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The threshold has been defined as the stimulus value which is perceptible in 50% of the trails.
Three methods of threshold measurement:
1.Method of constant stimuli
2.Method of limits
3.Method of adjustment
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Method of constant stimuli - Absolute Thresholds
Procedure: using the same set of stimuli(5~9 different values in the set) , each stimulus is present repeatedly, usually 100 times or more, but in a random order.
The percentage of detections as a function of stimulus intensity, φ is determined.
Yes or No response: the proportion( p ) of yes response is computed, and a graph called a psychometric function is constructed.
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Method of constant stimuli - Absolute Thresholds
Absolute threshold: the stimulus intensity for which the proportion of trials resulting in yes response is 0.5
Ogive function : S-shaped, is a cumulative form of normal distribution.
Fitting ogive function to threshold data range:
z-score
method of least squares
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Method of constant stimuli - Difference Thresholds
Standard stimulus(St):
fixed value
Comparison stimulus(Co):
the value is changed from trial to trial
Space error
Time error
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Method of constant stimuli - Difference Thresholds
Point of subjective equality(PSE):
The 0.5 point on the psychometric function , is perceived subjectively as equal to standard stimulus
Constant error(CE)
CE = PSE - St
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Method of constant stimuli - Difference Thresholds
Upper difference threshold(DL u )
the stimulus range from PSE to the .75 point
Lower difference threshold(DL l )
the stimulus range from PSE to the .25 point
DL = (Dl u + DL l) / 2
The method of least squares
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Method of limits - Absolute Thresholds
Ascending series : begin with very weak subthreshold stimulus, then increased by a small amount until the observer report the presence of the sensation
Descending series : the value of stimulus is decreased until the observer report the absence of the sensation
Threshold : the average of the transition points
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Method of limits - Absolute Thresholds
Error of habituation: the tendency for an observer to develop a habit of repeating the same response
Error of expectation: an observer may falsely anticipate the arrival of the stimulus at his threshold and prematurely report that the change has occurred before it really has
Minimize the errors:avoid using long trial series, vary the starting point
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Method of limits - Difference Thresholds
Upper limen(L u ): the point on the physical dimension where “greater” response change to “equal” response
Lower limen(L u ): the point where “less” response change to “equal” response
Interval of uncertainty(IU): the range on the stimulus dimension over which an observer cannot perceive a difference between the Co and St
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Method of limits - Difference Thresholds
IU = L u – L l
DL =1/2 IU = ½(L u – L l )
PSE = ½(L u + L l )
Error of habituation
Error of expectation
Space error
Time error
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Variation of the Method of Limits
Up-and-down/ staircase method (Cornsweet, 1962)
- efficient
- step size : must chosen with care
Threshold tracking method (Bekesy, 1947)
Forced choice method(Blackwell, 1953)
- temporal-forced choice
- spatial-forced choice
- threshold: two correct responses in succession
- response bias: the tendency to report the
presence of subthreshold or absent stimuli
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Method of adjustment - Absolute Thresholds
Procedure: set the stimulus intensity level either far below or far above threshold and ask the observer either to increase the intensity level until it is just perceptible or vice versa.
Can prevent boredom
Error of habituation
Error of expectation
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Method of adjustment - Difference Thresholds
Method of average error: the observer is instructed to adjust a Co until it seems equal to some St
Mean of X = PSE
CE = PSE – St
Standard deviation:
high degree of central
tendency when
discrimination is good
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Method of adjustment - Absolute Thresholds
Shortcomings:
- difficult to apply when stimulus are not
continuously variable
- difficult to apply when pairs of stimulus
cannot be presented simultaneously
- difficult to maintain constant conditions
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Application of classical psychophysical methods to problems of stimulus matching
Equal sensation contour: a stimulus critical value function, in which absolute threshold is plotted against some property of stimulus
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Application of classical psychophysical methods to problems of stimulus matching
Loudness enhancing effect : the increment in loudness of a sound caused by the presentation of another sound
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Conclusion
Sensation is treated as a concept which must be defined in terms of stimulus-response relationships.