Electroencephalogram EEG and neural synchrony

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Electroencephalogram (EEG) and neural synchrony
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• Since nervous excitation is accompanied by action
  potentials (currents),
• Galvani, 1791 Du Bois-Reymond, 1848
• Then maybe brain activity also gives rise to
  electrical potentials
• Human brain Berger, 1924-29 Adrian Matthews,
  1934
• Why the 70-year gap ?

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1). EEG potentials are very small

• Battery for my alarm clock 1.5 Volts
• Neuron action potentials
• Muscle action potentials
  60-100 mV
• Cardiac action potentials
• EEG 10-50 µV
• EEG potentials are 1000-10000 times
  smaller !!!!!!

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2). World War I
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Surviving college

• Terminology
• Identifying building blocks
• Trick is to try to be able to explain as much as
  possible by memorizing as little as possible
• In other words use your brain after all its how
  you got here in the first place!

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Review atoms

• What is net charge on an atom?
• ZERO

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Review Ions in solution

• What is net charge in solution?
• ZERO

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In order to be able to measure a charge

• Must do work (i.e. use energy) to separate
  positive charges from negative ones
• Neurons
• Semi-impermeable membrane
• Ion pumps
• How does a separation of charges arise in the
  brain?
• Hint Its not simply because the brain is
  composed of neurons

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Brain Some useful facts

• Cortex
• 1.5-4.5 mm thick
• 2200 cm2 (2.5 sq. feet)
• Two-thirds in sulci
• 1010-1011neurons
• 75 are excitatory

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Important organizational features of cortex

• Horizontal lamination
• Vertical columnation

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Cyto-architectural map of the human cerebral
cortex

• Brodmann areas

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Inputs-outputs to different layers are different
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EEG is generated by large pyramidal-shaped
neurons in layers II, III, IV

• Important features
• Oriented vertical to the cortical surface
• The inhibitory and excitatory inputs are
  spatially segregated over the surface of these
  neurons
• Soma only inhibitory inputs
• Dendrites excitatory and inhibitory inputs
  (excitatoryinhibitory6.51)
• Inhibitory inputs to dendrites and soma not
  generally the same

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How charge separation arises in a cortical neuron?

• Assume that excitation increased in dendrites
• OTHER CHOICES ARE POSSIBLE
• Generators are the excitatory and inhibitory
  post-synaptic potentials
• NOT ACTION POTENTIALS!!
• Convention Current flow is the direction that
  the positive ions flow

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Potential produced by a single neuron is too
small to measure by an electrode on the scalp

• Since the EEG-important neurons are vertically
  arranged, they can summate
• Hence EEG is generated by a huge population of
  neurons
• Cortical area required to produce a potential
  measured by a scalp electrode is 6 cm2
• There are 105 neurons per 0.008 cm2
• So the measured signal is from 108 neurons !!!

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EEG is a measure of neuronal synchrony

• Time averaged potential Pt
• If m dipoles oscillate in synchronization
• If m dipoles oscillate non-synchronously

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EEG and cortical anatomy

• The vertical-parallel arrangement of cortical
  pyramidal neurons is not always present
• Certain areas of brain, e.g. amygdala
• As a result of disease, e.g. developmental,
  infectious
• How would this affect the EEG?
• The dipoles will tend to cancel each other out
  and hence the potential will approach zero for a
  large enough neuronal population

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Dilemma posed by EEG for neuroscientists

• Focus of neuroscientists and computational
  neuroscientists is on action potentials
• EEG is the best measure of ongoing activity of
  the brain
• Does not measure action potentials
• Newer techniques, e.g. functional MRI and PET,
• Have poor temporal resolution
• Do not measure action potentials

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What does the EEG measure?
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Current flow through wires versus aqueous
solutions

• Wires ? negatively charged
• electrons move towards the positive
  charges
• Aqueous solutions ? the ions do the
  flowing

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How do we know that brain generates the EEG?

• Current flow follows the path of least resistance
• Thus expect that current over skull defect will
  be greater than in a person with no skull defect

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Adrian and Matthews (1934)

• Observation 1 alpha current density above skull
  defect is higher than in person with no defect
• If alpha is generated outside the skull, this
  would not be observed
• Observation 2 Less alpha current flows into
  frontal regions
• If alpha is generated by eye muscles, this effect
  would not occur