topic 1: psychophysical methods Flashcards
what is a threshold?
The patient’s limit, minimum quantity of a stimulus that can be detected
Distinguish between absolute and difference thresholds
Absolute threshold
The minimum quantity of a stimulus that can be detected against a dark background
Difference threshold
diff threshold
The minimum quantity of a stimulus that can be detected against a background that is NOT dark
The intensity of the background affects how bright the stimulus needs to be in order to be detected by the eyes!
what are the 5 factors resulting in variability in threshold results
- Background luminance (whether we are measuring an absolute or difference threshold)
- Perfect vs non-perfect observer (presence of neural noise)
- Subject attention (how much the person concentrates when performing the experiment)
- Subject motivation (how hard the person is trying to give an accurate response)
- Subject fatigue (how tired the person is)
What is the method of adjustment?
The subject adjusts the stimulus intensity either by
- Increasing the stimulus intensity from ‘can’t be seen’ to ‘seen’, (threshold is recorded when stimulus is first seen)
- Decreasing the stimulus intensity from ‘seen’ to ‘can’t be seen’ (threshold is recorded when stimulus first becomes invisible)
what errors are the method of adjustment prone to? briefly explain them
vulnerable to habituation and anticipation.
Habituation: occurs when subjects develop a habit of responding to a stimulus.
Anticipation: occurs when subjects prematurely report seeing the stimulus before the threshold has been reached.
how can we minimize these errors?
Habituation can be overcome by giving clear instructions, practice runs and demonstration. E.g. ‘press this clicker when you see the light, not when you hear the sound’.
This error can be minimised by changing the starting intensity for each trial.
What is the method of limits? what is the difference between ascending limits and descending limits?
In the method of limits, the subject must ***respond every time the stimulus is shown. The response is either ‘can see’ or ‘cannot see’.
ascending limits: Stimulus intensity increase from invisible to visible
descending limits: Stimulus intensity decreases from visible to invisible
Eg VA measurement (letters get smaller)
By starting at a level that the patient can see, it familiarizes the patient with the task and increases the clinician’s confidence that the patient understands how to perform the test.
what errors are the method of limits prone to?
habituation nd anticipation
what is staircase method?
The staircase method is a modified method of the combined ascending+descending method of limits.
the stimulus intensity starts at a level that is far from threshold (obviously visible or obviously invisible).
In staircase method, the stimulus intensity at several ‘reversal points’ are averaged to obtain the subject’s threshold. Below describes how a reversal point is obtained:
- At the beginning of the experiment, the stimulus is invisible. Its Intensity is increased until it is visible.
- Subject says ‘I can see it’ (Reversal point 1). The intensity of the stimulus at this point is recorded.
- The stimulus intensity is then decreased until it is invisible.
- Subject says ‘I cannot see it’ (Reversal point 2). The intensity of the stimulus at this point is recorded.
this repeated until 3-4 points are recorded
what is the method of constant stimuli? what is it good for? why is at bad?
In the method of constant stimuli, the stimulus intensity is presented to the subject randomly.
This method is typically used for research as it is good for controlling anticipation due to the randomness of the stimulus presentation.
However, it is time consuming (the same intensity is presented multiple times) and thus not commonly used clinically.
how do we conduct method of constant stimuli?
- Let’s use a hypothetical estimated threshold of 35
- 4 stimulus intensities above the estimated threshold chosen: i.e. 36, 37, 38, 39
- 5 stimulus intensities below the estimated threshold chosen: 34, 33, 32, 31, 30
- All together there are 10 different stimulus intensities including the estimated threshold itself: 30, 31, 32, 33, 34, 35, 36, 37, 38, 39
- The 10 different stimulus intensities are presented in random order in 20 presentations (each intensity presented 20 times)
6.The Subject just needs to respond if he sees the stimulus
The Psychometric function is plotted and the subject’s threshold value is obtained from the graph. ie. The stimulus intensity that is seen 50% of the time
what is the purpose of a psychophysical PROCEDURE? what are the 2 psychophysical procedures?
To minimise the variability in obtaining threshold, by make subjects commit to an answer
- Yes-no procedure
- Forced choice procedure
what is yes-no procedure?
he observer must judge the presence of absence of the stimulus. The correct response can range from 0 to 100%.
what is forced choice procedure?
the observer is shown a few windows (e.g. 2 or 4 choices), one of which will contain the stimulus. The observer has to indicate which window contains the stimulus.
Because an ‘I can’t see the stimulus’ is not an option, the possibility of reduced threshold value being recorded due to the observer’s own ‘strictness’ when performing the test will be reduced.
what is 2AFC procedure? where is the threshold value taken?
If there are 2 windows presented to the observer, it is called a 2-alternative forced choice experiment (2AFC).
the lowest percentage correct is 50%. It is not 0% because the observer has to give a response
subj is expected to randomly guess correctly 50% of the time. The threshold value is the midway point between chance performance and perfect performance, i.e. 75%.
