Psych 480: Fundamentals of Perception and Sensation

Dr. Keith S. Jones

Why do we perceive?

Realism Idealism

Elaboration?

The Ecological approach

Yes No

vs.

Information Processing approaches

Constructivist approaches

Computational approaches

MarrHelmholtz Rock

Gibson

A general IP model

Distal Stimulus

Proximal Stimulus

Transduction “Processing” Percept

(a lot can go wrong here)

(if elaborative)

Cognition(if elaborative)

Psychophysics

Distal Stimulus

Proximal Stimulus

Transduction “Processing” Percept

How do these relate to one another?

(if elaborative)

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?

Early attempts• 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

http://www.uni-leipzig.de/fechnerday/

Fechner’s attempt• 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

Example• 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)

Stevens’ attempt• Methods

– Magnitude estimation– Magnitude production– Cross-modality matching

• Steven’s Power Law– Perceived magnitude

= K Intensity power

brightness

line length

electric shock

Exponent = .5

Exponent = 1

Exponent = 3.5

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” psychophysicsSignal Detection Theory

If likelihood ratio (Psn/Pn) > ß, then person says “Yes”,else he or she says “No”

ß

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

Signal Detection Theory

Signal Detection Theory

CorrectRejection

FalseAlarm

Hit

Miss

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)

How it works ...

ß

d’ = zp(fa) - zp(hit)

Zfa = 2.5

Zhit = -.5

How it works ...

ß

ß = height(hit)/ height(fa)

Height = .34

Height = .05

Rules of Thumb• 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

Tests 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.