Introduction to the methodology of EEG recording Friday October 20th 2006

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 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

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

Generators of EEG signal And the miracle occurs

Modulators Age Vigilance Body temperature Hormonal cycles (women) 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 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 instrumentation

Overview Junction skin-electrode Analog conduction Differential amplifiers ADC Integration of triggers Transfer to the CPU / storage

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)

Transduction Eg. Ag / AgCl electrode: REVERSIBLE OXIDATION REDUCTION If an electron moves from the wire to the electrode toward the conductive gel: It reacts with AgCl e- + AgCl -> 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- -> e- + AgCl AgCl becomes insoluble one electron is liberated to the wire REVERSIBLE e- e-

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)

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 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…

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

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

Differential amplification Amplifier Input impedance Separate the differential input with a high resistance

Analog-to-Digital Conversion Sampling frequency: Nyquist and aliasing

Analog-to-Digital Conversion Sampling frequency ADC range Quantization

Acquisition and storage Data acquisition and storage Reasonable sampling rate Backup

Understanding instrumentation Quikcap Headbox Power unit System unit

C. Main ethical issues

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

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 1994;11:128-132.].

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

D. Physiological and electronical sources of noise

Interferences Physiological artifact Instrumental noise 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

Interferences Artifacts from the ocular domain

Interferences GOOD BAD With proper alignment of EOG electrodes, horizontal EOG do not pick up the signal from vertical eye movements GOOD BAD 1 s

Interferences 1 s Saccade / eye movements

Interferences Muscles

Interferences How life could be easy without muscles

Interferences (and with enough time to average thousands)

Interferences EKG

Interferences Respiratory

Interferences Movement

Interferences EDR/sweating

Interferences Physiological artifact Instrumental noise 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

Interferences A cell phone

Interferences Poor contact / Electrode pop

Interferences 60 Hz

E. Constraints for experimental protocols in EEG

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 system and selection of montage

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 percent

Montage equidistant (eg. EGI)

Montage 128 NSL

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

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

G. Management of connectics

Cables 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 the recording session

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

Am I sure that there’s no damage to the wires? Amplifier’s gain error 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. Digitization

J. Cleaning/decontamination  of sensitive equipment / equipment maintenance

Cleaning

Cleaning Equipment maintenance Troubleshooting / identification of noise source / Problem report