(i) Methods of Constant stimuli (ii) Method of Limits (iii) Method of Adjustment or Average Error

1. Classical Methods of
Psychophysics
Dr Rajesh Verma
Assistant Professor in Psychology
Govt. College Adampur, Hisar (Haryana)

2. Methods of Psychophysics
Fechner developed three tools to measure the
relationship between intensity of stimulus and
subsequent perception. These tools are called classical
methods of psychophysics and employed to measure the
stimulus threshold. The threshold has been defined as
the stimulus value which is perceptible in 50% of the
trials. The methods
are used to ascertain
the AL and DL of the
stimulus (Gescheider,
1997, p-45).

3. Methods of Psychophysics
The three methods are: -
(i) Methods of Constant
stimuli
(ii) Method of Limits
(iii) Method of
Adjustment or Average
Error

4. (i) Methods of Constant stimuli – In this method a
set of stimuli of which some are above the threshold and
some are below the threshold are used repeatedly
(Gescheider recommends of five and nine different
values) during the procedure. For example a subject is
made to hear a base sound of 60 db. Then individual
may be presented with five different levels of sounds (40
db, 80 db, 70 db, 30 db,
50 db) in random order
with equal increment.

5. From the recorded responses the percentage of
correct responses are calculated and plotted on a graph.
The values of stimulus intensity are plotted on the X
axis while the percentage of correct responses on Y axis.
A line joining the points
will be drawn and
subsequently AL is
calculated from the
graph. The method is
use to detect the
differential threshold
of JND.

6. (ii) Method of Limits – It means determining
the detection limit of a stimulus. In this method
both ascending or descending series of stimuli
can be used.
- Ascending Series means the presentation of
stimulus should start with low intensity and
gradually increasing with small equal
increments till it
is detected 50%
of total responses.

7. - Descending Series means the presentation of stimulus
should start with high intensity and gradually
decreasing the stimulus value in small equal increments
till it’s detection is perceived only in 50% of responses
or detection is negated
in 50% of responses.
This method is
generally used in
audiometry
(measurement of
hearing) (Gescheider,
1997, p-55).

8. In this method 3 variation techniques are used
(i) Up-&-down or Staircase – It is used to explore the
transition points (Points where subject reverses her/his
response). The average
of transition points is
said be as threshold.
Graph Courtesy: Manuel Kuehner (www.Bedienhaptik.de) [CC BY-SA
(https://creativecommons.org/licenses/by-sa/4.0)]

9. (ii) Threshold Tracking – First used by Bekesy
in 1947. In this variation technique the variation
in the intensity of stimulus is regulated by
subject. As long as subject keeps the button
pressed the stimulus will gradually decreases
and as long as the button is released the intensity
of stimulus will
increase. The
experiment can
be started either
with pressed or
released state of
button.

10. If started with pressed state then stimulus
will gradually decrease till it reaches a point of
disappearance. At this point the subject will
release the button and stimulus will start to
increase in its intensity
till the point where it
becomes detectable.
Again at this point
the button will be
depressed and so on
(Gescheider, 1997,
p-60).

11. (iii) Forced Choice – First used by Blackwell in
1953 (Vision) and Jones in 1956 (Taste and
Smell). It can be Temporal forced choice and
Spatial forced choice.
(a) Temporal Forced Choice – In temporal
forced choice the subject is presented with a set of
4 observations in a sequence. The subject is
required to tell
which observation
contains the
stimulus.

12. (b) Spatial Forced Choice – While in spatial
forced choice the information is presented to the
subject simultaneously. Usually, the subject is
presented with 4 quadrants out of which one
quadrant contains stimulus. Two successive
correct responses
is taken as
threshold of the
stimulus
(Gescheider,
1997, p-62).

13. (iv) Method of Adjustment – Also known as method
of average error. This method is basically used for
assessing the JND or Differential Threshold but also
can be employed for AL. In this method the subject
controls the intensity of stimulus. Initially the stimulus
is set to a very low or very high intensity, the subject
varies the intensity of
stimulus until it
becomes perceptible
or disappears
completely

14. The mean of settings suggested by the
subject for perception or disappearance is take
as AL. While assessing the DL the subject is
asked to adjust the intensity of stimulus until it
becomes differentiable. The mean of all stimulus
intensities set by the
subject is designated
as JND or
Differential
threshold.

15. Use of Classical Methods
These methods are used to measure
1. Absolute threshold or Absolute Limen (AL)
or Detection Threshold – The threshold of the
intensity of the stimulus required to perceive it.
2. Differential Threshold or Differential Limen
(DL) or JND –
The minimum
value required
to differentiate
between two
stimuli.

16. References:
1. Gescheider, G. A. (1997). Psychophysics: The
Fundamentals. Psychology Press.
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.

17. Next Discussion
Definition and Meaning of
Learning

18. Meet You Soon
With Next Video
vermasujit@yahoo.com
Thanks