Psychophysics

1 Introduction - Psychophysics

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Fundamental psychophysical problems

1. Detection: Is there anything there? 2. Identification: What is it? 3. Discrimination: Is the stimulus different from that

one? 4. Scaling: How much of X is there?

Adele Diederich Jacobs University Sensation and Perception Spring 2015

PresenterPresentation NotesImagine you are out in the dark

4) How far? Length, How much ? brightness

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Psychophysics

Gustav Theodor Fechner 1801 1887 Physicist and Philosopher Invented psychophysics, thought to be the true founder of experimental psychology Elemente der Psychophysik (1860) Pioneering work relating changes in the physical world to changes in our psychological experiences

http://en.wikipedia.org/wiki/Fechner


PresenterPresentation NotesGustav Theodor Fechner was one of the first interested in the relation between physically measurable properties of stimuli and sensation. As a physicist and philosopher, he is often considered as the founder of experimental psychology. His intention was to find a function that maps physical entities, e.g., intensity of a tone, onto a psychological scale, e.g., the sensation of loudness. His approach is called psychophysics and it still enjoys popularity among todays behavioral neuroscientists.

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Detection

Question: How much of a stimulus is necessary for an individual to be consciously aware of its presence (i.e., to see, to hear, etc.)?

Absolute threshold: Minimum amount of stimulation

necessary for a person to detect a stimulus 50% of the time

50% is an arbitrary threshold.


PresenterPresentation NotesBelow a critical intensity of the stimulus, a person would not be able to detect it. animals

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Psychometric Function

intensity < 12 never detected intensity > = 12 always detected Adele Diederich Jacobs University Sensation and Perception Spring 2015

PresenterPresentation NotesPerfect detector == ideal psychometric function

Ordinate (Y axis): proportion of stimulus presentations for which the observer ist expected to say yes to the question : Did you see (hear etc) the stimulus?

Abscissa (X axis); value of the stimulus magnitude

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Psychophysical Methods

Method of Limits Method of Adjustment classical Method of Constant Stimuli Forced Choice Adaptive Testing Signal Detection Theory (SDT) Magnitude estimation Reaction time


PresenterPresentation NotesHow can you determine or measure an absolute threshold?

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Methods of Limits

Trial 1: (99+98)/2 = 98.5 Trial 2: (99+100)/2 = 99.5

Trial 8: (98+97)/2 = 97.5

: :

Threshold: (98.5+99.5+ +97.5)/8 = 98.5


PresenterPresentation NotesName from Emil Kraepelin (1891)

Descending high intensity (down)

Ascending low intensity (up)Both are mixed

Why repeating? Bias

Threshold not fixed Why? (Joseph Jastrow, 1888) lapses of attentionslight fatigue other psychological changes And --> next slide

1 Name from Emil Kraepelin (1891) Descending high intensity (down) Ascending low intensity (up) Both are mixed

Why repeating?

Threshold is not fixed Why?

Method of Limits

8 Adele Diederich Jacobs University Sensation and Perception Spring 2015

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Endogenous (internal) noise: any background sensation other than the one to be detected

Exogenous (external) noise: experimentally controlled background noise caused by external stimuli other than the one to be detected


PresenterPresentation NotesWe tend to assume that in a threshold measurement experiment the only stimulus present is the stimulus we are asking our observers to detectNOT THE CASEA constantly present and ever-changing background of sensation exists no matter what stimulus we present to the observer

TRY:1) Place both of your hands over your ears to block out the rooms noises2) Sit in completely light-proof room in absolute darkness grayish mist = (cortical gray)Sometimes momentary bright pinpoint flashes

Any stimulus we ask an observer to detect must compete with the spontaneously generated fluctuating background

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Question

Problem with this method?


PresenterPresentation NotesLS 8: S knows how intense a stimulus to expect next (successively weaker/stronger)

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Method of Adjustment

Stimulus intensity is adjusted in a continuous manner (e.g. by turning a knob) until the observer can barely detect the stimulus.


PresenterPresentation NotesDifficult to evaluate

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Adele Diederich Jacobs University Sensation and Perception Spring 2015 12

Methods of Constant Stimuli

Fixed or constant set of stimuli is chosen beforehand, e.g.,

{7, 8, 9, 10, 11, 12} Order of stimulus presentation is random Each stimulus must be presented as many times as

the others (equally often).

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Method of Constant Stimuli

Threshold is arbitrarily defined at 50 %

psychometric function

PresenterPresentation NotesShow illustration of the method of constant stimuli (Figure 1.12).

1 Psychometric function


ideal reality

1 A psychometric function describes the relationship

between a parameter of a physical stimulus and the subjective responses of the subject.

Psychometric function


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Question

Problems with this method?

PresenterPresentation NotesExperimenter has little control over CRITERIA the subject applies

S refuses to say Yes unless the stimulus was obviousS willing to say yes at the slightest hint

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Method of Forced Choice

Present two or more alternatives Observer must pick one even if he/she thinks he/she

never saw the stimulus. (2AFC for two stimuli) Percentage correct is recorded for each stimulus

level and plotted as psychometric function.

PresenterPresentation NotesIf S does not know guess

Question not WHETHER but WHICH

1 Wolfe et al, http://sites.sinauer.com/wolfe4e/wa01.01.html

2AFC - Demo


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Question

What do you think the psychometric function looks like, the same as before?

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2AFC - Result

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2AFC - Result

1. The method gives a lower threshold than either the method of constant stimuli or the methods of limits.

2. The psychometric function does not fall to 0% for weak stimuli but levels off at 50%

PresenterPresentation NotesForced choice S must make a response; in the other methods the response can simply be a no, if S is unsure.

1) Many of the responses in the AFC may be guesses, but most S are surprised to learn (after the experiment) how good they were Thought guessing but better than chanceConsequence of guessing (many trials) threshold at 75% (in between) Fact that psychometric curve has its range limited between 50 and 100% makes it harder to measure thresholds (especially as the probability of getting EXACT 50% by guessing is small unless the # of trials is extremely large) to improve more alternatives e.g., 3AFC, 4AFC guessing = psychometric function between 25% an 100% threshold 62.5%

Adding even more choices may be confusing

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Can we separate sensitivity from response biases?

Sensitivity and Response bias


1 Example


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False alarm ? %

Hit ? %

Correct rejection ? %

Miss ? %

Stimulus

Noise Noise + Stimulus

Response Yes

No

100 % 100 %

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False alarm 40 %

Hit 90 %

Correct rejection 60 %

Miss 10 %

Stimulus N N + S

Response Yes

No

100 % 100 %

Laurie: Liberal responder

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False alarm 10 %

Hit 60 %

Correct rejection 90 %

Miss 40 %

Stimulus N N + S

Response Yes

No

100 % 100 %

Chris: Conservative responder

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False alarm Lose ?

Hit Win ?

Correct rejection Win ?

Miss Lose ?

Stimulus

N N + S

Response Yes

No

100 % 100 %

Payoff Matrix

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False alarm 10

Hit + 20

Correct rejection + 10

Miss 10

Stimulus N N + S

Response Yes

No

100 % 100 %

Change criterion with payoff matrix, e.g., Chris to say Yes more often

Criterion change

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False alarm Lose

Hit Win

Correct rejection Win

Miss Lose

Stimulus

N N + S

Response Yes

No

Payoff Matrix

Different payoffs change response criterion

PresenterPresentation NotesPayoff matrix change response criterion

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Chris: C, N, L Laurie: C, N, L

Results

Receiver Operating Characteristic (ROC) curve

Triangle: Results without payoffs

PresenterPresentation NotesNOTE: The intensity of the tone has remained constant. The only thing that changed is the subjects criterion. changes the subjects response.

The SD Experiments can tell us whether or not Chris and Laurie are equally sensitive to the tone.

1 are in RED Emphases in BLUE Questions and remarks in GREEN

Keywords


Introduction -PsychophysicsFundamental psychophysical problemsPsychophysicsDetectionPsychometric FunctionPsychophysical MethodsMethods of LimitsMethod of LimitsSlide Number 9QuestionMethod of AdjustmentMethods of Constant StimuliMethod of Constant StimuliPsychometric functionPsychometric functionQuestionMethod of Forced Choice2AFC - DemoQuestion2AFC - Result2AFC - ResultSensitivity and Response biasExampleSlide Number 24Slide Number 25Slide Number 26Payoff MatrixCriterion changePayoff MatrixResultsKeywords

Psychophysics

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