Coherence and Comodulation: Phase Synchrony and Magnitude Synchrony David A' Kaiser, Ph'D' StermanKa

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Coherence and Comodulation Phase Synchrony and
Magnitude SynchronyDavid A. Kaiser,
Ph.D.Sterman-Kaiser Imaging Laboratory, Inc.

• ISNR 16th Annual Conference - San Antonio, Texas
• August 28-September 1, 2008
• Saturday Aug 30 910-930am

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Anatomy is destiny FreudAnatomy is merely a
suggestion Anatomy
Functional connectivity and neuroplasticity
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Neural recruitment into larger functional groups

• Neurons fire around 80 times a second
  intrinsically (and up to 800 times a second
  during seizure).
• To process information of relevance to the
  organism, autorhythmicity is greatly suppressed
  and firing synchronized across neurons by means
  of inhibitory and excitatory influences.

(Hopfield, 1999 Goldensohn Purpura, 1963
Mountcastle, 1957 Casanova Tillquist, 2008)
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When autorhythmicity is suppressed in 2,000,000
cortical minicolumns (6 cm2), it can be detected
by scalp electrodes.
Mountcastle, 1957 1978 Cooper et al., 1965
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Voltage rhythms correspond well with mental and
physical behaviors
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High information STATE Low
information
Certain rhythms are generated by inhibitory
networks
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Generation of spindles (7-14 Hz)

• Length of inhibitory potential sets the frequency
  (which is mediated by GABA type A receptors) .
  The potential determines the time until another
  burst of spikes is generated by the TC neuron
  (Franks, 2008)

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The more neurons recruited into a rhythm, the
higher the spectral magnitude

• Spectral magnitude proportion of neurons in the
  functional group (rhythm)

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Time delay between brain areas recruited into the
same function (rhythm) is indicated by phase
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Detecting networks through timing and number

• Network organizes around event

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Synchrony between sites as indicated by phase
and magnitude relationships
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Phase and Magnitude consistency
Cross-spectral analysis Coherence is a phase
consistency function Comodulation is a
magnitude consistency function
between signals at a frequency across time Coh
average normalized cross-spectrum
amplitude Comod average normalized
cross-product amplitude
Coh ranges from 0.0 to 1.0 Comod ranges
from -1.0 to 1.0
Comodulation
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Shared information between EEG signals

• Magnitude
• Mean consistency (comodulation)
• Mean difference (asymmetry, unity)

• Phase
• Mean consistency (coherence)
• Mean difference (phase lag)

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Functional Connectivity from 5 to 35 years of age
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Four possible connectivity parameters
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Normalizing with Fisher z-transform (1921)
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Similarity of Coh and Comod

• Kaiser, 2008 (n43 children, 58 adults)

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(Kaiser, 2008)n 101

• Sowell ER, Peterson BS, Thompson PM, Welcome SE,
  Henkenius AL, Toga AW (2003). Mapping cortical
  change across the human life span. Nature
  Neuroscience, 6, 309-15.
• We used magnetic resonance imaging and cortical
  matching algorithms to map gray matter density
  (GMD) in 176 normal individuals ranging in age
  from 7 to 87 years. We found a significant,
  nonlinear decline in GMD with age, which was most
  rapid between 7 and about 60 years, over dorsal
  frontal and parietal association cortices on both
  the lateral and interhemispheric surfaces. Age
  effects were inverted in the left posterior
  temporal region, where GMD gain continued up to
  age 30 and then rapidly declined. The trajectory
  of maturational and aging effects varied
  considerably over the cortex. Visual, auditory
  and limbic cortices, which are known to myelinate
  early, showed a more linear pattern of aging than
  the frontal and parietal neocortices, which
  continue myelination into adulthood. Our findings
  also indicate that the posterior temporal
  cortices, primarily in the left hemisphere, which
  typically support language functions, have a more
  protracted course of maturation than any other
  cortical region.

Left posterior temporal lobe has longest
maturation (Sowell et al., 2003)
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Effect of age on connectivity

• Coherence increase with age (5-35 y, n101)

• Comodulation increases with age

Data are site-age correlations. (pink is
significant)
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Functional connectivity in childhood(5-20 years
of age)

• Structural changes
• Functional changes

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Functional connectivity in adulthood (20-35
years of age)

• Structural changes
• Functional changes

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College students show frontal plasticity compared
to older adults
Coherence
Comodulation
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Role of myelin in cerebral connectivity

• Without myelin sheath, 2 mph With
  sheath, 260 mph

Biggest 5 micron diameter pipes are posterior
but big pipes continue frontally throughout life
Corpus callosum cross-section
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Functional connectivity in adulthood (20-35
years of age)

• Red areas are last to myelinate
• Functional changes

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Global connectivity (alpha graphed)
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Scalp coherence may reflect RTN involvement in
cortical rhythms and comodulation the more
loosely organized corticocortical networks
Does phase and magnitude capture different
aspects of neurophysiology?
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Spectral parameters c.1994

• Absolute power
• Power asymmetry (A-B)
• Power ratio (A/B)
• Relative power
• Spectral entropy
• Spectral Correlation Coefficient (SCC)

• Coherence
• Phase lag
• Bicoherence
• Spectral Correlation

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Same spectral parameters, organized
Bicoherence
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Periodicity Table
(Kaiser, in press)
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Chemistry between periodicity types