Principles of Neural Organization

Lecture 2

Electrode, Microelectrode, Micron (1/1000th mm),membrane, nucleus, cytoplasm, Neuron, axon, dendrite, Schwann cell/glial cell, myelin sheath, node of Ranvier, Synapse, synaptic cleft, vesicle, neurotransmitter,receptors, ions, permeability, ion channels, voltage-dependent sodium channels, neural threshold, positive feedback, sodium (Na+), potassium (K+), sodium-potassium pump, electrochemical equilibrium potentials, sodium (Na+) +55mv, potassium (K+) -75mv, resting potential -70mv, polarization/ depolarization/ hyperpolarization, inhibitory post-synaptic potential (IPSP), Excitatory post-synaptic potential (EPSP),integration, axon hillock, action potential (AP), all-or-none, neuron threshold -55mv, saltatory propagation, AP propagation

KEYWORDS from Lecture 1

1 -- electrical stimulation (artificial depolarization)

2 -- spatial and temporal integration of EPSPs and IPSPs across the neurone’s

membrane resulting in the neuronal threshold being reached.• Generator potential

3 -- sensory stimulation (transduction)• mechanical (cytoskeleton)• chemical (receptors, second messengers)• light (hyperpolarization)

WHEN DO CELLS PRODUCE ACTION POTENTIALS?

• MODALITY• labelled lines• specific nerve energies

• INTENSITY• proportional to frequency• population (recruitment)

• DURATION• rapidly adapting (RA)• slowly adapting (SA)

• LOCATION• locate a site• distinguish two sites• mapping

Principles of Neural Coding

Johannes Műller 1826

DOCTRINE OF SPECIFIC NERVE ENERGIES

“regardless of how a receptor is stimulated it produces only one kind of sensory experience”

MODALITY

“vision”

“hearing”

“touch”

“labeled lines”

Intensity of stimulus

Res

po

nse

of

cell

threshold

INTENSITY

recruitment of additional cells with higher thresholds

DURATION

Introduction to structure of central nervous system...

Cortex

BRAIN STEM

PARIETALFRONTAL

INFEROTEMPORAL

CEREBELLUM

Somatosensory Cortex

Motor Cortex

Common structure of the senses

•Receptive fields•sense organ -> cortex via THALAMUS•heirarchy and parallel systems•topographic arrangement

OF A SINGLE PHOTORECEPTORTHE VISUAL RECEPTIVE FIELD

a single rod

The Visual Receptive Fieldof a single photoreceptor.Light outside this regionwill have no effect on thiscell.

screen

(A) Across-fibre pattern coding

One pattern codes one thing (eg. red)....

.. and another pattern codes something else (eg. blue).

(C) Specificity coding

One cell codes one thing (eg. red)....

.. and another cell codes something else (eg. blue).

Attribute coded by relative firing

(D) Channel coding

Attribute distributionof activity

(B) Population coding

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.

SUMMARY

Properties of neurones, action potentials, synapses

Transduction in sensory cells

Coding of modality, intensity, duration, location

Overview of structure of the brain and some of its maps

Common structure of the different senses

Coding mechanisms within the senses

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

Precisionhigh

Precisionlow

Accuracyhigh

Accuracylow

bias

bias

Method of limitsbias of expectationbias of habituationstaircase

Method of constant stimuli2AFC; 4AFC

Method of adjustmentrather variable“quick and dirty”

Figure 1.12 The results of an experiment to determine the threshold using the method of limits. The dashed lines indicate the crossover point for each sequence of stimuli. The threshold - the average of the crossover values - is 98.5 in this experiment.

METHODOF

LIMITS

Figure 1.13 Results of a hypothetical experiment in which the threshold for seeing a light is measured by the method of constant stimuli. The threshold - the intensity at which the light is seen on half of its presentations - is 180 in this experiment.

METHODOF

CONSTANTSTIMULI

SIGNAL DETECTION THEORY

• response bias• sensory noise• criterion• outcome matrix (hit/miss/false alarm/correct rejection)• receiver operating characteristic curves (ROC)• sensitivity (d’ or d prime)

RESPONSE

STIMULUS

“present” “absent”

present

absent

CORRECT

CORRECT

MISS

FALSEALARM

100%

100%

percentage of false alarms

per

cen

tag

e o

f h

its

more liberal

more conservative

Figure 1.14 The difference threshold (DL). (a) The person can detect the difference between a 100-gram standard weight and a 102-gram weight but cannot detect a smaller difference, so the DL is 2 grams. With a 200-gram standard weight, the comparison weight must be 204 grams before the person can detect the difference, so the DL is 4 grams. The Weber fraction, which is the ratio of DL to the weight of the standard is constant.

DIFFERENCETHRESHOLDS

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!

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

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

Consequences of Steven’s Law

• response compression• response expansion• linear on a log scale

Response compression

Response expansion