Part I: Discussion High Gamma Power Is PhaseLocked to Theta Oscillations in Human Neocortex' Canolty

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Part I Discussion High Gamma Power Is
Phase-Locked to Theta Oscillations in Human
Neocortex. Canolty et al Science (2006)
3131626-28

• Sridhar Devarajan
• CS379C, 11/29

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Background

• Origin of EEG
• Frequency band in EEG
• Delta (0-4Hz)
• Theta (4-8Hz)
• Alpha (8-12Hz)
• Beta (12-30Hz)
• Gamma (30-70Hz)
• High gamma (80-150 Hz)
• Rationale of filtering

Raw EEG
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Background Theta and Gamma Rhythms

• Theta rhythms
• Seen throughout neocortex
• Prominently reported from the hippocampus
• Manifest in hypnagogic states
• Spatial navigation and short-term memory
• Gamma rhythms
• Awake and aware rhythms
• Important for perception and consciousness
• High-gamma (HG) rhythms
• Probably have different physiological origins
  from theta and gamma rhythms
• Correlated with fMRI BOLD signals

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Experiment

• Subjects five patients undergoing neurosurgical
  treatment for epilepsy.
• Analyzed multichannel subdural electrocorticogram
  (ECoG) data from
• Looked for identifiable features of ongoing
  oscillatory activity
• Not an event-related analysis

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Experiment

• Theta and gamma amplitude and phase from EEG
  signal

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Theta/HG coupling findings

• Transient increase in high gamma power occurs at
  troughs of theta wave
• Theta/HG coupling strength is mainly a function
  of theta amplitude.

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Theta/HG coupling findings

• Similar tasks evoke similar spatial patterns of
  theta/HG coupling
• What are the implications?

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Part II Neuronal coherence
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Diversion Task switching

• Thought experiment
• Flashing green squares on screen
• Fixate on center cross
• Push a button quickly only when you see square on
  right
• If you hear a beep
• Push a button only when you see square to the
  left
• Next beep, reverse attention focus
• Immediate, strong cognitive control over the
  routing of information from sensory to motor
  areas
• Identical stimulus, different responses
• What could be happening at the neural level?

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Task switching A hypothetical dynamic routing
model
Right VA
Motor Area
Top-down cognitive/attentional mechanism
Left VA

• Top down control
• dynamically alters the gain of the response in
  the higher visual area to input from the
  attended lower visual area
• effectively modify the communication structure
  between these areas
• Happens over a very short time-scale for
  anatomical modifications to take effect

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Neuronal coherence A hypothesis

• Background oscillations modulate excitability
• Neurons at peak of these oscillations
• more likely to produce a spike
• more sensitive to synaptic input
• Only coherently oscillating groups (phase locked)
  can communicate effectively
• communication windows for input and output are
  open at the same time
• Coherently oscillating populations entrain
  non-oscillating populations

Fries, P. Trends in Cogn. Sci. (2005)
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Functional role for coherence

• Attention, Predicting reaction times

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The dynamic routing problem revisited

• Provides a putative mechanism for top-down
  mechanisms to alter the gain and impose a
  flexible effective communication structure on top
  of the fixed anatomical communication structure.
• Top down signal causes greater spike field
  coherence in the neuronal population
  corresponding to the attended region (say, the
  right VA). This entrains the Motor area into its
  rhythm thereby making
• Right VA (attended) to MA communication more
  effective
• Left VA (unattended) to MA communication less
  effective
• Cross-frequency coupling between distinct brain
  rhythms (HG/theta) may facilitate the transient
  coordination of cortical areas required for
  adaptive behavior in humans

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Problems with the coherence idea

• Exact mechanism by which Theta/HG coupling
  achieves flexible effective communication still a
  mystery
• Communication through neuronal coherence still
  remains a hypothesis (albeit a testable one)
• Origin and functional role of the different
  rhythms in the brain is debated
• Nature of the top down signal that increases
  spike-field coherence is unclear

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