EEG Generators

Electroencephalogram, EEG generators

EEG Physiological Basis

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II/III

IV

V

VI

•Sources:• EPSPs• IPSPs

• Sign of Signal: • Cortico-Cortical inputs• Thalamo-cortical inputs

• Signal Summation• Synchrony• Direction

• Action Potentials?

Outline for the session

• History and Introduction• Concept of field potentials and dipoles

• The net result !!

• Mechanisms of Synchronization

• Modulation of Synchronized rhythms – Why ?

• Summary


• History and Introduction

1875 - Richard Caton, a physician in England - Rabbit cortex electrical activity1890 - Adolph Beck of Poland

1929 - Hans Berger (1873 - 1941), an Austrian psychiatrist - The discoverer of human EEG (= ElectroEncephaloGram)- Alpha and Beta waves, eye closed and open, mental task- Sleep and Awake - Illness and EEG- Drug and EEG: Phenobarbital, morphine, cocaine- Telepathic transmission

Discovery of Brain Electrical Activity

EEG

Many neurons need to sum their activity in order to be detected by EEG electrodes. The timing of their activity is crucial. Synchronized neural activity produces larger signals.

Layers of the cerebral cortex

The 2 mm thick cortex can be divided into six distinct layers.

Each layer is distinguished both by the type of neurons that it contains and by the connections that it makes with other areas of the brain.

It is believed that the activation of the large pyramidal cells of layer V is what is reflected in most EEGs.

Pyramidal cells (layer V) are the primary output neurons of the cerebral cortex. Pyramidal neurons are glutamatergic and compose approximately 80% of the neurons of the cortex.

Cortical pyramidal neurons

They have a triangularly shaped cell body, a single apical dendrite extending towards the pial surface, multiple basal dendrites, and a single axon.

Pyramidal cells are grouped closely together and organized in the same orientation.

There are perhaps 105 neurons under a mm2 of cortical surface. EEG electrodes measure the space-averaged activity of >107 neurons.

A µvolt signal will only be detected if pyramidal neurons are synchronously activated and many small dipoles are combined.

EEGs reflect synchronous firing of pyramidal cells

Thalamo-Cortical fiber

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Equivalent Dipole

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The “equivalent dipole”

Note:- At a surface electrode, both positive and negative polarity may indicate depolarization (EPSPs) depending on the orientation of the dipole.

- EPSPs in superficial layers and IPSPs in deeper layers appear (at a surface electrode) as a negative potential.

Scalp EEG: positive polarity Scalp EEG: negative polarity

EPSP deep layer

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IPSP superficial layer

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EPSP superficial layer

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IPSP deep layer

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EPSP

IPSP

Synaptic Geometry

dipoles cancel dipoles reinforce

dipoles canceldipoles cancel

Possible dipole orientations

In a sphere (the head), dipoles can be radial (perpendicular to the surface of the skull), tangential (parallel to the surface of the skull)

or partially radial and partially tangential.

The activation of pyramidal neurons in layer V of the cortex that are oriented perpendicular to the surface of the skull will contribute to an EEG signal.

It takes approximately 100,000 adjacent neurons acting in temporal synchrony to produce a measurable change in electric field outside the head.

detectable electrical field

undetectable electrical field detectable

magnetic field

undetectable magnetic field

The Electroencephalogram

Two ways of generating synchronicity:

a) pacemaker; b) mutual coordination

1600 oscillators (excitatory cells)

un-coordinated coordinated

Normal EEG Rhythms

Alpha: 8-13 Hz

Beta: 14-30 Hz

Theta: 4-7 hz

Delta: <3.5

1 sec

THALAMUS

REVA

A

M

VL

VP LP

LGN

MGN

Pu

G. prae-centralis

G. post-centralis

Gyri frontales

Gyri orbitales

Gyritemporales

Gyriparietales

Corpus striatum

Th-cx

Th-cx

L-circ

L-circRE

RE

Aff

Dendro-dendr.

Cortex

Thalamo-cortical reentrant loops. Steriade, M. (1999). Coherent oscillations and

short-term plasticity in corticothalamic networks. TINS, Vol. 22 (8), 337-344.

Basic Circuitry:

Cortex

RE Dorsal Thal. = Relay Nuclei

‚Secondary neurons‘

Th-cx

Th-cx

L-circ

L-circ

RE

RE

Aff

Dendro-dendr.

Cortex

1,2 Afferent brainstem input to Th-cx (1), Activation of RE and Cortex (2)

Th-cx

Th-cx

L-circ

L-circ

RE

RE

Aff

Dendro-dendr.

Cortex

3 Excitatory processes in Cortex; Inhibition of primary L-circ; Inhibition of other RE cells

Th-cx

Th-cx

L-circ

L-circRE

RE

Aff

Cortex

The resulting effect is that during time 4, Th-cx are again under inhibitory control from L-circ neurons and, at the same time are activated from cortico-thalamic cells. Thus, only strong (converging and/or amplified) cortical feedback will trigger another excitatory activation wave into the cortex in time 5.

4 Excitatory feedback response from cortex. Disinhibition of primary L-circ neurons. Inhibition of secondary Th-cx neurons.

The strong inhibition of the secondary Th-cx

cell may lead to low threshold spikes

(LTS) and, thus, to a 10 Hz oscillation.

Th-cx

Th-cx

L-circ

L-circRE

RE

Aff

Cortex

5 The primary Th-cx cell may start a new excitatory burst into the cortex. At this stage (because released from the L-circ inhibition), a new afferent input will have a strong effect.

The secondary Th-cx cells remain under inhibition

RESULT: Center-surround ‚on-off‘ effect with a resulting strong focal activation of cortical target neurons.

Summary of findings: Afferent brainstem activation is missing and cortical activation is strong:

- Th-cx cells are hyperpolarized and oscillate with spindle frequency Note that a depol. current pulse during maximal hyperpol. leads to high frequency bursts. The result is increased oscillatory cortical activation leading to Delta activity.- The effect of increased cortical activation is even larger if stimulation patterns are oscillatory

Th-cxL-circ

RE

Missing brainstem afferents

Cortex Th-cx hyperpolarized,Sleep spindles

SLEEP: Spindles and Delta

Izhikevich Spiking Neuron Model (2003)

Claimed to be as realistic as Hodgkin-Huxley neurons.

As computationally efficient as simple integrate-and-fire.

Izhikevich, E. M. (2003). "Simple model of spiking neurons." IEEE Transactions on Neural Networks 14(6): 1569-1572.

Variables

v: Membrane potential.

u: Membrane Recovery variable, giving -ve feedback to v (higher value means neuron less likely to fire), representing Na+ and K+ ionic currents.

4 Parameters

c: Reset value for v; higher value means easier / more likely to fire again.

a: Speed of recovery of u; higher value means faster recovery.b: Sensitivity of u to v; higher values tightly couple the 2 variables resulting in possible sub-threshold oscillations and low threshold spiking.d: Additive reset value for u; higher value means harder / less likely for neuron to fire again.

Figure 3.15 Two Representations of Neural Circuitry (Part 2)

Why do we need so many oscillations ?

Fourier Transform: The inner product

Rodrigo Quian QuirogaSloan-Swartz Center for Theoretical Neurobiology

California Institute of Technologyhttp://www.vis.caltech.edu/~rodri

Frequency spectrum of a normal EEG.

Temporal spectra from frontal scalp

From Freeman et al. 2003

Roles of Neuronal Oscillation

• Memory processes are most closely related to theta and gamma oscillations;

- Lawrence M. Ward, TRENDS in Cognitive Sciences Vol.7 No.12 December 2003, 553-559

Neuronal Oscillations represent a dynamical interplay between cellular and synaptic mechanisms.

• Conscious awareness may arise from synchronous neural oscillations occurring globally throughout the brain;

• Gamma wave and epileptics seizure.

• Attention seems closely associated with alpha and gamma oscillations;

Rhythmic activity appears to play a major role in information processing in the brain

Object recognition

Feature extraction/abstraction

Associative learning

Selective attention

Novelty detection






• Summary

To summarize…

• Alpha rhythm

• Spindles

• Delta rhythm

• Theta rhythm

• Beta, gamma rhythm

EEG Generators

Health & Medicine

Transcript of EEG Generators