On behalf of my workshop at Kathmandu, I am writing few chapters here. I am grateful to Professor Sunity Shreshtha Hadda for her interest in my lecture.
Chapter 1: Introduction to Psychophysics
The Key issue in Operation research is optimization of resources to bring maximum
benefit and minimize the cost. This is done by changing physical attributes of
the product. Knowledge about psychophysics helps the operation research
scientist to select which attribute of the product will be changed so that the
change is perceptible, reliable and satisfying to the customers.
Change perception
Each product has
multidimensional physical attributes like weight, height, width, depth, color,
duration etc. The attributes are sensed by different sensory organs like visual
(eye), auditory, (ear), olfactory (nose),
gustatory (tongue) and cutaneous (skin) sensory organs. Besides the above five, kinesthetic sensation
(muscular sensation) also helps to sense change in product attribute like
weight. Each sensory organ has different threshold of perception and in
different units. Wavelength of color and
decibel of sound are the common units of threshold for perceiving change in
visual and auditory organs.
 |
| Figure 1.1 Change perception |
Threshold
Threshold is the limit wherein 50% of total trials one can
sense the change in product attribute.
Figure
1.1 shows that when stimulus intensity is 25 units, there is 50% probability to perceive the change in intensity level. Below
25 units, change in stimulus intensity cannot be perceived and after 25, change
is perceptible. Here 25 unit is
threshold.
In the above example, we have seen two scales – physical and
psychological. Stimulus intensity is measured by physical scale (X-axis) and
probability of perception is assessed by psychological scale (y-axis).
Psychophysics is the study of science that investigates quantitative relationship
between changes in physical events or
attributes and changes in corresponding psychological events. It helps us in
determining thresholds of perception. Study of threshold is used in designing torch light, bulb, car horns,
product weight, supersonic planes etc. There are three thresholds or limens –
absolute, differential and terminal.
Absolute threshold :
It is the lower limit of stimulus intensity, below which
presence of stimulus can not be experienced. For example, one can not see light
when it is far away as intensity of light is very low. Similarly, tone can not be perceived when sound intensity is below the decibel of absolute
threshold.
Terminal threshold
It is the upper limit of stimulus intensity, above which presence of stimulus can not be
experienced. For example, sound of supersonic
plane can not be sensed when it moves over the head. Terminal threshold
is just opposite of absolute threshold. Absolute is the lower limit and
terminal is the upper limit of stimulus intensity. The event occurs after differential
threshold. Below the terminal threshold, presence of stimulus can be
experienced.
Figure 1.2 Location
of Absolute and Terminal thresholds
Figure 1.2 shows two scales in both lower and upper
extremes. R scale is psychological and S scale is stimulus scales. In R-scale,
‘- - - - ‘ indicates no experience in stimulus. At the lower extreme, where in
‘- - - - -‘ ends, absolute threshold lies. Similarly at the upper level,
wherein ‘- - - - -‘ starts terminal threshold lies. In the S-scale, there is no
‘- - - - -‘ indicating continuous presence of stimulus. ‘>’ indicates
gradual increase in stimulus intensity.
Differential threshold
It is the limit of stimulus intensity, below which change in
stimulus intensity can not be experienced. Like absolute it is not the lower
limit of stimulus intensity. It is found after the absolute threshold. With
gradual change in stimulus intensity after absolute threshold, one event comes wherein change in stimulus
intensity can not be perceived 50% of total trials. This limit is called
differential threshold. It is the limit of just noticeable difference (jnd).
Figure 1.3 The three
thresholds. S indicates stimulus and R
indicates response continua
In Figure 1.3, the distance between S20 and R20 is
symmetrical. This indicates subject experiences change in stimulus intensity at
same level. But the distance between S40 and S44 is relatively higher than the
same between R40 and R44. The limit wherein subject can just discriminate
change in stimulus level is differential threshold.
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Chapter 2: Research Design
Research design of psychophysics can be understood with
following formula:
R = f ( O, S, T)
Formula suggests subject’s response to the change in
stimulus attribute is the function bof interaction among three things – Organism (O), stimulus value (S) and trials
(T).
Response: It
varies with stimulus modalities, the psychophysical methods followed. Classical
psychophysics follows categorical response scale. For example, ‘I can not see’
or ‘I can see’ or ‘I do not find difference’. Responses are symbolized for
recording.
RESPONSES
|
SYMBOLS
|
I can not see/ I can not feel/ I can not hear
|
-
|
I can see / I can feel/ I can hear
|
+
|
I do not find difference
|
=
|
Uncertainty
|
?
|
Organism : It may be any animal. But here organism is
person to whom the stimulus will be administered for judgment. Administration
and judgment depend on physical , psychological and demography characteristics
of subject. Physical characteristics are acquity of sensory organs, brain maturity, and neural functions. Psychological characteristics are ability
to discriminate and to integrate sensory experiences, perceptual organization, learning,
memorizing, reasoning for comparative judgments, motivation , interest, temperament and
personality patterns. Demographic characteristics are educational level,
socio-economic status.
Stimulus value: It is the unit of stimulus or product
attribute. For kinesthetic, it is the weight, for visual, it is wavelength of
color, for hearing, it is the decibel and for the skin, it is temperature.
Product attribute varies with stimulus characteristics and sensory modalities. Height, width, length, color are the
characteristics for visual experiencing the product. Tone, pitch, loudness are for auditory
experiences. Heat, cold, rough, smoothness are the characteristics when one
experiences product with skin or cutaneous sensory organs. One experiences food
characteristics through tasting some characteristics as sweet, sour, bitter,
salted. Another type of product characteristics are durability, reliability, usability and aesthetic. But
these are not very important in research on classical psychophysics.
Trials: Trials
are of ascending and descending in nature. Usually, administration of stimulus
follows descending order. For descending, stimulus value starts from the upper
side of base line level and for ascending stimulus value starts from lower side
of the base level.
Classical psychophysics is done within laboratory so that independent
variable can be administered systematically by controlling all the intervening
variables. When O is asked to judge the weight discrimination, he or she will
not be allowed to look at the weight and it will be done in the noise proof
room so that no sound can divert his attention. One checklist will be
administered before starting the experiment to know the current status of O.
Instruction should be standardized and be understood by the O.
Aim of
psychophysics is determination of thresholds. It can be done with three methods
as method of minimal changes, method of frequency and method of average error. In method of minimal changes stimulus are
presented systematically with minimum changes in stimulus value. Systematic
means first trial is descending and the next trial is ascending in order. But
in frequency method, stimulus values are random. For average error, O is
administered the stimulus in both non and random variation. The three methods
are discussed in the next chapter.
