PSYCHOPHYSICS
Navdeep Batth
Lecturer
D.A.V College For Girls
Yamuna Nagar
INTRODUCTION
• In tracing family tree of psychology one
discovers its major roots among group of
measuring techniques called psychophysical
methods.
• Gustav Fechner founded these methods.
• In his classic “elements der psychophysick”
(1860) he defined these methods.
DEFINITION
• Psychophysics is the exact science of
functional relation or relations of dependency
between body and mind.
• After Fechner J.P. Guilford (1956) in his book
“psychometeric methods” mentioned that
psychophysics is regarded as the science that
invesigates the quantitative relationship
between physical events and corresponding
psychological events.
• In a broad sense it can be said that much of
psychology in psychophysics depend on how
response depends upon stimulus.
• Other psychologists also studied and defined
psychophysics.
• Eysenck (1992) psychophysics concerns the manner
in which living organisms respond to the energetic
configuration of the environment.
CLASSICAL PSYCHOPHYSICS
• The classical psychophysics of Fechner, Muller
and Wundt was concerned mainly with the
determination of sensory threshold or limens.
• Number of psychophysical methods were
developed.
• Three methods explored by Fechner gained
particular importance.
• These methods are -:
1. Method of Limits
2. Method of Constant Stimuli
3. Method of Average Error
THE PSYCHOPHYSICAL
CONTINUA
• In psychophysical determinants e.g. finding
limens two types of continua are used a
physical continuum and psychological
continuum.
• Continuum means a closely graded series one
step merging imperceptibly into the other
forming a straight line signifying changes in a
single direction.
• Physical continuum – it is measureable in
physical limits and represents a single change
in some physical property like frequency of a
sound wave, amplitude of a sound wave,
weight in grams, length of a line, energy level
stimulus and so on.
• Psychological continuum – corresponding to
physical values are certain well recognized
aspects of sensory experience
• such as pitch, loudness, pressure, perceived visual
length and brightness of light.
• Physical values belong to stimulus continuum.
• Psychological values belong to response continuum.
• When we have applied numerical values to these
continua, we may call them as scales.
• Continuum have been demonstrated
diagrammatically -:
PROBLEMS OF
PSYCHOPHYSICS
• The problems of psychophysics are related to
the selective capacity of the organism his
ability to respond in different ways to different
stimuli.
• Some of the problems of psychophysics are.
• THRESHOLD –
• In ordinary language threshold refers to
boundary separating two specified regions or
areas.
• In psychophysics too it refers to a boundary wall on
the stimulus continuum separating the stimuli that
elicit one response from the stimuli that elicit other
response.
• The threshold varies from subject to subject and
even within same subject at different occasions.
• There are mainly three types of threshold values.
• Absolute threshold
• German word reiz limen (RL) is used to denote
absolute threshold.
• It is the value of the stimulus above zero at
which a sensation takes place and below
which one experiences no sensation.
• It is the value of a quantitative variable at
which a stimulus is just detectable (Eysenck,
1973)
• Hence reiz limen is therefore defined as the
value of a stimulus that is noticeable in 50%
observations and not noticeable in remaining
50% of observations.
• Persons differ in respect of value of R.
• Lower the threshold more sensitive is the
subject and vice versa.
• Differential Limen – According to D’Amato “
differential limen is the minimum amount of stimulus
change required to produce a sensation difference.”
• The point at which the subject reports the difference
in judgement determines the subject’s differential
threshold.
• This is also called just noticeable difference.
• It is denoted by JND
• It represents the smallest difference between
two stimuli which is just noticeable by the
subject.
• In order subject detect the change between
two stimuli is the experimenter has to
increase or decrease them by one JND unit.
• The value of JND to be increased depends on
discrimination power of the subject.
• Terminal Threshold –
• Our sense organs operate efficiently between
certain range of stimulus intensity.
• Individual cannot feel the presence of a
stimulus below the physical continuum of the
stimulus threshold.
• There is upper limit above which some stimuli
are not perceived by the individual.
• The upper threshold is called terminal
threshold.
• STIMULUS EQUALITY –
• If two similar stimuli are present under
somewhat different conditions then they
appear as different in psychological
continuum.
• This type of problem exist in all sense modality.
• ORDER DETERMINATION –
• When one is asked to arrange some stimuli in a
particular order with regard to some characteristics.
• The person does so not only on basis of that
characteristic.
• His personal feelings, past experiences, likes and
dislikes towards these stimuli influence order
determination.
• EQUALITY OF INTERVALS –
• This problem is much availed in psychological
measurement and evaluation.
• For measurement one has to determine equal
intervals to get valid result for perception and
sensation.
• When certain distractions are added to our
judgements as in case of Muller Lyer Illussion.
• Since the psychological dimension does not
parallel the physical dimension.
• A unit change in physical stimulus at different
places along its extensive range.
• This would not elicit equal differing
psychological experiences.
• Equal Ratio –
• It is concerned with the quantification of
experiences and responses.
• It refers to breakdown of a psychological
dimension into equal phenomenal units.
• Equal ratio is established throughout a
dimension .
• They are related to a physical stimulus change.
• STIMULUS RATING –
• range of characteristics influence the
judgment whenever judgment is made about
the stimulus.
• Just like to determine an object as pleasant or
unpleasant .
• Pleasantness and unpleasantness are not nondimensional .
• They are judged to vary in degrees.
METHOD OF LIMITS
•
•
•
•
This method is also called
Method of minimal changes
Method of serial exploration
Method of just noticeable difference
DETERMINATION OF AL
BY METHOD OF LIMITS
• AL is the minimum value of the stimulus that
will evoke a sensation.
• Determination of individual’s AL for pitch,
sensation is determined largely by the
frequency of a tonal stimulus.
• Variations are made in frequency of the tone
while intensity of the tone is held constant.
• Subject’s AL is roughly located by rapid initial
testing.
• Assumed to be somewhere near 25 Hz
• A range of stimulus is then chosen on either
side of the value.
• Higher frequencies are above the threshold
and lower values are below threshold.
• Stimuli are chosen so that they are separated
by a constant and relatively small difference in
frequency.
• A set of frequencies covering range of 17 to 33
Hz in steps of 1 Hz is selected.
PRESENTATION OF
STIMULUS
• The stimuli are presented to subject in
increasing and decreasing order.
• Some control measures are followed like
• half of the series is descending and half is
ascending.
• The starting point of each series is varies.
HYPOTHETICAL DATA
Stimulus
(Hz)
D
33
32
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
+
+
+
+
+
+
+
-
A
+
-
D
+
+
+
+
+
+
+
+
+
+
+
+
-
A
D
+
+
+
+
+
+
-
A
+
-
D
+
+
+
+
+
+
+
-
A
+
-
D
A
+
+
+
+
+
+
+
+
-
• The stimulus presented first is tone and S
responds that tonal character i.e. pitch is
present.
• A ‘+’ sign is entered on the datasheet in
appropriate column.
• Then next stimulus in the series is presented
and subject’s response is secured.
• This procedure is repeated till the subject
reports that the stimulus has lost its tonal
quality.
• A sign of ‘–’ is entered on the datasheet.
• This is transition point and here the series is
terminated.
• The AL is assumed to be midway between the
two values of the stimuli over which the
response reversal occurred.
CALCULATIONS
• Mean is calculated of descending series
TD = 26.5+21.5+25.5+21.5+24.5
5
= 119.5
5
= 23.9 Hz
TA = 25.5+22.5+25.5+25.5+21.5
5
= 120.5
5
= 24.1 Hz
• AL =Mean of ascending trials + Mean of descending trials
2
= 23.9+24.1
2
= 48
2
=24
CONSTANT ERRORS
 ERROR OF HABITUATION –
 Subject makes same response large number of
times before arriving at the threshold.
 Subject might become accustomed to making
the response .
 Response might be carried beyond the point
where reversal in sensation actually occours.
 On descending series error of habituation
tends to produce too small a value of T
• In ascending series it yields too high a value of
T.
• The negative difference between mean of
descending and ascending series implies error
of habituation.
• ERROR OF ANTICIPATION –
• Subject is aware sooner or later direction of
his sensation must change , he might
anticipate the change.
• The value of T is overestimated on descending series.
• Value of T is underestimated on ascending series.
• If the difference TD– TA is positive it implies
dominance of error of anticipation.
• These errors are referred to as constant errors.
• If unchecked they apt to bias the AL i.e. produce too
large or too small an absolute limen.
• It is perhaps apparent that if the same
stimulus were always used to begin D series
subject might be influenced by the fact that
on all such series positive responses intervene
between the series and the threshold.
• Such errors are controlled by randomization.
• The staring point of each series is randomized.
•
•
•
•
OTHER RELEVANT VARIABLES
Stimulus duration
Time between trials of series
Inter series interval
They are however controlled by holding
them constant.
DETERMINATION OF DL
BY METHOD OF LIMITS
• Difference limen is the minimum amount of
stimulus change required to produce a
sensation difference.
• In determining DL two stimuli are presented
to subject on every trial.
• One is standard stimulus (SS) and other is
variable stimulus (SV).
• Subject is to judge whether the magnitude of
the sensation evoked by variable stimulus is
greater than, less than or equal to that of
standard stimulus.
• When subject changes the judgment from greater (+)
to equal to (=) transition value (T+) is located midway
between these two judgments.
• It is upper threshold.
• When judgment is shifted from equal to (=) to less
than (-) another transition value (T-) is located.
• It is lower threshold.
• Participants DL lies between these two thresholds
DETERMINATION OF DL
FOR AUDIBLE PITCH
• Determination of standard stimulus and range of
variable values .
• Set of variable stimulus are taken .
• The variables should meet the following criterion –
a) All variables should always be perceivable.
b) The difference in the value of adjacent stimulus
should be “minimal” i.e. not easily discriminable.
c) The variable should include stimulus value that are
clearly higher and lower than standard stimulus.
• Determination of order of presentation to
control relevant variable (Rv)
• Counterbalancing technique is used both for
series direction and for the sequence
arrangement of standard and variable
stimulus.
• Total eight series / trials i.e. four ascending
and four descending.
• Series is arranged in double DADA sequence.
• In first four trials standard stimulus is
presented first on each trial.
• In last four trials variable stimulus is presented
first.
• Starting point is changed for each trial.
• Actual presentation of stimulus • The experiment starts with value of variable
well above standard.
• Then with minimal change in values of
variable stimulus the participant reports
variable to be greater than (+) than standard
stimulus.
• When participant reports the change in
judgment from greater (+) to equal (=).
• Upper threshold is determined by taking mid
value of these two values.
• Similarly lower threshold is determined when
participant changes the judgment from Equal
to (=) to less than (-) .
• Ascending series is started with value well
below the standard.
• Judgment is taken for all the eight trials/
series.
HYPOTHETICAL DATA
Stimul
us (Hz)
D
A
25
+
24
+
23
+
+
22
+
21
D
A
D
A
D
A
+
+
+
+
+
+
=
+
=
+
=
=
=
+
=
+
=
+
20
=
-
=
=
-
=
=
=
19
-
-
-
-
=
-
=
-
18
-
-
17
-
-
16
-
-
T+
21.5
22.5
21.5
20.5
22.5
20.5
21.5
20.5
T-
19.5
20.5
19.5
19.5
20.5
18.5
19.5
18.5
• Calculation of DL
• On each series two thresholds called upper
threshold (T+) and lower threshold (T-) are
calculated.
• Difference between upper and lower
threshold and lower threshold is called
“interval of uncertainty”
• IU = (T+) -(T-)
= UT-LT
= 21.38 – 19.5
= 1.88 Hz
• DL = IU
2
= 1.88 = 0.94 Hz
2
• It indicates that we must add atleast 0.94 Hz
frequency to the standard stimulus.
• So that subject can discriminate between
standard and variable stimulus.
• Point of subjective equality – it is the point
where the participant perceives variable
stimulus to be equal to standard stimulus.
• PSE =T+ + T2
= 21.38+ 19.5
2
= 20.44 Hz
• CE = PSE-Std
= 20.44 – 20
= .44 Hz
• When PSE is positive the standard stimulus is
overestimated.
• When PSE is negative the standard stimulus is
underestimated.
EVALUATION
• It has been used primarily for the
measurement of threshold.
• It is generally applicable where the stimulus
aeries is variable in small equal steps.
• It has been used to find intensity thresholds
for sounds, odour, taste,
colours,temperature,light and tactual
sensations.
CRITICISM
• There are certain errors like error of
habituation and anticipation.
• Error of fatigue and practice can also affect.
• These errors are minimized by order and
arrangement of the series.
• The stimulus error is partially not fully
eliminated.
• Size of step between stimulus
• Limiting DL is not identical with DL calculated
with other methods.
• Too much depends on terminal judgements
THANK YOU