eeg generators
DESCRIPTION
This presentation discusses the basic principles governing EEG Rhythm Generation, and discusses the various circuits that generate and maintain cerebral oscillations.TRANSCRIPT
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Electroencephalogram, EEG generators
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EEG Physiological Basis
I
II/III
IV
V
VI
•Sources:• EPSPs• IPSPs
• Sign of Signal: • Cortico-Cortical inputs• Thalamo-cortical inputs
• Signal Summation• Synchrony• Direction
• Action Potentials?
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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
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Outline for the session
• History and Introduction
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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
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Outline for the session
• History and Introduction• Concept of field potentials and dipoles
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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.
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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.
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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.
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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
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Thalamo-Cortical fiber
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Equivalent Dipole
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The “equivalent dipole”
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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
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Synaptic Geometry
dipoles cancel dipoles reinforce
dipoles canceldipoles cancel
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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.
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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
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Outline for the session
• History and Introduction• Concept of field potentials and dipoles
• The net result !!
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The Electroencephalogram
Two ways of generating synchronicity:
a) pacemaker; b) mutual coordination
1600 oscillators (excitatory cells)
un-coordinated coordinated
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Normal EEG Rhythms
Alpha: 8-13 Hz
Beta: 14-30 Hz
Theta: 4-7 hz
Delta: <3.5
1 sec
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Outline for the session
• History and Introduction• Concept of field potentials and dipoles
• The net result !!
• Mechanisms of Synchronization
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THALAMUS
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REVA
A
M
VL
VP LP
LGN
MGN
Pu
G. prae-centralis
G. post-centralis
Gyri frontales
Gyri orbitales
Gyritemporales
Gyriparietales
Corpus striatum
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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‘
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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)
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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
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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.
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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.
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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
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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.
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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.
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Figure 3.15 Two Representations of Neural Circuitry (Part 2)
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Outline for the session
• History and Introduction• Concept of field potentials and dipoles
• The net result !!
• Mechanisms of Synchronization
• Modulation of Synchronized rhythms – Why ?
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Why do we need so many oscillations ?
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Fourier Transform: The inner product
Rodrigo Quian QuirogaSloan-Swartz Center for Theoretical Neurobiology
California Institute of Technologyhttp://www.vis.caltech.edu/~rodri
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Frequency spectrum of a normal EEG.
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Temporal spectra from frontal scalp
From Freeman et al. 2003
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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;
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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
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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
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To summarize…
• Alpha rhythm
• Spindles
• Delta rhythm
• Theta rhythm
• Beta, gamma rhythm
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