Psychophysics

Psychophysics is the study of WHAT

How physical stimuli are related or translated into psychological experience

Can the relationship be modelled by a math equation

The relationship is described by the Weber’s law and Fechner’s law

What are the two basic concepts of psychophysics

• WHAT

• WHAT

What are the two basic concepts of psychophysics

• Absolute threshold

• Just Noticeable Difference (JND)

Absolute threshold is WHAT

Absolute threshold is the minimum amount if stimulation that an organism can detect

• Gustav Fechner was one of the early researchers who were interested in finding out the WHAT

• Researchers found out that there is no WHAT at which the subject jumps from no detection to completely accurate detection (unlike the case of neurons)

• Gustav Fechner was one of the early researchers who were interested in finding out the ABSOLUTE THRESHOLD.

• Researchers found out that there is no SINGLE STIMULUS INTENSITY at which the subject jumps from no detection to completely accurate detection (unlike the case of neurons)

The chance of detecting a stimulus increases gradually with WHAT

The chance of detecting a stimulus increases gradually with STIMULUS INTENSITY

Absolute threshold is defined as the WHAT

Absolute threshold is defined as the INTENSITY LEVEL at which the probability of detection is 50%

Signal detection theory: the detection of a stimulus depends on

1. WHAT

2. WHAT

Signal detection theory: the detection of a stimulus depends on

1. Intensity of the stimulus

2. Psychological state of the individual or context of the situation

1. Intensity of the stimulus

• If the intensity of a stimulus is low/ambiguous (a vague stimulus embedded in many background noises), decision outcome is affected by WHAT

1. Intensity of the stimulus

• If the intensity of a stimulus is low/ambiguous (a vague stimulus embedded in many background noises), decision outcome is affected by OTHER FACTORS

2. Psychological state of the individual or context of the situation

Two processes are involved

1. WHAT

2. WHAT

2. Psychological state of the individual or context of the situation

Two processes are involved

1. Sensory process

2. Decision-making process

Percievers response: Yes

Stimulus: Present

decision outcome = WHAT

Percievers response: Yes

Stimulus: Present

decision outcome = HIT

Percievers response: Yes

Stimulus: Absent

decision outcome = WHAT

Percievers response: Yes

Stimulus: Absent

decision outcome = FALSE ALARM

Percievers response: No

Stimulus: Absent

decision outcome = WHAT

Percievers response: No

Stimulus: Absent

decision outcome = CORRECT REJECTION -

Percievers response: No

Stimulus: Present

decision outcome = WHAT

Percievers response: No

Stimulus: Present

decision outcome = MISS -

Unbiased responses

Hit rate = WHAT

Miss rate = WHAT

False alarm = WHAT

Correct rejection = WHAT

Unbiased responses

Hit rate = 95%

Miss rate = 5%

False alarm = 5%

Correct rejection = 95%

Unbiased responses

WHAT rate = WHAT rate

WHAT rate = WHAT rate

Unbiased responses

HIT rate = CORRECT REJECTION rate

MISS rate = FALSE ALARM rate

Unbiased responses

Decision is based on WHAT information only. The result is not
affected by WHAT/WHAT factors

Unbiased responses

Decision is based on SENSORY information only. The result is not
affected by PSYCHOSOCIAL/INDIVIDUAL factors

biased responses YEA-sayer

Hit rate = WHAT

Miss rate = WHAT

False alarm = WHAT

Correct rejection = WHAT

Biased responses - YEA-sayer

Hit rate = 95%

Miss rate = 5%

False alarm = 65%

Correct rejection = 35%

Biased responses - YEA-sayer

High WHAT rate and high WHAT rate

Biased responses - YEA-sayer

High HIT rate and high FALSE ALARM rate

biased responses NAY-sayer

Hit rate = WHAT

Miss rate = WHAT

False alarm = WHAT

Correct rejection = WHAT

biased responses NAY-sayer

Hit rate = 35%

Miss rate = 65%

False alarm = 5%

Correct rejection = 95%

biased responses NAY-sayer

• High WHAT rate and high WHAT rate

biased responses NAY-sayer

• High CORRECT REJECTION rate and high MISS rate

A just noticeable difference (JND) is the WHAT

A just noticeable difference (JND) is the smallest DIFFERENCE in the amount of STIMULATION that a specific sense can detect.

Webers law

• The size of a Just Noticeable Difference (JND) is a WHAT of the size of the WHAT

Weber;s law

• The size of a Just Noticeable Difference (JND) is a CONSTANT PROPORTION of the size of the INITIAL STIMULUS

What is Webers fraction equation

ΔI/I = K

where

I = the smaller JND - look at image

Webers law

• Constant increments in stimulus intensity produce smaller and smaller increases in the perceived WHAT

Webers law

• Constant increments in stimulus intensity produce smaller and smaller increases in the perceived MAGNITUDE OF SENSATION

Fechner’s law

• Subjective sensation is proportional to the WHAT of the WHAT of the stimulus

Fechner’s law

• Subjective sensation is proportional to the LOGARITHM of the INTENSITY of the stimulus

Fechner’s law equation

S = K log R

K = constant

S = sensation (measured in JND

R = Reiz (stimulus)

Hypothetical example of just noticeable difference (JND)

Arithmetic progression

ADDING a fixed amount

Geometric progression

MULTIPLYING by a fixed amount

Implication of Fechner’s Law

• Psycho = WHAT

• Psychological reality correlates with a WHAT - WHAT

Implication of Fechner’s Law

• Psycho = PHYSICAL

• Psychological reality correlates with a PHYSICAL REALITY - PSYCHOPHYSICAL PARALLELISM

Implication of Fechner’s Law

• Experimenter WHAT one variable and WHAT another

Implication of Fechner’s Law

• Experimenter CONTROLS one variable and MEASURES another

Implication of Fechner’s Law

• The birth of WHAT

Implication of Fechner’s Law

• The birth of EXPERIMENTAL PSYCHOLOGY