Introduction to the methodology of EEG recording

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Introduction to the methodology of EEG recording

• Friday October 20th
• 2006

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Summary

• A. Generators and modulators of EEG signal
• B. Understanding instrumentation
• C. Main ethical issues
• D. Physiological and electronical sources of
  noise
• E. Constraints for experimental protocols in EEG
• F. Configuration of EEG recording system and
  selection of montage
• G. Management of connectics
• H. Paste-up and problem solving during the
  recording session
• I. Digitization
• J. Cleaning/decontamination  of sensitive
  equipment / equipment maintenance

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A. Generators and modulators of EEG signal
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Generators of EEG signal

• When reached by an input, the presynaptic neuron
• Releases some neurotransmitter in the synaptic
  cleft
• The dendritic process of the post-synaptic
  neuron
• shows a local change in its membranes
  permeability
• generates a primary (intracellular) current from
  the locus of the synapse to the soma
• Generates a secondary/return extracellular
  current to close the loop

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Generators of EEG signal

• Cortical pyramidal neurons, arranged in layers
• The movement of the ions is creating an open
  field (no cancellation)
• When a local community of tens of thousands of
  neurons are activated simultaneously by some
  input, a signal can be detected as far as at the
  surface of the scalp
• This signal is EXTREMELY tiny, and requires many
  precautions when measured

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Generators of EEG signal

• And the miracle occurs

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Modulators

• Age
• Children
• Changes in frequency content due to the size of
  the loops in the anatomical networks
• Changes in the conduction time due to
  myelinization
• Change in the amplitude of the signal due to
  myelinization
• Adult
• Increased variability over 40
• Vigilance
• Chronopsychology (more details next)
• Drugs
• Caffeine
• Body temperature
• Hormonal cycles (women)
• Laterality

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Modulators

• Circadian rhythms
• Global power is maximal during the afternoon
• Theta power has two peaks at 4pm and midnight
• Induced alpha is maximal in the afternoon
• Beta is maximal between 5pm and 7pm
• The modulation is dependent on the location of
  the electrodes

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B. Understanding instrumentation
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Overview

• Junction skin-electrode
• Analog conduction
• Differential amplifiers
• ADC

• Integration of triggers
• Transfer to the CPU / storage

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Transduction

• The living tissues contain free ions
• The wire is conveying electrons
• The transfer of the signal from one material to
  the other requires a chemical transformation
• Oxidation or reduction (AC)

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Transduction

• Eg. Ag / AgCl electrode
• OXIDATION
• If an electron moves from the wire to the
  electrode toward the conductive gel
• It reacts with AgCl
• e- AgCl -gt Ag Cl-
• Cl- becomes hydrated and enters the conductive
  paste
• REDUCTION
• If ion moves from the conductive gel to the
  electrode
• It reacts with solid Ag
• Ag Cl- -gt e- AgCl
• AgCl becomes insoluble
• one electron is liberated to the wire
• REVERSIBLE

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Transduction

• Eg. Ag / AgCl electrode
• The Ag/AgCl electrode is non-polarizable (or
  minimally polarizable)
• POLARIZATION
• The anion (Cation) is unable to move freely
  across the gel/electrode border
• The concentration of ions at the border is
  altered.
• Ions concentrate over the border with the
  electrode and create a steady potential
  (bi-layer, capacitance)
• This steady potential hampers the movement of the
  charges
• This is important since the biopotential we
  intend to measure is in the range of 1/1000 of
  the half-cell potential (local potential at the
  junction between the conductive paste and the
  electrode)

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Analog conduction

• As soon as the potentials are digital, they are
  immune to noise (not to deletion)
• Between the cap and the ADC, the minuscule
  currents are traveling through the cables and in
  the amplifier.
• Contamination through movements of the cables
• Contamination by cross-talk inside the amplifier
  and at the multiplexer of the ADC

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Differential amplification

• We amplified to push the deflection of the pens
  (mechanical)
• We amplify to bring the signal in the range of
  the ADC (usually 0-1 to 0-5 V)
• But more importantly, the purpose is differential
  amplification

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Differential amplification

• Principle of differential amplification the CMR
• (Signal noise) (noise)
• Take a scalp electrode (say C3) and a fixed point
  (GND)
• Measure one potential difference
• Take a reference electrode (say MA1) and a fixed
  point
• Measure a second potential difference
• (Signal noise) (noise) a very clean signal

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Differential amplification

• The ability of the amplifier to reject the common
  mode noise is called the CMRR

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Differential amplification

• Amplifier Input impedance
• Separate the differential input with a high
  resistance

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Analog-to-Digital Conversion

• Sampling frequency Nyquist and aliasing

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Analog-to-Digital Conversion

• Sampling frequency
• ADC range
• Quantization

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Acquisition and storage

• Data acquisition and storage
• Reasonable sampling rate
• Backup

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Understanding instrumentation

• Quikcap
• Headbox
• Power unit
• System unit

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C. Main ethical issues
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Electrical safety

• Security for the subject and security for the
  equipment
• Faulty connections
• Additional devices (response pads, sensors)
• Ground loops
• Static discharges
• Chassis leakage
• EMI in crossing wires
• Isolation amplifiers (Neuroscan system) are
  regulated by IEC 601-1 specifications.
• Additional devices connected to Neuroscan have to
  be detailed in the application to the EEG
  committee
• Order to plug or unplug the components

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Infection risk

• Most of the supplies, especially those in contact
  with the subject (eg. needles), are disposable
• Any supply in contact with the subject does not
  return to the main.
• eg. the gel is sampled in a cup. Do NEVER refill
  a syringe in the main container.
• Moderate skin preparation a subject should never
  be bleeding as a result of skin preparation.
• Inspection for the presence of blood after
  experiment (to choose the decontamination
  procedure)
• Decontamination of non-disposable equipment
• Is regulated by American Electroencephalographic
  Society. Report of the Committee on Infectious
  Diseases. J Clin Neurophysiol 199411128-132..

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Infection risk
Object Classification Use of Item Decontamination required after cleaning
Critical Enters vascular system or sterile body tissues Sterilization and holding in sterilized state. High level disinfection is not sufficient
Semi-Critical Comes in contact with non-intact skin or intact mucous membranes High level disinfection (by heat or chemicals)
Non-Critical Comes in contact with intact skin Intermediate or low level disinfection
Spaulding's classification of devices/medical
instruments
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D. Physiological and electronical sources of noise
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Interferences

• Physiological artifact
• Ocular domain
• Muscular domain
• EKG
• Respiratory
• Movement
• EDR/sweating
• Subjects instruction and online monitoring
• Instrumental noise
• EMI wireless or line noise (60 Hz)
• Sway of the cable
• Electrodes poorly attached (pop)
• Electrode noise
• Amplifier noise
• Flicker noise (DC recordings!)
• Amplifier blocking
• Shielding and guarding

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Interferences

• Artifacts from the ocular domain

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Interferences

• With proper alignment of EOG electrodes,
  horizontal EOG do not pick up the signal from
  vertical eye movements

GOOD
BAD
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Interferences

• Saccade / eye movements

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Interferences

• Muscles

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Interferences

• How life could be easy without muscles

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Interferences

• (and with enough time to average thousands)

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Interferences

• EKG

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Interferences

• Respiratory

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Interferences

• Movement

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Interferences

• EDR/sweating

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Interferences

• Physiological artifact
• Ocular domain
• Muscular domain
• EKG
• Respiratory
• Movement
• EDR/sweating
• Subjects instruction and online monitoring
• Instrumental noise
• Flicker noise (DC recordings!)
• EMI wireless or line noise
• electrode noise
• amplifier noise
• Sway of the cable
• Electrodes poorly attached (pop)
• Amplifier blocking
• Shielding and guarding

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Interferences

• A cell phone

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Interferences

• Poor contact / Electrode pop

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Interferences

• 60 Hz

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E. Constraints for experimental protocols in EEG
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Protocols

• Paradigms
• Evoked response
• Steady-state paradigms
• A single source of variation between conditions
  All other things being equal
• A good Stimulation/recording coupling time
  accuracy in analog and digital stimuli/triggers
• Subject screening
• Day-before instruction
• Accepting or rejecting a volunteer
• artifacts instruction, task instructions,
• Online monitoring of data quality and management
  of breaks

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F. Configuration of EEG recording system and
selection of montage
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Configuration

• Configuration of data recorder (scan-acquire
  mode)
• Sampling frequency
• DC/AC recording (DC and EDR resident on the skin
  DC and choice of electrodes)
• Triggers
• Selection of montage
• Only referential recording
• Reference electrodes
• Ground electrode
• Ancillary recording (EOG, surface EMG, EKG)

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Montage 10 percent
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Montage equidistant (eg. EGI)
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Montage 128 NSL
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Montage reference

• Choice of the reference electrode
• Cephalic/non cephalic
• Well-attached
• Single electrode or pair of electrode
• Pair physically or digitally linked
• Position of the ground
• In midline for ERL
• Remontage

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Next sessionpractical sessionG. Management of
connectics H. Paste-up and problem solving
during the recording session I. Digitization
J. Cleaning/decontamination  of sensitive
equipment / equipment maintenance
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G. Management of connectics
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Cables everywhere

• Main cables are present in continuous
• Cables being specific to an experiment are
  removed after each experiment

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H. Paste-up and problem solving during the
recording session
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Transduction

• Practical recommendation
• Do not mix electrodes metal
• Do not mix gels
• homogenous contact skin-gel-electrode

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• Electrodes impedance
• Poor impedance
• Is the electrodes surface clean (eg did I
  perforate a crust into the electrodes hole while
  injecting conductive gel?). Did I rub enough the
  scalp? Did the subject come with inadequate hair
  condition? Is the electrode filled enough?
• Infinite impedance
• Am I sure that theres no damage to the wires?
• Amplifiers gain error
• Poor signal on some electrodes

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I. Digitization
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J. Cleaning/decontamination  of sensitive
equipment / equipment maintenance
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Cleaning
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• Cleaning
• Equipment maintenance
• Troubleshooting / identification of noise source
  /
• Problem report