Comodulation and Coherence in Normal and Clinical Populations

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Comodulation and Coherence in Normal and Clinical
Populations
Chagall Birthday
David A Kaiser, Ph.D. Rochester Institute of
Technology
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Defining my terms

• Raw EEGs are voltages across time
• In time domain, we estimate
• amplitude (positive and negative values)
• at a sample rate (only positive)
• In frequency domain, we estimate
• magnitude (only positive)
• phase (positive and negative)
• at a frequency
• Spectral power is magnitude squared

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How similar are two signals?

• Cross-correlation reveals similarities in time
  between signals. (e.g., Barlow, 1951 Brazier
  Casby, 1951)
• Cross-spectral analysis reveals similarities in
  frequency next slide

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How similar are two signals?

• Cross-spectral analysis reveals similarities in
  frequency.
• Signals may be similar in phase, magnitude, or
  both
• phase analysis coherence (Goodman, 1957
  Walter, 1961)
• magnitude analysis comodulation (Pearson, 1896
  Kaiser, 1994)

For example, does cortical activity become more
or less similar after treatment?
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Cross-spectral analysis
Coherence estimates phase consistency
Comodulation estimates magnitude consistency
between signals at each specified
frequency across time Coh average normalized
cross-spectrum amplitude2 Comod average
normalized cross-product amplitude
Coh range from 0.0 to 1.0 Comod range
from -1.0 to 1.0 Confusing point Tukey called
coherency the square root of coherence
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Comodulation was invented to examine low spatial
resolution concerns of EEG topography (e.g.,
volume conduction, Nunez, 1990)
Does surface EEG reflect cortical potentials
well? - if not, all neighbors will be equally
correlated with each other - if so, correlations
will be stronger within functionally-related areas
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Signals are

• comodulated if their magnitude relationship
  is stable

• coherent if their phase relationship is
  stable

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• Coherence analysis provides
• Coherence (Coh)
• Phase delay (/-180o)
• Comodulation analysis provides
• Comodulation (Comod)
• Proportion Site 1/Site 2

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Functional model for dominant frequency
...suggests common response Multiple networks
(related but dissimilar) organize neural
activity

...suggests common generator Single network
organizes neural activity
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• Comodulated but not coherent
• Pacemaker network partly segregated by cortical
  feedback
• Complex recruitment
• Coordination

• Coherent but not comodulated
• Pacemaker network unified
• Primitive recruitment
• Synchronization

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• Feedback system
• Modulators
• Corticothalamic projections
• Slow, diffuse, weak
• Sustained consciousness (i.e., self-)

• Fastforward system
• Drivers
• Thalamocortical projections
• Fast, focal, strong
• (Momentary) Consciousness

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Why comodulation analysis is performed on
magnitude (mV) and not on power (mV2)

• Brief history of power spectral analysis in EEG
• Dietsch (1932) analyzed 7 EEG signals using
  Fourier (1831).
• Cooley Tukey (1965) invented FFT algorithm,
  reducing computer workload, allowing practical
  spectral applications
• Dumermuth Fluhler (1967) applied FFT to EEG
• BUT ...

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• Why assume brain rhythms and mental activity are
  related by a power function? Are changes in brain
  behavior actually associated with larger changes
  in mental behavior (i.e. reason for using the
  power spectrum)?
• Might brain and mind activity be more linearly
  related at this level of investigation
  (i.e., reason for using the magnitude spectrum)?

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Comodulation versus Coherence Data sets
THANKS to Jolene Ross Jim Caunt for ADHD, some
of the AS data Coralee Thompson for normal
children
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EEG Comodulation and Coherence values are often
very similar!
Eyes Closed Replications Within subject, n20
EC1 v 2 r .91 Coh r .84 Comod Being more
reliable also can mean less sensitive to state
differences

• Dark bars Comod Red/green bars Coherence

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How to read our Comod Coh maps

• Rho DATA Z-SCORE from norms

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Typical Atypical(25 college
students) (1 college
student)
If you build it (adult pattern of frontal lobe
myelination), it still takes time for them to
come
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MTBI patient Rage Disorder
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Autobiography dysfunction associated with reduced
right anterior temporal pole connectivity
(Aspergers, Schizophrenia)
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Hypermodulations after stroke, 74yF (seen during
math task only!)
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Autism as severe global disconnectivity
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Asperger v Child Norms8-12 Hz (in Std Error)
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ADHD v Child Norms 8-12 Hz (in Std Error)
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Global Comodulation by age19-site mean of 18
comparisons each site
Life is about making connections...
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but not too fast!... Beta hypercoherence
between occipital and medial frontal cortex, esp.
right-sided, during rest for Asperger children
(resembles adult pattern)
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in fact, slowing down the rate of connections at
some times in your life may even do you some
good!One form of Intelligence (neotenous)
resists the natural neural integration
trajectory
Neural behavioral (Ph.D) indices of neoteny