertainty regarding their decision
What does lax threshold result in. In forced choice
Relatively low threshold (high sensitivity)
What causes the threshold criteria to be potentially complicated for interpretation of experimental results in forced choice?
Observer criterion
In this methodology, the effects of the observers criteria are minimized by forcing a choice between several alternatives, one of which contains the stimulus
Forced choice method
Forced choice method: steps
- stimulus randomly presented in one of the two windows
- the other window does not contain a stimulus-it is blank
- the observer is forced to choose which window contains the stimulus
- I cannot see the stimulus is not acceptable
- the role of the observers threshold criteria is reduced
What is a perfect example of forced choice method
Vector vision CSV chart
-two force choice method
If the experiment forces the observer to choose between two alternatives, it is referred to as
Two-alternative forced choice (2AFC) experiment
A psychometric function for a 2AFC experiment
- The lowest percentage correct is 50%, not 0, because the observer is expected to guess correctly 50% of the time
- threshold is typically assumed to be the point midway between chance performace (50) and perfect performance (100)
- the threshold for a 2AFC experiment is 75
Stimulus in which the stimulus is randomly presented in one of four windows, the observer is forced to choose the correct window
Four alternative forced choice (4AFC) method
4AFC experiment
- the psychometric function for this experiment shoes chance performance of 25%
- threshold is taken as the point midway between chance performance and 100%, which is 62.5%
Why is 4AFC better than 2 AFC
Because the 4AFC psychometric function is steeper, the threshold can be ascertained with more confidence
Advantages of 4AFC method
- Compared to other psychophysical methods, forced choice cynically results in lower thresholds
- when observer are forced to guess, they often do remarkably well despite claims they cannot see the stimulus
- used effectively to determine VA and other visual capabilities of infants
What is the chance performance for a 2AFC
50%
What is the steepness of the curve for a 2AFC
Not steep
What is the threshold for a 2AFC?
75%
What is the chance performace for a 4AFC
25%
What is the threshold of a 4AFC
62.5 %
Steepness of 4AFC
Steeper than 2AFC, better
