The branch of psychological science that study relationship between psychological phenomenon and physical properties of a stimulus

1. Introduction to
Psychophysics
Dr Rajesh Verma
Asst. Prof in Psychology
FGM Govt. College Adampur, Hisar, Haryana

2. Meaning
The word Psychophysics is made up of Psychology
and Physics.
Psychology = Psychological variables (Specially
Sensation and Perception) of an individual.
Physics = Physical Properties of Stimulus
Means studying the
relationship between
psychological phenomenon
and physical properties
of stimulus.

3. Definition
The branch of psychological science that study
relationship between psychological phenomenon and
physical properties of a stimulus.
The relationship between stimuli and the sensations
they evoke has been studied in a discipline, called
psychophysics (NCERT, XI).
“The analysis of perceptual processes by
studying the effect on a subject's
experience or behaviour of
systematically varying the properties of
a stimulus along one or more
physical dimensions” (Bruce et. al.,
1996 quoted by Wikipedia).

4. Introduction
The term psychophysics was coined by Gustav
Theodore Fechner in 1860 to describe mathematically
the relationship between body and mind. He was
German physicist and philosopher who used this in a
book (in German) named as ‘Elements of
Psychophysics’. Prior to his work
while working at Leipzig University,
Ernst Heinrich Weber proposed the
concept of JND based upon the senses
of touch and light which is commonly
referred as Weber’s law.

5. It states that the amount required to
produce JND is a constant proportion of the
value of the original stimulus or reference
intensity. The concept of modern Psychophysics
is based on the work Weber and Fechner.
In psychophysics the intensity of stimuli
that activate the five
senses are objectively
quantified.

6. The two important concepts of psychophysics: -
(i) Absolute threshold or limen (AL) – It is
also known as Detection Threshold. The
minimum amount, intensity, value or weight of
the stimulus to get noticed or perceived by
human sensory system. AL varies across
individuals and situations depending upon
various psycho-
physical and
situational
factors.

7. So to measure AL the number of trials
forms the base. When the stimulus is perceived
correctly in 50% of total trials than the
particular intensity or value of the stimulus is
referred as AL. Ear drum, and three tiny bones
(Ossicles)

8. (ii) Differential threshold of limen (DL) – It is also
known as Just Noticeable Difference (JND) or
Discrimination Threshold. The smallest difference in the
values of the two stimuli that helps in detecting the
difference between them. As AL, DL also measured
using number of trials i.e. the minimum amount of
change in the value of a stimulus which is capable
enough to be detected
correctly in 50% of
the trials.

9. Methods of Psychophysics
To measure the relationship between
intensity of stimulus and perception Fechner
developed following three methods. : -
(i) Methods of Limits
(ii) Method of
Constant Stimuli

10. (iii) Method of Adjustment
These methods are used to measure the
threshold of the intensity of the stimulus
required to perceive it which is known as
Absolute threshold or Absolute Limen (AL).

11. References:
1. NCERT, XI Psychology Text book.
2. Bruce. V., Green, P. R. & Georgeson, M. A. (1996). Visual
perception (3rd ed.). Psychology Press.
3. https://www.britannica.com/science/Webers-law.
4. https://en.wikipedia.org/wiki/Psychophysics.
5. https://www.britannica.com/science/psychophysics.

12. Next Discussion
Problems of
Psychophysics

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