Localizing source of a given field

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Localizing source of a given field
From butler.cc.tut.fi/malmivuo/bem/eegmeg
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Current is measured as a difference in electrical
potential, in a grounded system
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• TWO MONTAGE FORMS
• Two channels
• Bipolar Compare with other pairings to infer
  likely source
• Monopolar Treat one channel or linked channels
  as reference (inactive).
• Linked ears, mastoids, etc
• Multiple channels
• Average eliminate commonalities by subtracting
  average contributions of all channels
• Laplacian triangulate source by field dynamics
  with
• 2d geometry (Hjorth)
• 3d spheroid geometry (Laplacian Spherical
  Harmonic Expansion LORETA)
• 3d neurogeometry (Brodmann solution)

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Which montage to use?
1 such as linked ears OK
unless widespread
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Necessity of Laplacian transform for focal
activity
Incorrect interpretation of RgtL frontal medial
activation in referential montage
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Energies erupting at the surface
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Triangulation of source(s) with multiple
electrodes
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Laplacian is good for focal sources
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Brain energies may be multiple and distributed
across regions, rarely focal
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Laplacian is fooled by diffuse or distributed
sources
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Effect of Laplacian on Network Properties
Energies must be localized for accurate
corrections, etc.
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Need for Laplacian in network measures
Incorrect interpretation of increased shared
activity during pink site (1) silence with
referential montage
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• Were finding that Laplacian SHE transform may
  obscure networks identified through correlational
  techniques (coh, comod) so another means to
  localize energies must be found to identify
  coherence and comodulation accurately.
• Right now, Hjorth laplacian may be a good
  compromise. If not, use Referentialfor comod and
  coh. Still use Laplacian for phase and unity.

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98 of EEG energy is below 40 HzKeep in mind for
artifacts
SKIL 3 database 1 42 Hz
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Power vs Magnitude

• Power
• Magn

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Natural Log Magnitude

• Ln Natural log (base e, not base 10)
• e 1 1/1! 1/2! 1/3! 2.718

Nonlinear measures misbehave
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Why analyze periodicities instead of
voltage?Spectra appears to reflect neural coding
associated with mental events such as the
desynchronization-behavior relationship
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Why comodulation (etc) are performed on magnitude
(mV) instead of power (mV2) in SKIL

• Dietsch (1932) analyzed EEG using discrete
  Fourier transforms (1831).
• Fast Fourier Transform (FFT) algorithm invented
  (Cooley Tukey, 1965) , allowing practical
  spectral applications
• Dumermuth Fluhler (1967) applied FFT to EEG
• Why assume brain rhythms-to-mental activity
  corresponds with a power function? Science
  demands conservative linear assumption until
  proven otherwise.
• Scientific question, not theoretical

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• Motor quiescence rhythm (SMR and Sleep Spindle)
  mature by 3 months of age

• Sensory quiescence rhythm (Alpha) requires 10
  years or so to mature to adult frequency

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Deciphering Gross Neural codingBrainwave
frequencies and approximate mentation

• 0.1 2 Hz DELTA Damage in awake brain
• 2 4 Hz DELTA Cortical free-running
• isolation
• 4-7 Hz THETA Hyperpolarized
  thalamocortical
• engagement, and
  others
• 8-12 Hz ALPHA Sensory quiescence,
  preparatory recruitment
• 12-15 Hz SMR Motoric quiescence over
  
• motor strip sleep
  spindle
• 12 Hz BETA Active external attention,
• Perhaps serve
  information
• regulation in
  moderately
• sized networks

 
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Eyes Closed Eyes Open
Rest Baselines
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Activation and thalamocortical network
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Steriade et al
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Brain rhythm training, particularly SMR, is
entropy manipulation

• We have to reduce entropy (chaotic neural
  chatter) before a system can respond effectively
• High entropy - all background, all is not you
• Low entropy all is you

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Inhibitory networks produce population rhythmicity
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Low information High informationWell-prepared
to respond Poorly-prepared to respond
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Idling rhythm ? responsive quiescence

• Actively turned off so easier to recruit
  neurons in such states than active processing
  states

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When is theta alpha? Dominant frequency
determination
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Dominant Frequency
4-8 Hz
8-12 Hz
Healthy Adult
Healthy Child
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1 Hz Brain Maps
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Topographic MapsBetter focal localization with
laplacian derivations
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Macrostate model of investigating brain function

• Mental operations of rest and challenges produce
  unique neurophysiological states.
• Neurophysiological states may be identified by
  their distinct, reliable EEG patterns (Gevins,
  1984 1986)
• Energy needs of these macrostates may be measured
  by QEEG and fMRI-etc modalities

Normal rest and abnormal response to challenge
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Support of macrostate concept
Replications Across challenge types
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Topographic specificity occurs during challenge

• Moving joystick Watching movie

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Laterality differences

• Most of us are left-brained but movies balance us

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Women listen, Men watch
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Obstacles or opportunities for neurometric
assessment

• TRAIT
• Gender
• Handedness
• Age
• Education
• Lifespan experience
• Neurological history
• Bilingual

• STATE
• Task competence
• Task strategies
• Time of Day
• Drugs
• Sleep debt
• Recent experience
• Clinical Diagnosis

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Challenges constrain, more likelyforce brain
into specified macrostate
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Neurometric Analysis(Comparing someone to a QEEG
Database to identify statistical abnormality,
commonly gt 2 Std deviations, plus or minus)

• Baseline conditions
• Eyes closed
• Eyes open
• Motor control
• Stimulus control
• Task conditions
• Reading
• Math
• Problem solving

Abnormality not seen in this ADHD child until
stressed
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Functional model for dominant frequency
...suggests distributed generator Complex
recruitment Coordination Feedback system -
Modulators Corticothalamic projections Slow,
diffuse, weak

...suggests focal generator (pacemaker) Primitive
recruitment Synchronization Fastforward system -
Drivers Thalamocortical projections Fast, focal,
strong
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Hyperconnectivity with (right) cingulate in
Autistic children- indicates impaired mirror
neuron system?
Subcortical hyperconnectivity may reduce
cortico-cortical connectivity
E.g., Mizuno et al. (2006). Partially enhanced
thalamocortical functional connectivity in
autism. Brain Research, 1104 160-74.
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Reduced right inhibition of motor system in ADHD
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Adult Child Databases as of Sep 2007