PSYCHOLOGY 386/925
Laboratory in Behavioural Neuroscience
Electricity and Electronics
This week:
•  Electricity and electronics
•  Neural basis of EEG
•  EEG Recording
!  Why would anyone (in this department) wish to know some
basic “physics” about electricity?
!  To understand EEG
!  To understand what you are doing (e.g., measuring
something called impedance) in an EEG lab
•  electrodes,
amplification
John McDonald
Psychology, Simon Fraser University
Psychology 386/925
Electricity and Electronics
Electricity and Electronics
!  Electrons: negatively charged particles that orbit nuclei of
atoms. Movement and behaviour affected by magnetic fields,
chemical reactions, electrostatic fields, etc.
!  Electric field: region in which one electron can affect (force)
another
!
!
!
More details: Electric fields contain electrical energy with energy density
proportional to the square of the field intensity.
Online example: www.falstad.com/emstatic/ (local applet here).
Online tutorials:
!
Electric fields 1: www.youtube.com/watch?v=laGSICm_agM
!
Electrid fields 2: www.youtube.com/watch?v=puTZvhOFpRA
Psychology 386/925
jmcd@sfu.ca
jmcd@sfu.ca
!
Voltage: Electrical force needed to make electrons move.
!  Refers to both the electromotive force (e.g. battery voltage) and
the potential difference between two points.
!  Often referred to as electrical potential
!  potential energy per unit of charge
!  Measured in volts
!
Voltage polarity (positive, negative): Determines
the direction in which electrons will flow. In simple
electrical circuit, electrons flow from negative
battery terminal through circuit to positive
terminal.
!
Online tutorial:
www.youtube.com/watch?v=LkIai_KXGxg
Psychology 386/925
jmcd@sfu.ca
Electricity and Electronics
Electricity and Electronics
!  Current: The movement of electrons through a conductive
material
!  Measured in amperes (1 amp = 6 x 1018 electrons flow
per second). Current flows only if the electrons are
pushed in one direction.
!  Alternating current (ac): A periodic reversal of current flow.
For ac-powered circuits, the polarity of the voltage source
changes at a specific rate or frequency, measured in cycles
per second, or hertz (Hz).
!  Resistance: The opposition to current flow.
!  The units of resistance are ohms.
!  Conductance is the inverse of resistance (1/R), or how
easy it is to pass charges.
!  Ohm’s law: E = IR (think: V=CR)
!  E = voltage, I = current, R = resistance
!  Tells us that voltage is equal to current
multiplied by resistance
!  Direct current (dc): Current flow that does not
reverse directions because voltage source does
not reverse polarity.
Psychology 386/925
jmcd@sfu.ca
Electricity and Electronics
Psychology 386/925
jmcd@sfu.ca
Human Electrophysiology
!  Impedance: complex “resistance”
!  The total opposition offered to the flow of an alternating
current. The higher the impedance, the lower the current.
!  Includes resistance, capacitance and inductance!
!  Measured in Ohms (like resistance)
!  Depends on the frequency of the ac signal
This Week:
1.  Electricity and electronics
2.  Neural basis of EEG
3.  EEG Recording
!  electrodes, amplification, A/D conversion
"  Can’t measure impedance with an Volt-
Ohm meter available at Radio Shack. This
could be dangerous!
Psychology 386/925
jmcd@sfu.ca
Psychology 386/925
jmcd@sfu.ca
Neural Basis of EEG
Neural Basis of EEG
!  Post-Synaptic Potentials (PSPs):
!  Ionic flow alters potential
!  Causes electric field and current
in interior of dendrite
!  current flows into cell if Na+
channels open; Excitatory PSP
!  Synapse usually in dendrite
!  current flows out of cell if K+ or Clchannels open; Inhibitory PSP
!  Synapse usually at soma
!  Strength of current source
decreases with distance: 1/r2
Psychology 386/925
jmcd@sfu.ca
Neural Basis of EEG
!  Action Potentials (APs):
!  All-or-none response down axon
once voltage at hillock reaches -40
mV
!  Strength of AP current decreases
faster than PSP current with
distance: 1/r3
!  EEG and MEG reflect primarily PSPs!
!  AP electrical fields decline more
rapidly in time and space
!  Q: how do we know EEG arises
from PSPs and not APs?
Psychology 386/925
Pyramidal cell
dendrites
Cerebral Cortex
cell body
axon
incoming
fiber
Synapse
Synapse"
Depolarization"
Extracellular
(volume)
currents"
Intracellular
currents"
jmcd@sfu.ca
Neural Basis of EEG
Closed Field
Pyramidal cell
dendrites
Cerebral Cortex
cell body
axon
incoming
fiber
Synapse
Open Field
Psychology 386/925
jmcd@sfu.ca
Psychology 386/925
jmcd@sfu.ca
+
Sink
apical
dendrite
dendrites
+
soma
Source
soma
A. Herdman
Psychology 386/925
jmcd@sfu.ca
Dipole
A. Herdman
Psychology 386/925
jmcd@sfu.ca
Extracellular
Potentials
In
Out
Dipole
Magnetic
Field Lines
+
A. Herdman
Psychology 386/925
jmcd@sfu.ca
A. Herdman
Psychology 386/925
jmcd@sfu.ca
Human Electrophysiology
Magnetic
Electric
This Week:
1.  Electricity and electronics
2.  Neural basis of EEG
3.  EEG Recording
!  electrodes, amplification, A/D conversion
Out
In
A. Herdman
Psychology 386/925
jmcd@sfu.ca
Overview of Methodology
Analog
Filtering
Analog-Digital
Conversion
Amplification
Signal-to-Noise
Enhancement
Psychology 386/925
jmcd@sfu.ca
Electrodes
!  Brain electricity recorded by EEG system
!  Scalp and EEG system connected by electrodes
Montage
Selection
averaging
digital filtering
artifact rejection
Display
Connection is mui importante!
waveforms
topographical maps
source analysis
Component
Analyses
Measurement
amplitudes
latencies
Psychology 386/925
jmcd@sfu.ca
Psychology 386/925
jmcd@sfu.ca
Electrode Placement
Scalp-Electrode Interface
!
!
Psychology 386/925
Standard electrode placement
and nomenclature
!  10-20 system
!  10-10 system
!  Also: equal distance designs,
e.g., geodesic electrode net
(www.egi.com)
How many electrodes?
!  Spatial resolution
!  Spatial Nyquist
jmcd@sfu.ca
Scalp-Electrode Interface
!
!
!
!  Impedances:
!  Electrode impedance (low)
!  Amplifier input impedance (high)
!  Reducing electrode impedances
!  Procedures
!  Recommendations
!  Types of electrodes
!  Metals
!  Sizes
Psychology 386/925
jmcd@sfu.ca
Electrode impedances
!  Electrode impedances should be low
!  If high, current from other sources (e.g., lights) will be
induced in the electrode circuits
!  Picton et al. (2000). Guidelines: < 10 kOhms
!  Maybe higher with modern EEG amplifiers
Electrical double layer - build
up of charges at metal
interfaces
Affects (filters) signals,
depending on:
!  Signal frequency
!  Size of electrode
!  Type of metal
!  Current passing through
(impedances)
Eliminate with ‘reversible’
electrode such as Ag/AgCl
!  Layer of AgCl blocks ions
from escaping metal
High
Low electrode
<<<
input
impedance
impedance
Psychology 386/925
jmcd@sfu.ca
Psychology 386/925
jmcd@sfu.ca
Impedances
Scalp-Electrode Interface
Impedances (k Ohms)
Electrode
input amplifier
ratio (%)
1
10 M Ohms
.01
2
2
100 M Ohms
1
G Ohms
.002
.0002
5
5
100 M Ohms
1
G Ohms
.005
.0005
10
10
100 M Ohms
1
G Ohms
.01
.001
20
20
100 M Ohms
1
G Ohms
.02
.002
50
100
100 M Ohms
1
G Ohms
.05
.005
100
100
100 M Ohms
1
G Ohms
.1
.01
Psychology 386/925
!  Impedances:
!  Electrode impedance
!  Amplifier input impedance
!  Reducing electrode impedances
!  Procedures
!  Recommendations
!  Types of electrodes
!  Metals
!  Sizes
Picton et al (1995). Figure 2
jmcd@sfu.ca
Scalp-Electrode Interface
!  Impedances:
!  Electrode impedance
!  Amplifier input impedance
!  Reducing electrode impedances
!  Procedures
!
!  Recommendations
!
!  Types of electrodes
!
!  Metals
!
!  Sizes
Psychology 386/925
jmcd@sfu.ca
Overview of Methodology
Electrodes off skin!
Electrolyte
Hold electrodes in place
Abrade skin
!  Benefits #
!  Risks
Analog
Filtering
Analog-Digital
Conversion
Amplification
Signal-to-Noise
Enhancement
Montage
Selection
averaging
digital filtering
artifact rejection
Display
waveforms
topographical maps
source analysis
Component
Analyses
Measurement
amplitudes
latencies
Psychology 386/925
jmcd@sfu.ca
Psychology 386/925
jmcd@sfu.ca
Recording EEG
Electrode Montage
!  Remember: voltage measured between two sources (potential
difference)
!  What electrodes are needed?
!  Electrode pair
!  active vs. “passive” (note: passive not possible)
!  active vs. reference
!  “Ground” (not earth ground)
!
How are electrodes connected?
!  Referential montage, or
common reference
!  inactive reference?
!  Mastoid, ear
!
Psychology 386/925
jmcd@sfu.ca
Overview of Methodology
Analog
Filtering
Analog-Digital
Conversion
Amplification
Signal-to-Noise
Enhancement
Montage
Selection
averaging
digital filtering
artifact rejection
Psychology 386/925
!  Nose
!  Noncephalic
!  Linked mastoids
!  Average
Bipolar montage
jmcd@sfu.ca
SA Instrumentation Amp
Display
Electrode
monitors
waveforms
topographical maps
source analysis
Selectable
gain
Component
Analyses
Power &
LED Indicators
Measurement
amplitudes
latencies
Psychology 386/925
jmcd@sfu.ca
Psychology 386/925
jmcd@sfu.ca
SA Instrumentation Amp
Differential Amplification
Input 1 (electrode 1)
Input 1
Input 2
Electrode cap
inputs
7 µV
Input 2 (electrode 2)
Amplifier
0 µV
7 µV
Mode
ref, open, gnd
7 µV
7 µV
7 µV
Impedance
meter
Psychology 386/925
jmcd@sfu.ca
Differential Amplification
-7 µV
Psychology 386/925
0 µV
14 µV
jmcd@sfu.ca
Differential Amplification
!  Electrode-amp coupling
!  Capacitor or direct (DC)
!  Noise
!  Common mode rejection ratio (CMRR)
!  Ground
Fp1
Fpz
Fp2
M1
(Common reference)
Psychology 386/925
jmcd@sfu.ca
Psychology 386/925
jmcd@sfu.ca
Signal loss in a Differential
Amplifier
Overview of Methodology
Analog
Filtering
Analog-Digital
Conversion
Amplification
Signal-to-Noise
Enhancement
Montage
Selection
averaging
digital filtering
artifact rejection
Display
waveforms
topographical maps
source analysis
Component
Analyses
Measurement
amplitudes
latencies
Illustration by
Wolfgang Teder-Sälejärvi
Psychology 386/925
jmcd@sfu.ca
Analog Filters – SAI Amps
Low-pass filter
30, 80, 160, 3000 Hz
High-pass filter
0.01, 0.1, 1, 10 Hz
Psychology 386/925
jmcd@sfu.ca
Psychology 386/925
jmcd@sfu.ca