Introduction - University of Texas at Dallasotoole/PSY_4362/Ch01_Lecture-f.ppt.pdf · Introduction 1 •Sensation and ... –Magnitude estimation (Stevens, 1950’s) •Signal Detection

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1Introduction

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• Sensation and perception

– The processes by which physical energyimpinging on the senses is converted intoelectrochemical energy by the senses, transmittedto the brain for adaptive interaction with theenvironment.

1 Introduction

• Early Philosophy of Perception

• Nativism and Empiricism

• Psychophysics

• Biology of Perception

1 Early Philosophy of Perception

• Plato’s “The Allegory of the Cave” (380 BCE)

1 Early Philosophy of Perception (cont’d)

• perceptual system as a filter

• What kind of filter?

– perception and thereby “sense of reality“ evolved:

• survival

• individual animals rely on different types ofenergy

• sensory ability “tailored to” the environment

• we are selective perceivers


• some species sense energies that humans cannot:

– bees see ultraviolet lights

– rattlesnakes sense infrared energy

– dogs and cats can sense sounds with higherfrequencies

– birds, turtles, and amphibians use magnetic fieldsto navigate

– elephants can hear very low-frequency sounds,which are used to communicate

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1 Some Animals are Able to Sense Stimuli that Humans Cannot 1 Early Philosophy of Perception (cont’d)

• Heraclitus (540–480 BCE): “You can never step intothe same river twice.”

– both world and perceiver are constantly changing

– idea that perceiver cannot perceive the sameevent in exactly the same manner each time


• Adaptation: A reduction in response caused by prioror continuing stimulation

– light over time ….

• sun to dark basement

• dark basement to sun


• Democritus (460–370 BC): The world is made up ofatoms that collide with one another, and thesensations caused by these make contact with oursense organs

– perception is the result of the physical interactionbetween the world and our bodies

– primary qualities and secondary qualities

• touch versus vision


• Sensory transducer:

– receptor that converts physical energy from theenvironment to neural activity

1 Nativism and Empiricism

• Approaches to perception

– nativism

• the mind produces ideas that are not derivedfrom external sources

– innate and created ideas

– empiricism

• experience from the senses is the only sourceof knowledge

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1 Nativism and Empiricism (cont’d)

• Descartes’ (1596–1650) dualist view of the world

– mind–body dualism: originated by Descartes, theidea positing the existence of two distinctprinciples of being in the universe: spirit/soul andmatter/body


• dualism

– mind and body exist “separably”

• monism

– mind and matter are formed from, or reducible to,a single ultimate substance or principle of being


• materialism

– physical matter is the only reality, and everythingincluding the mind can be explained in terms ofmatter and physical phenomena. Materialism is atype of monism.


• empiricism:

– experience from the senses is the only source ofknowledge


• Hobbes (1588–1678) believed that everything thatcould ever be known or even imagined had to belearned through the senses


• Locke (1632–1704) sought to explain how allthoughts, even complex ones, could be constructedfrom experience with a collection of sensations

– tabula rasa

• recovery from blindness

– What can you “see”?

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• Berkeley (1685–1753) studied waysin which perception is limited by theinformation available to us throughour eyes

– all of our knowledge aboutthe world comes fromexperience, even ifperception is limited

– “distance of objects”

–infer distances fromcues in the image

– “To be is to be perceived”

1 The Dawn of Psychophysics

• Fechner (1801–1887) invented “psychophysics,”founder of experimental psychology

– work relating changes in the physical world tochanges in our psychological experiences

1 The Dawn of Psychophysics (cont’d)

• psychophysics

– the science of defining quantitative relationshipsbetween physical and psychological (subjective)events


• Weber (1795–1878) discovered that the smallestchange in a stimulus, such as the weight of an object,that can be detected is a constant proportion of thestimulus level—“Weber’s Law”


• JND (Just Noticeable Difference): The smallestdetectable difference between two stimuli, or theminimum change in a stimulus that can be correctlyjudged as different from a reference stimulus. Alsoknown as difference threshold

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• How much weight must you add to a:

– tea cup ?

– 10lb bag of sugar?

• How much light must you add to a:

– flashlight in basement?

– flashlight in outdoor sunlight?

• Knock on door (with/without background)

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Weber Fraction

k = ΔI/Ik = constant

I = intensity of the standard

Example100 gram weight103 gram weight3 grams = JNDk = 3/100 = .03

True: yes if the standard is not too close to the threshold


• Fechner’s Law:

– A principle describing the relationship betweenstimulus magnitude and resulting sensationmagnitude such that the magnitude of subjectivesensation increases proportionally to the logarithmof the stimulus intensity

– S = k log R

• S = perceived stimulus intensity (sensation)

• R = stimulus intensity (reality)

1 Fechner’s Law

1Psychophysical Methods

1 Overview

• Classical Threshold Theory

– Absolute threshold (Fechner, 1860)

• Difference Threshold

– Weber (1834)

• Above Threshold

– Magnitude estimation (Stevens, 1950’s)

• Signal Detection Theory

– Green and Swets (1960’s)

1Quantitative

• Physical stimulus

– Detection -> absolute threshold

• Two physical stimuli

– Discrimination -> difference threshold

• Above detection/discrimination

– Psychophysical function

• magnitude of physical

• magnitude of the perceptual

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1Detection


• DETECTION

– absolute threshold

• minimum amount of stimulation necessary for aperson to detect a stimulus 50% of the time

– e.g.,

» how much intensity do you need todetect a light at 500µ

1Absolute Detection Threshold

• Questions– Least intense sound I can hear?

– Least intense light I can see?

– Smallest concentration of chemical I can taste?

1Assumption

• Intensity exists for each observer, above which theobserver detects the stimulus, below which, theobserver does not detect the stimulus

100

0

Physical intensity

% detect

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Methods for Determining Detection Threshold

• Method of limits

• Methods of constant stimuli

• Method of adjustment

Methods

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1Method of Limits

• 5-9 stimuli varying in intensity– Least never detected

– Most always detected

• present in ascending/descending order

• observer responds when he/she detects

• data - function

– Percent detection as f(stimulus intensity)

• threshold - arbitrary but often 50% or 75%

Method of Limits 1 Method of Limits

1Method of Limits

• Example use???

• Eye doctor….

• Potential problems?

1Method of Constant Stimuli

• 5-9 stimuli varying in intensity– Least never detected

– Most always detected

• present in random order

• observer reports which he/she detects

• data - function

– Percent detection as f(stimulus intensity)

Method of Common Stimuli

1 Method of Constant Stimuli 1Method of Adjustment

• observer adjusts an analog device that controlsstimulus intensity until he/she just detects stimulus…

• Multiple trials can be used

• Example

– Von Bekesy audiometer

Method of Adjustment

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1Evaluation

• Accuracy– constant stimuli

• Speed

– adjustment

1Discrimination


• DISCRIMINATION

– minimum difference at which two stimuli can bedistinguished

• examples

– two-point threshold

– two different frequencies

– two different colors

1Methods

• Discrimination threshold

• Reliably discriminate between two stimuli

100

0

Physical intensity change

% different

1Weber’s Weight Study

• Smallest difference in weight that can be reliablydiscriminated

• Just noticeable difference (JND)

• Weight difference that was discriminable depends onthe reference weight

1Above Threshold

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1Above Threshold

• Directly relating the physical and the psychological

– Magnitude Estimation Method

– Steven’s Power Law to describe findings

1Magnitude Estimation

• Present a standard stimulus

• Rate it as having an intensity of x

• Present a comparison stimulus

• How intense is it?

• Possible results….

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Physical Intensity

Psychological Intensity

Linear response

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Physical Intensity


Response Compression

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Physical Intensity


Response Expansion

1 Magnitude Estimation

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Steven’s Power Law

ψ = kµn

ψ = perceptionk = constant

µ = Physical stimulus intensity

n = 1 linearn > 1 response expansion

n < 1 response compression

1Signal Detection Theory

1What is wrong with Classical Absolute Threshold

Theory?

100

0

Physical intensity

% detect

100

0

Physical intensity

% detect

Subject A

Subject B

1Who is the better detector?

• Subject B detects stimulus intensities smaller thanthose detected by Subject A

• But…..

• The problem of guessing?????

1Decision Bias

• Can you trust a subject’s response?

• Not really…..

1Decision Bias Factors

• individual differences (in personality)

– liberal responders

– conservative responders

• task demands

– conservative

– liberal

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Response = f(detection ability and decision making)

We want to know detection ability independentof decision making factors (response bias)

1Method -> Practical

• Trick subject

• include null (“dummy” trials)

• measure bias directly

– the number of “dummy trials” the S falls for

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reality

response

stimulus no stimulus

“yes”

“no”

hit false alarm

misscorrectrejection

=100% =100%

=bias “yes”

=bias “no”

1 Things to remember

• reality adds up to 100%

• accuracy is on the diagonal

• bias is in the rows

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response

signal + noise noise

“yes”

“no”

hit false alarm

misscorrectrejection

d’ = z(hit rate) - z(false alarm rate)1

Signal Detection Theory

• Model of the detector and/or discriminator

– Green and Swets (1966)

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1Model Components

• Amorphous perceptual continuum

Perceptual Continuum

1Model Components

• Noise model - normally distributed

• Instantaneously varying….

• physiological?


N

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• Add constant strength signal to the noise


N N+S

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• Add constant strength BIG signal (gunshot)


N N+S

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• Add constant strength TINY signal (pin drop)


N N+S

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• Decision threshold β, or criterion C


N N+S

Say “yes”Say “no”

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1Model Components

• Decision threshold β


N N+S


Liberal responder

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• Decision threshold β


N N+S


Conservative responder

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hits

“hits” - yes to a stimulus event (S+N)

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False alarms

“false alarms” - yes to non-event (N)

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Correct rejections

“correct rejection” - no to a non-event (N)

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Misses

“misses” - no to a stimulus event (S+N)

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1Model Measures

Perceptual ContinuumSay “yes”Say “no”

d’ = z(hit rate) - z(false alarm rate)

d’

1Model Measures


C = -.5(z(hit rate) + z(false alarm rate))

d’

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d’ is the perceptual distance between themean of the noise and the signal+noise distributions….

Perceptual magnitude of the signal for the detector

1Most important thing

• A response bias free measure of the accuracy ofdetection

• Because you measure bias and parcel it out

Response = f(detection ability and decision making)

Response = f(d’ and C)

1How response data changes

with β shift


d’

hit and false alarm?A

100 60

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d’

hit and false alarm?B

90 40

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d’

hit and false alarm?C

80 10

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d’

hit and false alarm?D

60 0

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p(hit)

p(false alarm)

AB

C

D

Receiver Operating Characteristic (ROC) Curve

1 Receiver Operating Characteristics (ROC) 1Model

• Why this works…..

• Assumptions about normality….

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1Concrete Example

• Air port metal detectors…..

• Noise - non-gun-carrying passenger

• Signal - passengers with guns

• Detect

• Theory

• Threshold...

1DetectionExample 1

• Ice cream and low fat ice cream

• Experiment = 200 trials

– 100 with fat

– 100 without fat

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response

signal + noise noise

“yes”

“no”

hit60

false alarm10

miss40

correctrejection90

d’ = z(hit rate) - z(false alarm rate)1

Model Measures

Perceptual ContinuumSay “high fat”Say “low fat”

d’ = z(high fat rate) - z(low fat rate)

d’

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response

high fat low fat

“high fat”

“low fat”

hit60

false alarm10

miss40

correctrejection90

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• d’ = z(hit rate) - z (false alarm rate)

• d’ = z (.60) - z(.10)

• 1.53 = .25 -- 1.28

• d’ is in z-score units

• High d’?

• Low d’?

• Negative d’?

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Conclusions….

1Final Considerations

• Cautions

– Normality assumptions?

• Non-parametric versions (A’)

– Non-linearity in the extreme values

– corrections for these

1 Signal Detection Theory (Part 1) 1 Signal Detection Theory (Part 2)

1 Signal Detection Theory (Part 3) 1 Signal Detection Theory (Part 4)

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1Biology of Perception

1 Biology of Perception

• Doctrine of specific nerve energies

– (Müller, 1801–1858):

• nature of a sensation depends on whichsensory fibers are stimulated, not on how fibersare stimulated

1 Biology of Perception (cont’d)

• Cranial nerves: Twelve pairs of nerves that originatein the brain stem and reach the periphery throughopenings in the skull

– Pure sensory

• olfactory (I) nerves

• optic (II) nerves

• Auditory (vesitbular-cochlear) (VIII) nerves

– Eye movements

• Oculomotor (III)

• Trochlear (IV) nerves

• Abducens (VI) nerves

1 The Twelve Cranial Nerves


• Helmholtz (1821–1894): Studied activity of neurons;how fast they transmit signals

– Resonators, ophthalmoscope

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• Synapse:

– the junction between neurons that permitsinformation transfer


• Neurotransmitter: A chemical substance used inneuronal communication at synapses

1 The Action Potential

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Introduction - University of Texas at Dallasotoole/PSY_4362/Ch01_Lecture-f.ppt.pdf · Introduction 1 •Sensation and ... –Magnitude estimation (Stevens, 1950’s) •Signal Detection