EEG Generators

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Information about EEG Generators
Health & Medicine

Published on February 5, 2014

Author: drrahulkumarsingh

Source: slideshare.net

Description

This presentation discusses the basic principles governing EEG Rhythm Generation, and discusses the various circuits that generate and maintain cerebral oscillations.

Electroencephalogram, EEG generators

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?

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

Outline for the session • History and Introduction

Discovery of Brain Electrical Activity 1875 - Richard Caton, a physician in England - Rabbit cortex electrical activity 1890 - 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

Outline for the session • History and Introduction • Concept of field potentials and dipoles

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.

Cortical pyramidal neurons 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. 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.

EEGs reflect synchronous firing of pyramidal cells 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.

The “equivalent dipole” + - + + + + + + + + + + + + + -- + - Equivalent Dipole Thalamo-Cortical fiber

Scalp EEG: positive polarity EPSP IPSP + EPSP deep layer + - EPSP superficial layer + + - - IPSP superficial layer Scalp EEG: negative polarity - + - IPSP deep layer - + 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.

Synaptic Geometry dipoles cancel dipoles reinforce dipoles cancel dipoles 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.

detectable electrical field undetectable electrical field undetectable magnetic field detectable magnetic field 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.

Outline for the session • History and Introduction • Concept of field potentials and dipoles • The net result !!

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

Outline for the session • History and Introduction • Concept of field potentials and dipoles • The net result !! • Mechanisms of Synchronization

THALAMUS

Gyri orbitales Gyri frontales Co rp u G. praecentralis G. postcentralis RE Gyri temporales VA VL VP LP Pu LGN MGN Gyri parietales A M ss tri atu m

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 Cortex Dorsal Thal. = Relay Nuclei RE L-circ Th-cx Dendro-dendr. Th-cx RE L-circ Aff ‚Secondary neurons‘

1,2 Afferent brainstem input to Th-cx (1), Activation of RE and Cortex (2) Cortex L-circ RE Th-cx Dendro-dendr. Th-cx RE L-circ Aff

3 Excitatory processes in Cortex; Inhibition of primary L-circ; Inhibition of other RE cells Cortex L-circ RE Th-cx Dendro-dendr. Th-cx RE L-circ Aff

4 Excitatory feedback response from cortex. Disinhibition of primary L-circ neurons. Inhibition of secondary Th-cx neurons. 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. Cortex The strong inhibition of the secondary Th-cx cell may lead to low threshold spikes (LTS) and, thus, to a 10 Hz oscillation. L-circ RE Th-cx Th-cx RE L-circ Aff

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. Cortex RE L-circ Th-cx Th-cx RE L-circ Aff

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 SLEEP: Spindles and Delta RE Cortex Th-cx hyperpolarized, Sleep spindles L-circ Th-cx Missing brainstem afferents

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)

Outline for the session • History and Introduction • Concept of field potentials and dipoles • The net result !! • Mechanisms of Synchronization • Modulation of Synchronized rhythms – Why ?

Why do we need so many oscillations ?

Fourier Transform: The inner product Rodrigo Quian Quiroga Sloan-Swartz Center for Theoretical Neurobiology California Institute of Technology http://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; • Attention seems closely associated with alpha and gamma oscillations; • Conscious awareness may arise from synchronous neural oscillations occurring globally throughout the brain; • Gamma wave and epileptics seizure. Neuronal Oscillations represent a dynamical interplay between cellular and synaptic mechanisms. - Lawrence M. Ward, TRENDS in Cognitive Sciences Vol.7 No.12 December 2003, 553-559

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

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

To summarize… • Alpha rhythm • Spindles • Delta rhythm • Theta rhythm • Beta, gamma rhythm

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