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ERPs and Intracranial Recordings(2/2)
초고주파 및 항공전자통신 연구실
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Contents
Methodology
└ Recording
└ Data Analysis
Applications
Conclusion
1/11
Microwave & Avionics LAB.
Methodology
Recording
Effects of Anesthetic Agents
Altering cerebral metabolism, which in turn can affect the EEG by either
exciting or depressing it
Each anesthetic agent alter the evoked response differently
2/11
Microwave & Avionics LAB.
Methodology
Data Analysis
Artifact Rejection
Like deriving scalp-recorded ERPs, eliminating of artifact from the EEG data
set is critical for intracortical ERPs
Four sources of artifact
– EEG equipment
– Electronic noise
– Noisy electrodes and leads
– Noncerebral physiologic potentials
The rejection of such artifacts can best be accomplished with computer
algorithm in conjunction with the visual inspection and the manual rejection
of contaminated trials
3/11
Microwave & Avionics LAB.
Methodology
Data Analysis
Artifact Rejection
External electrical interference
: electrostatic, electromagnetic, and radiofrequency pulses
⇒ low- and high-pass filter
Artifact from electrode and leads
: poor contact between the electrode and the patient’s cortex
⇒ reapply to the brain
The most common artifacts are those that arise from the patient
: intraoperative data is not free from physiologic artifact
⇒ creating a stable metal/liquid interface can minimize this artifact
Anethetics can also introduce artifacts into the data
⇒ visually inspecting the data and manually removing
4/11
Microwave & Avionics LAB.
Methodology
Data Analysis
Signal Averaging
The most basic method for analyzing
ECoG data is to derive ERPs, which are
simply the time-locked average across
many trials
Time-locking is typically done to a stimulus
(auditory tone) → a series of positive and
negative voltage fluctuation
The ERPs recorded from the cortical surface
are much greater in amplitude than those
recorded from the surface of the scalp
5/11
Microwave & Avionics LAB.
Methodology
Data Analysis
Signal Averaging
Scalp-recorded ERPs provide a noninvasive measure, but they do lack in
spatial resolution
The latest brain imaging technique(fMRI, ERPs) do not provide adequate
information regarding the neural generators of a particular brain potential
⇒ high-density recording arrays in conjunction with sophisticated data
analysis technique(dipole source localization) : forward solution
In interpreting the waveform one should consider the stimuli used to elicit
the ERP and the properties of the scalp-recorded ERP in order to obtain a
clear understanding of what the intracranially recorded ERP represents
6/11
Microwave & Avionics LAB.
Methodology
Data Analysis
Time-Frequency Analysis
Allows for the investigation of the event-related spectral perturbations of
the brain
Induced gamma activity may correlate with feature binding in humans
Used in scalp EEG, mainly moving-window fast Fourier transforms(FFTs) and
wavelet-based methods, are used for intracranial data
Intracranial recordings have also provided information on high frequency
“ripple” (100~500Hz) in hippocampal and epileptic human cortex, and have
shown that successful memory performance is accompanied by rhinalhippocampal gamma band coupling
7/11
Microwave & Avionics LAB.
Application
The Effect of the Skull on EEG
The skull is thought to act a low-pass filter
signal frequency increases, there is a reduction in the degree to which
signal phases align and thus the signals at higher frequencies tend to
cancel out
Amplitude in the scalp EEG is a function of amplitude plus coherence in the
ECoG, and at the scalp surface both amplitude and coherence decrease
with frequency
8/11
Microwave & Avionics LAB.
Application
The Size of Functional Units in Cortex
Concerns elucidating the size of functional units in cortex
The local domain of EEG synchrony, at least for gamma rhythms, in about
1cm2
In conjunction from converging evidence from other domain, this suggests
that the size of specialized cortical “modules” is on the order of a square
centimeter or so
9/11
Microwave & Avionics LAB.
Application
The Size of Functional Units in Cortex
The use of ECoG
Diameter and spacing of the electrodes are such that 1cm2 region of cortex
are sampled, and many tens of cortical columns are sampled beneath each
electrode
The results of the study(Freeman et al., 2000)
1cm2 size for the domains of local gamma synchrony
The optimal array would have ”an electrode interval of 1.25mm and 8X8
spatial window of at least 10mm”
10/11
Microwave & Avionics LAB.
Conclusion
Intracranial EEG
Provides excellent temporal and spatial resolution
it is still not without problems
Does not employ healthy, normal working brains
Limited to recording from restricted brain regions
Imaging technique(fMRI)
Provide excellent spatial resolution
Noninvasive investigations
Revealed a mosaic of interacting cortical modules that specialize in different
functions
11/11
Microwave & Avionics LAB.
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