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### Psych 480: Fundamentals of Perception and Sensation

Dr. Keith S. Jones

Idealism

vs.

The Ecological

approach

Information

Processing

approaches

Gibson

Elaboration?

Yes

No

Constructivist

approaches

Computational

approaches

Helmholtz

Rock

Marr

A general IP model

Distal

Stimulus

(a lot can go wrong here)

Proximal

Stimulus

(if elaborative)

Transduction

“Processing”

Percept

Cognition

(if elaborative)

Psychophysics

Distal

Stimulus

How do these relate to one another?

Proximal

Stimulus

(if elaborative)

Transduction

“Processing”

Percept

Cognition

(if elaborative)

Activity (2 pts)

- How would you measure the magnitude of a perception?
- Describe your measurement technique.
- Explain how it accomplishes your goal.
- What problems, if any, exist with your technique?

Gustav Fechner (1860)

Concepts

Absolute threshold

Difference threshold

aka “JND”

“Weber’s Law”

I/I=K

Methods

Method of constant stimuli

Method of limits

Method of adjustment

Early attemptshttp://www.uni-leipzig.de/fechnerday/

His measurement system

Use a person’s absolute and difference thresholds as a counting system.

Assumes that

All JNDs are subjectively equal

Weber’s law is true

Fechner’s law

Perceived magnitude = K log Intensity

Fechner’s attemptExample

- Fechner’s law
- Perceived magnitude = K log Intensity
- Assume
- absolute threshold = 50 units
- difference threshold = 10%.

Problems

- Weber’s law is NOT always true
- It breaks down at the extremes
- Stevens (1957) argued that JNDs are not subjectively equal.
- He argued that they might be for metathetic dimensions (e.g., pitch, color), but not prothetic dimensions (e.g., loudness, brightness)

The tower of Babel was never finished because the workers could not reach an understanding on how they should build it; my psychophysical edifice will stand because the workers will never agree on how to tear it down. (Fechner, 1877)

Methods

Magnitude estimation

Magnitude production

Cross-modality matching

Steven’s Power Law

Perceived magnitude = K Intensity power

Stevens’ attemptExponent = .5

brightness

Exponent = 1

line length

Exponent = 3.5

electric shock

Problems

- If you use a category rating scale (i.e., a likert scale), then data follow a logarithmic function, not a power function.
- Stevens argues that these scales are inherently biased b/c the categories are not subjectively equal.
- There is also evidence to suggest that magnitude procedures are affected by context as well.
- Exponent inconsistency over time.

The debate rages on ...

- Many applied settings have adopted Stevens’ Power Law as a standard.
- However, from a theoretical standpoint, work is on-going.
- Krueger, L.E. (1989). Reconciling Fechner and Stevens: Toward a unified psychophysical law. Behavioral and Brain Sciences, 12, 251-320.

“New” psychophysics

Signal Detection Theory

ß

If likelihood ratio (Psn/Pn) > ß,

then person says “Yes”,

else he or she says “No”

http://acad.cgu.edu/wise/sdt/sdt.html

Calculating d’ and ß

- Based on the p(hit) and p(false alarm), you can calculate d’.

d’ = zp(fa) - zp(hit)

- Based on the height of the curves, you can calculate ß.

ß = height(hit)/ height(fa)

For d’, if it is

d’ < 1.5 = difficult

d’ = 1.6 to 2.5 = moderately difficult

d’ = 2.6 to 3.5 = moderately easy

d’ > 3.5 = easy

For ß,

three or less is common

If greater than three, person is conservative

Rules of ThumbTests of TSD

- Vary signal probability
- Should change ß w/o changing d’
- Pay-off matrices
- Systematically affect ß by changing operators strategies w/o changing d’
- Vary quality of signal
- Should affect d’ w/o affecting ß
- 2nd chance experiment
- If person is incorrect, then must be below threshold so giving a 2nd choice shouldn’t matter. If TSD is correct, then 2nd choice should beat chance b/c people differentially weight different possibilities.

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