Psychophysics 4

Research Methods

Fall 2010

Tamás Bőhm

Psychophysical functions

• How the threshold changes with some stimulus dimension?

• Run several threshold measurements over a range of values along that dimension

• Each point corresponds to a threshold value

• More complete account of sensory characteristics

200 Hz

Psychophysical functions

Newby 1972

Psychophysical functions

con

tra

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spatial frequency

Highest sensitivity at ~3 cycles/degree

Psychophysical functions

• Why are we the most sensitive to contrast at 3 cycles/degree?

• Is it because of reading?

(The spatial frequency of typical letters is ~3 cycles/deg)

• No: several primates has the peak at a similar spatial frequency.

Uhlrich et al. BBR 1981

Psychophysical functions

1 month 2 months 3 months 6 months Adult

Psychophysical methods• Threshold measurements: detection of small intensities (absolute thr.) and

discrimination of small intensity differences (difference thr.)Is it intense enough to see? How small a difference can you see?

– Fechner’s 3 methods• Method of constant stimuli• Method of limits• Method of adjustment

– Modifications of Fechner’s methods• Staircase method• Modifications of the method of constant stimuli (adaptive, no standard)

– Forced choice, objective methods– Sensory decision theory (SDT)– Psychophysical functions from psychometric data

• Direct scaling: growth of sensation with intensityHow bright do you see a light?

– Magnitude estimation and the power law• Multidimensional scaling: degree to which stimuli are comparable along

some dimensionsAlong which dimensions do you judge the similarity of two stimuli?

Direct scaling

• Measuring the growth of sensation with intensity• Scaling: observers are asked to judge the

sensation magnitude on a scale• Direct: judgments ‘directly’ reflect sensation

magnitude

• Subjective but quantitative it can reveal regularities in judgments

Subject A Subject B

Magnitude estimation(S.S. Stevens)

1. Stimuli with various intensities are presented several times, in randomized order

2. Observer assigns numbers to each stimulus

3. First stimulus is assigned an arbitrary number

4. Further stimuli are assigned numbers representing the subjective impression

No limit on the scale!

Intensity Response

2 30

3 35

4 38

1 28

4 40

5 45

2 32

6 52

3 36

6 50

1 26

5 47

Magnitude estimation

• Actual numbers vary a lot from observer to observer

• But the relationship among the numbers are similar across observers

• Quick and easy

Observers

A B C

1 95 4 27

2 100 5 30

3 110 6 35

4 120 7 40

5 140 9 47

6 165 11 55

Variants of magnitude estimation

• Presenting a standard stimulus and an assigned number (modulus)

Play a 60 dB tone and tell the observer that this corresponds to 100. Further stimuli are judged relative to the standard.

• Cross-modality matching: equate sensation strengths from different sense modalities

Adjust the loudness of a sound until it matches the brightness of a light.

http://psych.hanover.edu/JavaTest/Media/Chapter2/MedFig.MagEst1.html

Power law (S.S. Stevens)

• The perceived strength of a stimulus increases in proportion to its physical intensity raised to some power:

E.g. for loudness:S = kI0.67

Power law

a<1: large increases in intensity produce small increases in sensation

a>1: small increases in intensity produce large increases in sensation

a=1: linear relationship; change in intensity corresponds to sensation change

Power law

Power law:

S = kIa

Take the logarithm of both sides:

log(S) = a log(I) + log(k)

K=log(k) is constant:

log(S) = a log(I) + K

This is the equation of a line(in the form of y = ax + b)a: slope; K: x-intercept

Power law

slope=1

slope=3.5

slope=0.33

Power lawContinuum Exponent (a) Stimulus conditionLoudness 0.67 Sound pressure of 3000 Hz

tone Vibration 0.95 Amplitude of 60 Hz on finger Brightness 0.33 5° target in dark Lightness 1.2 Reflectance of gray papers Visual length 1 Projected line Visual area 0.7 Projected square Taste 1.4 Salt Smell 0.6 Heptane Cold 1 Metal contact on arm Tactual roughness 1.5 Rubbing emery cloths Tactual hardness 0.8 Squeezing rubber Heaviness 1.45 Lifted weights Viscosity 0.42 Stirring silicone fluids Electric shock 3.5 Current through fingers Vocal effort 1.1 Vocal sound pressure Duration 1.1 White noise stimuli