Methods of Stimulus Presentation Flashcards
Method of ascending limits
The magnitude of the stimulus intensity is set well below threshold so that the observer cannot detect it when presented. The intensity of the stimulus is increased systematically until the observer reports that it’s visible.
The intensity of the stimulus is reduced again and then systematically increased again.
Several trials are repeated and the threshold for the stimulus magnitudes for each trial are averaged.
Ex: absolute light detection
Example of method of ascending limits
Absolute light detection
Method of descending limits
A trial begins with a clearly visible stimulus. Stimulus magnitude is decreased systematically until the stimulus can no longer be seen.
Ex: visual acuity (This method is commonly done in low vision)
Example of method of descending limits
Ex: visual acuity (This method is commonly done in low vision)
Two disadvantages of both methods of ascending and descending limits
Anticipation and habituation (getting used to the pattern) can lead to an incorrect threshold value.
The effects of anticipation can be minimized by changing the initial stimulus intensity for each trial.
Staircase method
Stimulus magnitude is increased until the observer indicates its visible. The magnitude is then decreased until the observer can no longer detect it.
Forced choice method
Forcing the observer to indicate which of two or more intervals contain the stimulus. The observer cannot say “I don’t see the stimulus,” which reduces the influence of the observer’s criterion (strict or lax)
Methods of constant stimuli is commonly used for
Research and infant testing. Can be time consuming. Therefore, clinical procedures don’t typically use this method.
Methods of constant stimuli
A pre-selected range of stimulus magnitudes is used during testing. Some above and below the expected threshold.
For each trial, the stimulus magnitude is randomly chosen from the range of magnitudes available.
Method of constant stimuli for measuring absolute thresholds
Select range of light from invisible to clearly visible with 4-7 evenly spaced points in between. Show to patient in random order, with forced choice (either there or not). For each intensity level, the percent correct detection rate is calculated and plotted.
Detection rate is Y and stimulus intensity is X.
% often chosen as threshold performance for a 2-alternative forced choice test.
75%
Blank trials. What are they and what is their importance?
Trials in which there is no stimulus present at all, but the subject is asked whether the stimulus is present.
Used to determine the proportion of hits, misses, false positives, and correct rejections, which are used to determine the reliability of the data.
Hit
Miss
False positive
Correct rejection
Hit- Subject says stimulus was present when it was present.
Miss- Subject says stimulus was not present when it was presented.
False positive- Subject says stimulus was present when it was not presented.
Correct rejection- subject says stimulus was not present when it was not presented.
Method of adjustment
The subject him/herself adjusts the stimulus magnitude unit the stimulus is just barely visible (or invisible.)
Several trials performed, and averaged.
Threshold can be quickly determined since the subject is in control.
Can be influenced by feedback
Con of method of adjustment
Can be influenced by anticipation by the observer.
Example of method of adjustment
Patient adjusting their axis
2 types of bias that can cause erros
Lax threshold- Observer says stimulus is present when it is not presented. Results in high false positives.
The frequency of seeing curves doesn’t start at 0%, it starts at the fall positive rate.
Strict threshold- The subject says the stimulus is not present when it is presented. Results in high false negatives.
The frequency of seeing curve never reaches 100%, because even at high stimulus magnitude, the observer does not always indicate that he/she sees the stimulus.
Lax threshold
Observer says stimulus is present when it is not presented. Results in high false positives.
The frequency of seeing curves doesn’t start at 0%, it starts at the fall positive rate.
strict threshold
The subject says the stimulus is not present when it is presented. Results in high false negatives.
The frequency of seeing curve never reaches 100%, because even at high stimulus magnitude, the observer does not always indicate that he/she sees the stimulus.
Response bias
Participants in psychophysical experiments can be biased towards a certain response.
Ex: For every correct response, you get a dollar.
This would cause the participant to respond more often, resulting in more false positives.
Ex: For every incorrect response, you lose a dollar
This would cause the participant to respond less, resulting in more false negatives.
Strict criteron for forced choice testing.
high or low threshold?
High or low sensitivity?
Wont say they see it unless they’re really sure
High threshold and low sensitivity
Lax criteron for forced choice testing.
high or low threshold?
High or low sensitivity?
Will say they see it even when uncertain
Low threshold and high sensitivity
Is better performance indicated by a high or low threshold?
Low
Four alternative faced choice. What should the threshold intensity be?
25% chance of choosing correct interval just by guessing.
Choose the percentage that is halfway between guessing and perfect.
Ex: 75%/2 is 37.5(halfway) + 25 (guessing) = 62.5% threshold value
With more alternative faced choices, the curve ____, decreasing the change of error.
Steepens
