Sensation & Perception

Lecture 2: Psychophysical MethodsAndy Clark

September 29, 2004

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Review…

• Large percentage of brain (subcortical/cortical) devoted to perceptual processing

• Perception is (re) construction of physical reality

• Instances in which perception does not correspond to reality (illusions) provide useful insight into structure of system

How to Study?

Phenomenology

• Philosophical Movement– Study of structures of experience

(consciousness)– OBSERVATIONAL– SUBJECTIVE– LACKS FIRM MEASURE

How to Study Empirically?

Psychophysics

• Study of the relationships between physical energy and sensory capabilities

– Weber (1834)– Fechner (1860)

After Descartes, 1644

Attended Stimulus

TransductionAction

Perception

Processingrecognition

Stimulus on

receptorsEnvironmental

Stimulus

B

C

A

Levels of Analysis

• A (stimulus-perception)– Psychophysics

• Classical methods, Magnitude Estimation, TSD

• B (stimulus-processing)– Physiology (extra- & intracellular recordings)

• C (physiology-perception)– fMRI, awake-behaving monkeys

Classical Psychophysical Methods

• Grew out of research into Absolute Detection

• Absolute Detection-description of sensory events in terms of the minimum amount of stimulus energy required to elicit them– i.e. dimmest visible light– ‘sensory threshold’

Classical Psychophysical Theory

• Sensory Threshold– Herbart (1824) critical boundary, above which

neural activity signals the presence of a sensory event –ABSOLUTE

Psychophysical Theory

• Threshold is a statistical concept, affected by:– Chance variation in the nervous system– Intensity of signal– Observer’s criterion

Psychophysical Theory

Method of Limits

• Example: Visual Detection Thresholds– Begin experiment by showing subject a dim

light• Slowly raise luminance until subject indicates

detection– Start over by initially displaying bright light

• Slowly lower luminance until subjects can’t detect

Method of Limits

* *

Start

Start

N

N

N

Y

N

Y

Y

Y

Etc. etc…

Method of Limits

• Threshold=Mean of ‘crossover’ values96 Y Y95 Y Y

94 Y Y Y93 N N Y

92 N Y N91 N N

90 N N

Method of Adjustment

• Subject able to manipulate value of stimulus parameter of interest (brightness in our example)

• Lower/Raise until just barely detectable– Repeat # of times, average values to estimate

threshold

Problems ?

• Yes Hysteresis

– “path dependence”

– Ex. Method of Limits– Threshold value will differ dependent upon

whether experimenter started with: dimbright

brightdim

Method of Constant Stimuli

• Create stimulus set a priori

• Value of parameter of interest varies slightly throughout set

Method of Constant Stimuli

• Display stimuli in random order• For each trial subject indicates their

perception (i.e. yes/no for detection case)• Plot subjects responses as probability of

detection versus stimulus value• Threshold = value for which subject detects

stimulus on 50% of the trials

Method of Constant Stimuli

0

1

.5

% “

Yes

” re

spon

ses

luminance

Method of Constant StimuliPsychometric function

% “

Yes

” re

spon

ses

luminance0

1

Extract 2 measures:

1. Absolute Threshold

Method of Constant StimuliPsychometric function

% “

Yes

” re

spon

ses

luminance0

1

Extract 2 measures:

2. “Sensitivity” (slope)

Sensitivity

• Weber (1834)– Measure the smallest detectable change in

stimulus energy• Just Noticeable Difference (JND)

– Worked with discrimination of lifted weights– Studied relationship between JND for intensity

and base intensity level

Sensitivity

• Weber’s Law• JND=S*K or JND/S=K

– Where S=value of standard, K=Weber’s constant, JND=just noticeable difference

• Increases in intensity that are just noticeably different to an observer are constant fraction of stimulus intensity

• Holds for suprathreshold stimuli

Sensitivity

• Ex. Brightness

% Y

es R

espo

nses

(Q=S

timul

i Diff

eren

t?)

Brightness

Magnitude Estimation

• Observer rates stimuli in relation to some stimulus standard

• Ex. Brightness• Observer rates standard light with

Brightness of 10• Successive estimations proportional to

standard light twice as bright – 20half –5 etc. etc.

Magnitude EstimationLine length

Brightness

Shock

•Response compression

•Response expansion

Magnitude Estimation

• Steven’s Power Law

P=perceived magnitude, K=constant, S=stimulus intensity

When estimates plotted on log/log scale, functions become linear, exponent determines the slope

* nP K S