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 potential 3 -- 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 PARIETAL FRONTAL 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 patterns Population coding only codes one thing 2 stimuli --> smaller ignored integration of activity means all neurones involved Good for coding a single parameter such as direction Specificity coding can code more than one thing 2 stimuli always coded as separate each neurone acts alone (therefore vulnerable) Good for coding patterns Channel 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 1method of limits method of constant stimuli method of adjustment Section 2signal detection theory DISCRIMINATION THRESHOLDS Section 3 Weber’s Law Fechner’s Law Steven’s Power Law PSYCHOPHYSICS

PRECISION vs ACCURACY Method of limits bias of expectation bias of habituation staircase Method of constant stimuli 2AFC; 4AFC Method of adjustment rather variable “quick and dirty” 1. Multiple presentations METHOD OF LIMITS 2. Staircase

PRECISION vs ACCURACY Method of limits bias of expectation bias of habituation staircase Method of constant stimuli 2AFC; 4AFC Method of adjustment rather variable “quick and dirty” perfect performance chance performance half way between METHOD OF CONSTANT STIMULI

SIGNAL DETECTION THEORY

time low firing rate high probability SPONTANEOUS ACTIVITY

low firing rate high probability GENTLE STIMULUS time

low firing rate high Probability SIGNAL + NOISE “yes” “no” CRITERION

low firing rate high Probability “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 CORRECT HITS CORRECT MISS FALSE ALARM 100%

RESPONSE STIMULUS “yes”“no” present absent CORRECT MISS FALSE ALARM 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 percentage of hits 10% 75% RECEIVER OPERATOR CHARACTERISTIC (ROC)

percentage of false alarms percentage of hits 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 percentage of hits your sensitivity RECEIVER OPERATOR CHARACTERISTIC (ROC)

DETECTION THRESHOLDS Section 1method of limits method of constant stimuli method of adjustment Section 2signal detection theory DISCRIMINATION THRESHOLDS Section 3 Weber’s Law Fechner’s Law Steven’s Power Law PSYCHOPHYSICS

DIFFERENCE THRESHOLDS WEBER’S LAW ΔIΔI I is 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 Fractions Taste0.08 8% Brightness0.088% Loudness0.055% Vibration0.044% Line length0.033% Heaviness0.022% Electric shock0.011% Fechner’s law (1860) sensation magnitude proportional to logarithm (stimulus intensity) assumption: all jnd’s are the same stood for 100 years!

stimulus intensity perceived magnitude 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 same stood 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 1 power equal to 1 Perceived magnitude  (intensity) h Perceived magnitude  (intensity) h

Ernst Weber ( ) Increase in intensity Intensity = constant

Gustav Fechner ( ) Perceived magnitude  log (intensity)

S.S. Stevens ( ) Perceived magnitude  (intensity) h

Somatosensory System section 3

Why? Perception--- body parts --- touch --- special --vibrissae antennae pain braille temperature 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’S CORPUSCLE (RA) MERKEL’S DISK (SA) RUFFINI CORPUSCLE (SA) PACINI CORPUSCLE (very RA)

MERKEL’S DISK (SA) Free nerve ending PACINI CORPUSCLE (very RA) Nerve ending around hair (RA) HAIRY SKIN MEISSNER’S CORPUSCLE (RA) RUFFINI ENDING (SA)

SA RA SA

RA SA very RA SA

Somatosensory pathway Trigeminal system from face DORSAL COLUMNS 1st 2nd 3rd 4th CROSS OVER

Somatosensory Cortex

Afferent fibres SA RA PC Cortical cells in area 3b (SA)

Effect of Lateral inhibition

Lateral inhibition improves 2-point discrimination

POINT LOCALIZATION THRESHOLDS

PRESSURE THRESHOLDS