Introduction to the methodology of EEG recording

1. Introduction to the methodology of EEG recording Emmanuelle Tognoli The Human Brain and Behavior Center For Complex Systems and Brain Sciences Florida Atlantic University tognoli@ccs.fau.edu

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 • 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. e- e- 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 -> 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

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)

16. Differential amplification • Principle of differential amplification: the CMR • (Signal + noise) – (noise) • Take a scalp electrode (say F3) and a fixed point (GND) • Measure one potential difference • Take a reference electrode (say M1) 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 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

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;11:128-132.].

26. Infection risk Spaulding's classification of devices/medical instruments

27. D. Physiological and electronical sources of noise

28. 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

29. Interferences • Artifacts from the ocular domain

30. 1 s Interferences • With proper alignment of EOG electrodes, horizontal EOG do not pick up the signal from vertical eye movements GOOD BAD

31. 1 s Interferences • 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 • 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