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Principles of Perceptual Measurement
 Introduction
 Scientific basis of perceptual measurement
 Classical psychophysics
 Modern psychophysics

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Introduction
 Can we measure the energy from physical

stimuli in our world?


Yes (e.g., measure the intensity of a light stimulus)  Can we measure someone’s psychological

experience of a physical stimulus?


No (e.g., do you see a red light as the same
‘red’ as another?)

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Scientific Basis of Perceptual Measurement
 Quantitative relationships and their benefits:


Provides an estimate of the perceptual quality of a stimulus in numerical terms





Allows it to be compared with other stimuli

Allows comparisons among different individuals and different species
Allows comparisons across different sensory modalities 4

Scientific Basis of Perceptual Measurement
 Is there a general relationship between a

physical stimulus and perception?


There are multiple possible functions that indicate an increase in our perception of a stimulus as the physical intensity of the stimulus increases

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Examples of these relationships - 1


Linear function:
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As stimulus intensity increases, perceived intensity increases by a constant amount across the range.

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Examples of these relationships - 2


Exponential function:
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As stimulus intensity increases, changes in perceived intensity increases slowly at low values but increases dramatically after a certain point.

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Examples of these relationships - 3
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Logarithmic function:
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As stimulus intensity increases, changes in perceived intensity increases dramatically at low values but increases much more slowly after a certain point.

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Examples of these relationships - 4
“Ogive” function – smoothly tapering function that has an upper and lower limit, and a quasi-linear region of change in the middle – very characteristic of perceptual functions at relatively high intensities quasi-linear region at relatively low intensities

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Approach to Perceptual Measurement
 Is there a general relationship between a

physical stimulus and perception?


General approach to obtaining the relationship between physical events and perceptual experience: 



Ask humans to rate the perceived intensity of a stimulus at various physical intensities.
Measure the smallest change in stimulus input that causes a just discriminable change in sensation. Classical Psychophysics :
Some Terminology

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 Absolute threshold:

Minimum intensity level of a stimulus before it is registered by the brain in the form of a perceptual event  Subthreshold:
 Stimuli below absolute threshold that are not detectable by the sensory system
 Suprathreshold:
 Stimuli above absolute threshold where sensation takes place
 Difference threshold:
 The smallest change in stimulus intensity required to produce a discriminable change in sensation


Classical Psychophysics :
Requirements
 To plot the function relating changes in

physical stimulus intensity with changes in perceptual intensity, it is necessary to have:


Absolute threshold:




Provides first data point for plotting function

Difference threshold:


Used to estimate the changes in slope at suprathreshold levels

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Classical Psychophysics :
History

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 Fechner (1860):
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Set out to establish a set of experimental methods to be used in psychophysics:
 The study of the quantitative relationships between physical events and psychological experience Believed in a general relationship between physical and perceptual qualities:
 Could be obtained knowing the absolute and difference thresholds Gustav Theodor Fechner
(1801–1887)

Classical Psychophysics:
How to do it 1

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 Psychophysical methods:
 Method of adjustment:
 Participant adjusts the physical