Psychophysics

PSYCH 333 Exam 1

What is psychophysics?

Study of relationship between physical stimuli and perception

Two main components of psychophysics?

Physical world + perceptual experience

Key figures in psychophysics?

Ernst Weber and Gustav Fechner

What are the three main topics in psychophysics?

• Thresholds

• Scaling

• Signal Detection Theory

What does each topic study?

• Thresholds → what can you detect

• Scaling → how strong it feels

• SDT → sensitivity vs decision bias

What is the absolute threshold?

Smallest stimulus detected

How is absolute threshold defined?

Detected 50% of the time

Is absolute threshold fixed?

No, it is probabilistic

What is the difference threshold?

Smallest detectable difference between stimuli

Another name for difference threshold?

Just Noticeable Difference (JND)

Why are thresholds not fixed?

Due to noise

What is noise?

Variability in responses across trials

Types of noise?

• External (stimulus variability)

• Internal (neural variability)

Method of adjustment?

Participant controls stimulus (fast, less accurate)

Method of limits?

Experimenter changes stimulus

Method of constant stimuli (VERY IMPORTANT)?

Random intensities + many trials → builds psychometric function

What is a psychometric function?

Graph of stimulus intensity vs % detected

Where is threshold on this graph?

50% detection point

What is the staircase method?

Increase intensity if missed, decrease if detected

What is forced choice?

Participant chooses when/where stimulus occurred

Why use forced choice?

Reduces guessing bias

What does Weber’s Law state?

JND increases with stimulus intensity

Formula for Weber’s Law?

ΔI / I = k

What does k represent?

Weber fraction (constant ratio)

Key idea of Weber’s Law?

Sensitivity depends on proportion, not absolute change

Example of Weber’s Law?

100g → +2g
200g → +4g
(same ratio)

What problem does scaling solve?

Measuring perceived intensity above threshold

Formula for Fechner’s Law?

S = c ln I

Key idea of Fechner’s Law?

Perception increases slower than stimulus

Limitation of Fechner’s Law?

Doesn’t always match real perception

Formula for Stevens’ Law?

S = cIⁿ

What determines perception in Stevens’ Law?

Exponent (n)

What happens when n < 1?

Compression (perception grows slower)

What happens when n > 1?

Expansion (perception grows faster)

What happens when n = 1?

Linear relationship

Weber vs Fechner vs Stevens?

• Weber → detecting differences (ΔI/I = k)

• Fechner → logarithmic scaling (S = c ln I)

• Stevens → power scaling (S = cIⁿ, most accurate)

What is JND?

Smallest noticeable difference

What does JND follow?

Weber’s Law

What is noise?

Trial-to-trial variability in detection

Why are thresholds probabilistic?

Because of internal + external noise

What is the key takeaway of psychophysics?

Relationship between physical stimulus and perception is NOT linear