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

2. 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

3. A. Generators and modulators of EEG signal

4. 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

5. 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

6. Generators of EEG signal
And the miracle occurs

7. 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

8. 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

9. B. Understanding instrumentation

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

11. 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)

12. 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-

13. 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)

14. 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

15. 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…

16. 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

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

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

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

20. Analog-to-Digital Conversion
Sampling frequency
ADC range
Quantization

21. Acquisition and storage
Data acquisition and storage
Reasonable sampling rate
Backup

22. Understanding instrumentation
Quikcap
Headbox
Power unit
System unit

23. C. Main ethical issues

24. 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 specifications.
Additional devices connected to Neuroscan have to be detailed in the application to the EEG committee
Order to plug or unplug the components

25. 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: ].

26. 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

27. D. Physiological and electronical sources of noise

28. 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

29. Interferences
Artifacts from the ocular domain

30. 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

31. Interferences
1 s
Saccade / eye movements

32. Interferences
Muscles

33. Interferences
How life could be easy without muscles

34. Interferences
(and with enough time to average thousands)

35. Interferences
EKG

36. Interferences
Respiratory

37. Interferences
Movement

38. Interferences
EDR/sweating

39. 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

40. Interferences
A cell phone

41. Interferences
Poor contact / Electrode pop

42. Interferences
60 Hz

43. E. Constraints for experimental protocols in EEG

44. 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

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

46. 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)

47. Montage 10 percent

48. Montage equidistant (eg. EGI)

49. Montage 128 NSL

50. 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

51. 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

52. G. Management of connectics

53. Cables everywhere Main cables are present in continuous
Cables being specific to an experiment are removed after each experiment

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

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

56. 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

57. I. Digitization

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

59. Cleaning

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