PSYCHOPHYSICSNavdeep Batth

LecturerD.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 Limits2.Method of Constant Stimuli3.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 non-

dimensional .• 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 DATAStimulus

(Hz)D A D A D A D A D A

33 + +32 + +31 + + +30 + + + +29 + + + +28 + + + + +27 + + + + +26 - + + + + + + +25 - + - - + - +24 - + - + - -23 - + + - + -22 - + - - + - +21 - - - - - - -20 - - - - -19 - - - - -18 - - -17 - -

• 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 DATAStimulus (Hz)

D A D A D A D A

25 + +

24 + + +

23 + + + + +

22 + = + = +

21 = = = + = + = +

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+ + T-

2 = 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

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