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Modifications of Fechner’s methods, forced choice Research Methods Fall 2010 Tamás Bőhm.

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1 Modifications of Fechner’s methods, forced choice Research Methods Fall 2010 Tamás Bőhm

2 Modifications of Fechner’s methods Method of limits: only the last stimulus in each series contribute to the threshold estimate Method of constant stimuli: only the stimuli nearest to the threshold intensity contribute significantly to the estimate Rest of the stimuli: tell us little about the threshold but takes a lot of time to test  inefficient New procedures and technologies can make the methods more efficient Method of limits  staircase method  interleaved staircase Method of constant stimuli  adaptive methods  difference thresholds with no standard stimulus Light intensity Trials

3 Staircase method (Békésy / Dixon&Mood) Stimuli is concentrated around the threshold  efficient Serial ordering of stimuli: observer may become aware of the scheme; this may influence responses  interleaved staircase 1.Start with an intensity well above the expected threshold region and decrease until the observer declares it invisible 2.Reverse the direction of stimulus change and increase intensity until the response changes again 3.Go on until 6-7 reversals 4.Threshold estimate: average of stimulus intensities at the reversals

4 Interleaved staircase Observer is unlikely to become aware of the scheme Can be extended to more than two staircases Two staircases One starts from above, the other from below the expected threshold region Strict alternation: –Odd numbered trial: staircase A –Even numbered trials: staircase B Random alternation: –E.g. BABBAAAABB

5 Pretesting and adaptive methods Method of constant stimuli: little information is gained from stimuli that is always or never detectable Pretesting: determine the region where the threshold is expected  stimulus intensities can be selected from this region only Adaptive methods: choice of the next stimulus depends on previous responses of the subject If the observer could see all the light spots so far then, for further trials, we should decrease the intensity of the stimuli. –Even sensitivity fluctuations during the experiment can be handled. –Sophisticated computer-controlled methods

6 Omitting the standard stimulus Difference threshold measurement with the method of constant stimuli: the standard can often be omitted (and the precision of judgments remains the same)  efficient McKee (1981): –difference threshold for visual velocity –standard stimuli could be presented neither simultaneously (observers may judge disposition instead of velocity) nor temporally (a stimulus lasts long) –What if we don’t present the standard at all? –Instead: observers watch a few presentations of all 7 velocities beforehand (familiarization) –During the test: observers judge if the stimulus is faster or slower than the mean of all the seven velocities –Observers made accurate perceptual comparisons

7 Forced-choice, objective methods Subjective methods (yes/no paradigm): experimenter cannot judge the correctness of the response „Can you see the spot of light?” No way to verify the response. Objective methods: experimenter can evaluate the correctness of the response “Where is the spot of light?” Observer proves that he/she could see the light e.g. by identifying another stimulus characteristic (position, orientation, etc). Forced-choice: subjects are forced to decide + decisions can be checked objectively

8 Forced-choice, objective methods On which side of the fixation cross do you see the light? LEFTRIGHT

9 Forced-choice, objective methods Which direction does the egg point to? LEFTRIGHT Kovács et al , Gerván et al. 2007

10 Forced-choice, objective methods Differences from non-forced-choice: –Stimulus presentation: often times another characteristic needs to be varied –Observer’s task: identify this characteristic What if the observer does not detect the stimulus? –He/she still needs to decide: guessing –If observers guess: ~50% of the responses correct  chance level Terminology: –If there are two possible responses: 2-alternative forced-choice (2AFC) –Similarly: 3AFC, 4AFC, … What is the chance level in a 3AFC, 4AFC, nAFC design?

11 Forced-choice, objective methods Valid range of responses: chance level – 100%

12 Forced-choice, objective methods Why use forced-choice? 1.Subliminal perception: Observers often claim they do not see anything, still they produce results well above chance There is more sensory information available than the observer is aware of Forced-choice judgments do not require conscious experience  benefit from subliminal percepts

13 Forced-choice, objective methods Why use forced-choice? 2.Criterion: –Observers use a criterion to make a decision –Criterion differences across observers Schizophrenic and elderly people adopt strict criterions –Criterion differences within observers May decrease after training (observer wants to see it) –Forced-choice: no decision about detection  results not affected by criterion differences


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