Lecture 12 - Psychophysics

Question 1

What are psychophysics?

Answer 1

The study of relationships between
the properties of physical stimuli
and the subjective experience of those stimuli.
* Sensory threshold = the weakest intensity
that is detectable e. g. 50% of the time.
* Difference threshold or just-noticeable
difference (j.n.d.)

Measurement
Psychometrics focused on:
talents, skills and traits.
Psychophysics focused on:
relationship between
the physical intensity of a stimulus
and mental intensity of it’s
subjective experience.

Question 2

Weber

Answer 2

Prof. of anatomy, physiology and physics at
University of Leipzig!!. “De Tactu”
Aesthesiometer; adjustable compass
just detectable distance between two needles:
1–40mm, depending on the density of innervations
Just noticeable differences j.n.d.
specific to the modality of stimulus
(touch, weight, tone etc), tone

Ø Weber’s Law: The j.n.d. is a constant proportion of the
standard stimulus.
(Rn - Rs) ÷ Rs = k
Rs = standard stimulus Rn = new stimulus
e.g. for a lifted weight k=1/50, for a sound k=1/10

Weber’s k fraction for weight sensation

Probably don’t need to know but:
Weber compared two stimuli, the standard stimulus (Rs) and
the new stimulus (Rn) to measure the j.n.d.
just-noticeable-difference between their intensities.
Weber’s Law: The j.n.d. represents a constant proportion (k)
of the standard stimulus.
* How are the physical intensities represented in the mental experience?
* The j.n.d.’ s are not physically identical (in magnitude) but they appear equal
The j.n.d. values are specific to the sensory modality, such as weight or sound.
For example, for a lifted weight, k = 1/50
but for sound, k = 1/10

Question 3

Fechner

Answer 3

Gustav Theodor Fechner 1801 – 1887
Prof. of medicine and physics at Leipzig Univ.
Studies on colour vision and psychophysics.
Personal crisis, switch to spiritualism.
“Elemente der Psychophysik” 1860.
Psychophysics - a new science of the mind:
measuring the mental, subjective experiences
through physical stimuli.
The j.n.d. is a unit of mental experience.
Physical logarithmic scale - a scale of mental experience. Fechner’s Law
Sensory threshold in audiological examination
the weakest intensity, detectable 50% of the time
* Difference threshold = minimal detectable change in the physical
intensity of a stimulus. It’s a just noticeable difference (j.n.d.) between
the original stimulus and the new one.

Fechner: The j.n.d.’ s as units along a sensory scale
Physical versus subjective magnitude of the j.n.d.’ s
Logarithmic scale corresponding with subjective sensation

Fechner’s law: S = k log R
The magnitude of subjective sensation is directly proportional to the
logarithm of
the physical magnitude of the stimulus
S = subjective magnitude of sensation
k = constant fraction
R = physical magnitude of a stimulus
Fechner S = R(1+k)ⁿ S = k log R
The magnitude of sensation is directly proportional to the logarithm of the
physical intensity of the stimulus.
R Physical intensity on the logarithmic scale
R Physical intensity on the arithmetic scale

Question 4

Weber versus Fechner

Answer 4

Weber: a simple arithmetic scale
Fechner: a sophisticated logarithmic scale
Weber: the line representing the relationship between the arithmetically
growing intensity of stimuli and their sensation is curved
Fechner: the line representing the relationship between the logarithmically
growing intensity of stimuli and their sensation is straight and