Principles of Neural Organization

Lecture 3

KEYWORDS from Lecture 2

ACTION POTENTIALS 1 -- electrical stimulation (artificial depolarization)2 -- spatial and temporal integration of EPSPs and

IPSPs… Generator potential3 -- sensory stimulation (transduction), mechanical (cytoskeleton), chemical (receptors, second messengers), light (hyperpolarization)

• modality (Müller's doctrine of specific nerve energies 1826; labelled line); • intensity (APs/sec; frequency coding; population coding;

thresholds); • duration (rapidly and slowly adapting neurones) • location (absolute, two-point discrimination, topographical

coding)

KEYWORDS from Lecture 2 (cont’d)

Pacinian corpuscle, adequate stimulus, receptive fields,

thalamus, cortex, sulcus, gyrus, brainstem, topographic (maps) representation, superior colliculus, inferior colliculus (those are the names of the bumps on the brain stem that deal with vision and hearing respectively), Brodmann, phrenology,

areas of cortex: primary sensory areas(olfactory, somatosensory, visual, auditory), motor cortex, association cortices (parietal, inferotemporal, frontal)

BRAIN STEM

PARIETALFRONTAL

INFEROTEMPORAL

CEREBELLUM

Across pattern coding• can code more than one thing at the same time• can code ‘similarity’• 2 stimuli coded as two stimuli (if sufficiently different)• Good for coding patternsPopulation coding• only codes one thing• 2 stimuli --> smaller ignored • integration of activity means all neurones involved• Good for coding a single parameter such as directionSpecificity coding• can code more than one thing• 2 stimuli always coded as separate• each neurone acts alone (therefore vulnerable)• Good for coding patternsChannel coding• only codes one thing• 2 stimuli perceived as 1 (different from either alone - metamer)• Good for extracting a single parameter in the presence of

other potentially confusing factors.

Psychophysics

section 2

DETECTION THRESHOLDS

Section 1 method of limitsmethod of constant stimulimethod of adjustment

Section 2 signal detection theory

DISCRIMINATION THRESHOLDS

Section 3 Weber’s LawFechner’s LawSteven’s Power Law

PSYCHOPHYSICS

PRECISION vs ACCURACY

Method of limitsbias of expectationbias of habituationstaircase

Method of constant stimuli2AFC; 4AFC

Method of adjustmentrather variable“quick and dirty”

1. Multiple presentations

METHOD OF LIMITS

2. Staircase

PRECISION vs ACCURACY

Method of limitsbias of expectationbias of habituationstaircase

Method of constant stimuli2AFC; 4AFC

Method of adjustmentrather variable“quick and dirty”

perfect performance

chance performance

half way between

METHOD OF CONSTANT STIMULI

SIGNAL DETECTION THEORY

time

low firing rate high

pro

bab

ility

SPONTANEOUS ACTIVITY

low firing rate high

pro

bab

ility

GENTLE STIMULUS

time

low firing rate high

P

rob

abili

ty

SIGNAL+

NOISENOISE

“yes”“no”

CRITERION

low firing rate high

P

rob

abili

ty

“yes”“no”

medium stimulus

gentle stimulus

stronger stimulus

SIGNAL DETECTION THEORY

• sensory noise• criterion• stimulus magnitude• outcome matrix (hit/miss/false alarm/correct rejection)

RESPONSE

STIMULUS

“yes” “no”

present

absent

CORRECTHITS

CORRECT

MISS

FALSEALARM

100%

100%

RESPONSE

STIMULUS

“yes” “no”

present

absent

CORRECT

CORRECT

MISS

FALSEALARM

100%

100%

75% 25%

90%10%

SIGNAL DETECTION THEORY

• sensory noise• criterion• stimulus magnitude• outcome matrix (hit/miss/false alarm/correct rejection)• receiver operator characteristic curve (ROC)

percentage of false alarms

per

cen

tag

e o

f h

its

10%

75%

RECEIVER OPERATOR CHARACTERISTIC (ROC)

percentage of false alarms

per

cen

tag

e o

f h

its

more liberal

more conservative

RECEIVER OPERATOR CHARACTERISTIC (ROC)

SIGNAL DETECTION THEORY

• sensory noise• criterion• stimulus magnitude• outcome matrix (hit/miss/false alarm/correct rejection)• receiver operator characteristic curve (ROC)• from which we can measure your sensitivity

percentage of false alarms

per

cen

tag

e o

f h

its your sensitivity

RECEIVER OPERATOR CHARACTERISTIC (ROC)

DETECTION THRESHOLDS

Section 1 method of limitsmethod of constant stimulimethod of adjustment

Section 2 signal detection theory

DISCRIMINATION THRESHOLDS

Section 3 Weber’s LawFechner’s LawSteven’s Power Law

PSYCHOPHYSICS

DIFFERENCETHRESHOLDS

WEBER’S LAWΔI

Iis constant

change in stimulus magnitude

stimulus magnitude

The difference threshold

• just noticeable difference (jnd)• Weber’s law (1834)

the just noticeable increment is a constant fraction of the stimulus

Weber FractionsTaste 0.08 8%

Brightness 0.08 8% Loudness 0.05 5% Vibration 0.04 4% Line length 0.03 3% Heaviness 0.02 2% Electric shock 0.01 1%

• Fechner’s law (1860)sensation magnitude proportional to

logarithm (stimulus intensity)assumption: all jnd’s are the samestood for 100 years!

stimulus intensity

per

ceiv

ed m

agn

itu

de

1 jnd

2 jnd

4 jnd

5 jnd

6 jnd

3 jnd

Perceived magnitude

log (intensity)

The difference threshold

• just noticeable difference (jnd)• Weber’s law (1834)

the just noticeable increment is a constant fraction of the stimulus

• Fechner’s law (1860)sensation magnitude proportional to

logarithm (stimulus intensity)assumption: all jnd’s are the samestood for 100 years!

• Steven’s law (1961)(“To honour Fechner and repeal his law”)

sensation magnitude proportional to (stimulus intensity) raised to a power

…but Stevens noticed that not everything went like that!

Stevens introduced the idea of “magnitude estimation”

so this is the Weber-Fechner law…

Perceived magnitude

(intensity) h

Stevens’ Power Law

Response compression

Response expansion

power less than 1

power more than 1power equal to 1

Perceived magnitude (intensity) h

Perceived magnitude (intensity) h

Ernst Weber (1795-1878)

Increase in intensityIntensity = constant

Gustav Fechner (1801-1887)

Perceived magnitude

log (intensity)

S.S. Stevens (1906-1973)

Perceived magnitude

(intensity) h

Somatosensory System

section 3

Why?• Perception --- body parts

--- touch--- special -- vibrissae

antennaepainbrailletemperature

• Protection• Temperature regulation• signals (flushing/muscle arrangement)

somatosensory

How?• Receptors• Neural pathways• Neural codes(remember those ‘common features’…)

somatosensory

Coding in the somatosensory system

• detection

• identify modality (Müller's doctrine of specific nerve energies 1826; labelled lines);

• identify properties and spatial form

• magnitude intensity (APs/sec; frequency coding; population coding; thresholds);

• location (absolute, two-point discrimination, topographical coding)

• movement

GLABROUS (non-hairy) SKIN

MEISSNER’SCORPUSCLE(RA)

MERKEL’SDISK (SA)

RUFFINICORPUSCLE(SA)

PACINICORPUSCLE(very RA)

MERKEL’SDISK (SA)

Free nerveending

PACINICORPUSCLE(very RA)

Nerve endingaround hair(RA)

HAIRYSKIN

MEISSNER’SCORPUSCLE(RA)

RUFFINIENDING(SA)

SA

RA

RA

SA

RA SA very RA SA

Somatosensorypathway

Trigeminalsystem fromface

DORSAL COLUMNS

1st2nd

3rd

4th

CROSS OVER

Somatosensory Cortex

Afferent fibres

SA

RA

PC

Cortical cells in area 3b (SA)

Effect ofLateralinhibition

Lateral inhibition improves 2-point discrimination

POINT LOCALIZATIONTHRESHOLDS

PRESSURETHRESHOLDS