Modifications of Fechner’s methods, forced choice

Research Methods

Fall 2010

Tamás Bőhm

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

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Ligh

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ityTrials

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 until6-7 reversals

4. Thresholdestimate:average of stimulus intensities atthe reversals

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

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

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

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

Forced-choice, objective methods

On which side of the fixation cross do you see the light?

LEFT RIGHT

Forced-choice, objective methods

Which direction does the egg point to?

LEFT RIGHT

Kovács et al. 1993-, Gerván et al. 2007

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?

Forced-choice, objective methods

• Valid range of responses: chance level – 100%

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

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 observersMay decrease after training(observer wants to see it)

– Forced-choice: no decision about detection results not affected by criterion

differences