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)
