classical methods of psychophysics (Three methods)

(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
Fechnerdevelopedthreetoolstomeasurethe
relationshipbetweenintensityofstimulusand
subsequentperception.Thesetoolsarecalledclassical
methodsofpsychophysicsandemployedtomeasurethe
stimulusthreshold.Thethresholdhasbeendefinedas
thestimulusvaluewhichisperceptiblein50%ofthe
trials.Themethods
areusedtoascertain
theALandDLofthe
stimulus(Gescheider,
1997,p-45).

3. Methods of Psychophysics
Thethreemethodsare:-
(i)MethodsofConstant
stimuli
(ii)MethodofLimits
(iii)Methodof
AdjustmentorAverage
Error

4. (i)MethodsofConstantstimuli–Inthismethoda
setofstimuliofwhichsomeareabovethethresholdand
somearebelowthethresholdareusedrepeatedly
(Gescheiderrecommendsoffiveandninedifferent
values)duringtheprocedure.Forexampleasubjectis
madetohearabasesoundof60db.Thenindividual
maybepresentedwithfivedifferentlevelsofsounds(40
db,80db,70db,30db,
50db)inrandomorder
withequalincrement.

5. Fromtherecordedresponsesthepercentageof
correctresponsesarecalculatedandplottedonagraph.
ThevaluesofstimulusintensityareplottedontheX
axiswhilethepercentageofcorrectresponsesonYaxis.
Alinejoiningthepoints
willbedrawnand
subsequentlyALis
calculatedfromthe
graph.Themethodis
usetodetectthe
differentialthreshold
ofJND.

6. (ii)MethodofLimits–Itmeansdetermining
thedetectionlimitofastimulus.Inthismethod
bothascendingordescendingseriesofstimuli
canbeused.
-AscendingSeriesmeansthepresentationof
stimulusshouldstartwithlowintensityand
graduallyincreasingwithsmallequal
incrementstillit
isdetected50%
oftotalresponses.

7. -DescendingSeriesmeansthepresentationofstimulus
shouldstartwithhighintensityandgradually
decreasingthestimulusvalueinsmallequalincrements
tillit’sdetectionisperceivedonlyin50%ofresponses
ordetectionisnegated
in50%ofresponses.
Thismethodis
generallyusedin
audiometry
(measurementof
hearing)(Gescheider,
1997,p-55).

8. Inthismethod3variationtechniquesareused
(i)Up-&-downorStaircase–Itisusedtoexplorethe
transitionpoints(Pointswheresubjectreversesher/his
response).Theaverage
oftransitionpointsis
saidbeasthreshold.
GraphCourtesy:ManuelKuehner(www.Bedienhaptik.de)[CCBY-SA
(https://creativecommons.org/licenses/by-sa/4.0)]

9. (ii)ThresholdTracking–FirstusedbyBekesy
in1947.Inthisvariationtechniquethevariation
intheintensityofstimulusisregulatedby
subject.Aslongassubjectkeepsthebutton
pressedthestimuluswillgraduallydecreases
andaslongasthebuttonisreleasedtheintensity
ofstimuluswill
increase.The
experimentcan
bestartedeither
withpressedor
releasedstateof
button.

10. Ifstartedwithpressedstatethenstimulus
willgraduallydecreasetillitreachesapointof
disappearance.Atthispointthesubjectwill
releasethebuttonandstimuluswillstartto
increaseinitsintensity
tillthepointwhereit
becomesdetectable.
Againatthispoint
thebuttonwillbe
depressedandsoon
(Gescheider,1997,
p-60).

11. (iii)ForcedChoice–FirstusedbyBlackwellin
1953(Vision)andJonesin1956(Tasteand
Smell).ItcanbeTemporalforcedchoiceand
Spatialforcedchoice.
(a)TemporalForcedChoice–Intemporal
forcedchoicethesubjectispresentedwithasetof
4observationsinasequence.Thesubjectis
requiredtotell
whichobservation
containsthe
stimulus.

12. (b)SpatialForcedChoice–Whileinspatial
forcedchoicetheinformationispresentedtothe
subjectsimultaneously.Usually,thesubjectis
presentedwith4quadrantsoutofwhichone
quadrantcontainsstimulus.Twosuccessive
correctresponses
istakenas
thresholdofthe
stimulus
(Gescheider,
1997,p-62).

13. (iv)MethodofAdjustment–Alsoknownasmethod
ofaverageerror.Thismethodisbasicallyusedfor
assessingtheJNDorDifferentialThresholdbutalso
canbeemployedforAL.Inthismethodthesubject
controlstheintensityofstimulus.Initiallythestimulus
issettoaveryloworveryhighintensity,thesubject
variestheintensityof
stimulusuntilit
becomesperceptible
or disappears
completely

14. Themeanofsettingssuggestedbythe
subjectforperceptionordisappearanceistake
asAL.WhileassessingtheDLthesubjectis
askedtoadjusttheintensityofstimulusuntilit
becomesdifferentiable.Themeanofallstimulus
intensitiessetbythe
subjectisdesignated
asJNDor
Differential
threshold.

15. Use of Classical Methods
Thesemethodsareusedtomeasure
1.AbsolutethresholdorAbsoluteLimen(AL)
orDetectionThreshold–Thethresholdofthe
intensityofthestimulusrequiredtoperceiveit.
2.DifferentialThresholdorDifferentialLimen
(DL)orJND–
Theminimum
valuerequired
todifferentiate
betweentwo
stimuli.

16. References:
1.Gescheider,G.A.(1997).Psychophysics:The
Fundamentals.PsychologyPress.
2.Bruce.V.,Green,P.R.&Georgeson,M.A.(1996).
Visualperception(3rded.).PsychologyPress.
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
[email protected]
Thanks