Human Brain and Behavior Laboratory Center for Complex Systems and Brain Sciences Introduction to the methodology of EEG recording Friday October 20th

Human Brain and Behavior Laboratory

Center for Complex Systems and Brain Sciences

Introduction to the Introduction to the methodology of EEG methodology of EEG

recordingrecordingFriday October 20th

2006



Summary 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



A. Generators and modulators of EEG A. Generators and modulators of EEG signalsignal



Generators of EEG signal 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

membrane’s 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




• 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




• And the miracle occurs



Modulators 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



Modulators 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



B. Understanding instrumentationB. Understanding instrumentation



Overview Overview

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

• Integration of triggers

• Transfer to the CPU / storage



Transduction 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)




• Eg. Ag / AgCl electrode:– OXIDATION

• If an electron moves from the wirewire to the electrodeelectrode toward the conductive gelconductive gel:

– It reacts with AgCl » ee-- + AgClAgCl -> AgAg + ClCl--

– Cl- becomes hydrated and enters the conductive paste

– REDUCTION• If ion moves from the conductive gel to

the electrode:– It reacts with solid Ag

» AgAg + ClCl-- -> ee-- + AgCl AgCl – AgCl becomes insoluble– one electron is liberated to the wire

• REVERSIBLE

e-e-

e-e-




• 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)



Analog conduction 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



Differential amplification 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…




• 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




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




• Amplifier Input impedance– Separate the differential

input with a high resistance



Analog-to-Digital Analog-to-Digital ConversionConversion

• Sampling frequency: Nyquist and aliasing



Analog-to-Digital Analog-to-Digital ConversionConversion

• Sampling frequency • ADC range• Quantization



Acquisition and storageAcquisition and storage

• Data acquisition and storage– Reasonable sampling rate– Backup



Understanding Understanding instrumentationinstrumentation

• Quikcap • Headbox• Power unit• System unit



C. Main ethical issues C. Main ethical issues



Electrical safetyElectrical 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



Infection riskInfection 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 NEVERNEVER 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 1994;11:128-132.].



Infection riskInfection 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



D. Physiological and electronical sources D. Physiological and electronical sources of noiseof noise



Interferences 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




• Artifacts from the ocular domain




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

1 s

GOOD BAD




• Saccade / eye movements

1 s




• Muscles




• How life could be easy without muscles




• (and with enough time to average thousands)




– EKG




– Respiratory




– Movement




– EDR/sweating




• 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




• A cell phone




• Poor contact / Electrode pop




• 60 Hz



E. Constraints for experimental protocols E. Constraints for experimental protocols in EEGin EEG



Protocols 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



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



Configuration 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)



Montage 10 percentMontage 10 percent



Montage equidistant (eg. Montage equidistant (eg. EGI)EGI)



Montage 128 NSLMontage 128 NSL



Montage referenceMontage 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



Next session:Next session:practical sessionpractical session

G. Management of connecticsG. Management of connectics H. Paste-up and problem solving during the recording H. Paste-up and problem solving during the recording

session session I. Digitization I. Digitization

J. Cleaning/decontamination of sensitive equipment / J. Cleaning/decontamination of sensitive equipment / equipment maintenanceequipment maintenance



G. Management of connecticsG. Management of connectics



Cables everywhereCables everywhere

• Main cables are present in continuous

• Cables being specific to an experiment are removed after each experiment



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




Practical recommendation:– Do not mix electrodes’ metal– Do not mix gels– homogenous contact skin-gel-electrode



• Electrodes impedance– Poor impedance

• Is the electrode’s surface clean (eg did I perforate a crust into the electrode’s 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 there’s no damage to the wires?• Amplifier’s gain error• Poor signal on some electrodes



I. DigitizationI. Digitization



J. Cleaning/decontamination of J. Cleaning/decontamination of sensitive equipment / equipment sensitive equipment / equipment

maintenancemaintenance



Cleaning Cleaning



• Cleaning

• Equipment maintenance

• Troubleshooting / identification of noise source /

• Problem report

Documents

Human Brain and Behavior Laboratory Center for Complex Systems and Brain Sciences Introduction to the methodology of EEG recording Friday October 20th