Measuring the Threshold

Just Noticeable Difference (JND)

A just noticeable difference (JND) is the SMALLEST difference in the amount of STIMULATION that a specific sense can DETECT

Just Noticeable Difference (JND)

• Each unit represents a WHAT increment

Just Noticeable Difference (JND)

• Each unit represents a CONSTANT increment

Weber’s Law

The size of a Just Noticeable Difference (JND) is a CONSTANT proportion of the SIZE of the initial stimulus

Weber’s fraction

∆𝐼 / 𝐼 = 𝐾

Weber’s Law

• Constant increments in stimulus intensity produce WHAT in the perceived WHAT of sensation

Weber’s Law

• Constant increments in stimulus intensity produce SMALLER INCREASES in the perceived MAGNITUDE of sensation

Fechner’s Law

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

Hypothetical example of just noticeable difference (JND)

• Arithmetic progression (WHAT)

• Geometric progression (WHAT)

Hypothetical example of just noticeable difference (JND)

• Arithmetic progression (adding a fixed amount)

• Geometric progression (multiplies by a fixed factor)

Implications of Fechner’s Law

• S = WHAT

• Psycho = WHAT

Implications of Fechner’s Law

• S = K Log R

• Psycho = PHYSICAL

Implications of Fechner’s Law

• Psychological reality correlated with a WHAT - WHAT parallelism

Implications of Fechner’s Law

• Psychological reality correlated with a PHYSICAL REALITY - PSYCHOPHYSICAL parallelism

Stevens’ Power Law

• Modern investigators have found that Weber’s Law is not WHAT for all SENSES.

Stevens’ Power Law

• Modern investigators have found that Weber’s Law is not TRUE for all SENSES

Example:

• The perception of pain is the OPPOSITE of what Fechner’s Law predicts. As shown in the graph, a slight increase in stimulus magnitude results in a great increase in perceived magnitude.

Stevens’ Power Law - Equation

P = KS^n

where

P = Perceived magnitude

K = a constant

S = stimulus intensity