This presentation looks at EEG signal generation, pyramidal cells, recording of EEG, source localisation, polarity, analysis of dipole, derivations, montages,

1. Principles of Polarity in EEG
Dr Pramod Krishnan, MD (Int Med), DM Neurology (NIMHANS)
Fellowship in Epilepsy (SCTIMST) (LMU, Munich)
World Sleep Federation Certified Sleep Medicine Specialist.
Consultant Neurologist and Epileptologist
Head of the Department of Neurology,
Manipal Hospital, Bengaluru.

2. Introduction
• EEG is a graphic representation of the difference in voltage
between two different scalp locations plotted over time.
• The scalp EEG signal generated by cerebral neurons is modified
by:
1. Electrical conductive properties of the tissues between the
electrical source and the recording electrode on the scalp.
2. Conductive properties of the electrode.
3. Orientation of the cortical generator to the recording
electrode.

3. Scalp
Skull
Arachnoid mater
Subarachnoid space
Cerebral cortex
Pia mater
Dura
mater
Amplifier
Electrode
Efferent axon
The spontaneous EEG signal in the routine scalp recorded EEG arises from
cortical pyramidal cell post synaptic potentials.
Pyramidal cells
Synapses

4. Pyramidal cells have unique
biophysical advantages:
1. Firing in synchrony (ie, they
summate in time)
2. Similar radial orientation (ie they
are aimed in the same direction
towards the cortical surface). So,
summation is better.
3. Long duration of potentials
making them ideal for producing
a large enough signal to be
conducted through the skull and
recordable at the scalp.
Pyramidal cell of the cortex

5. SINK: At synaptic site of EPSP, positive ions enter the cell to depolarize the local membrane.
SOURCE: More distal portion of cell, current flows out of the cell passively completing a
closed circuit.
•Primary current: Current flow within the cell (intracellular). (MEG Generation)
•Secondary or return current: Current flow outside the cell (extracellular). (EEG generation)
Sink Source +
+
+
+
+
+
+
+

6. Pyramidal dipole.
• Since the pyramidal cell PSPs
generate a circuit of current
flow throughout the length of
each cell, one end of the
pyramidal cell has the opposite
polarity from the other.
• A μvolt EEG signal is recorded
when numerous pyramidal cells
are synchronously activated
and many small dipoles are
combined.

7. • Basket cells, though numerous do not produce electrical activity
detectable on scalp recordings, because of their structure/orientation.
• However, pyramidal cells, because of their orientation and
synchronous firing, show electrical activity strong enough to be
recorded by scalp electrodes.

8. The recording electrode that detects the highest amplitude signal is overlying
the cortical source.

9. • Source is in a fissure.
• Source is primarily in a sulci.
• Cortical anatomy and
orientation have been altered
due to developmental
malformation, trauma, surgery.
• Skull thickness is altered
(surgical defects, thickening
due to Pagets disease etc).
Exceptions to this simple model

10. Pyramidal cell orientation in the cortex

11. -μv
+μv
SP1 P2
+ +
+
+
+
+
+
+
+
+ +
++++
+
+
+
+
+
+ +
S: Amplitude of voltage.
P1, P2: Electrodes.
The current that reaches the
electrode from the cortical
source is equivalent to the
area contained by the lines
that begin at the cortex and
converge on the electrode.
For P2, the source projects
both negativity from the
outer cortex (area between
solid lines) and positivity
from the inner cortex (area
between dashed lines).
Therefore the voltage of the
signal is reduced.
Volume conduction theory for a cortical source that
occupies a gyrus

12. • Scalp EEG can be imagined as
a constantly changing relief
map in which the map is
composed of hills and valleys
whose heights and depths
represent a particular voltage.
• The more electrodes are used,
the more accurately they will
map the actual contours of the
scalp potential.

13. Differential amplifier
Input
terminal 1
Input
terminal 2
Fp2
C4
Oz
Physiologic electrical currents are detected using differential amplifiers.
Input 1- Input 2= Recorded EEG signal (hence differential)

14. Common mode rejection
• The advantage of differential amplifiers is that they cancel
external noise.
• The most common source of noise are other electrical devices.
• Eg, When current is supplied at 50-60 cycles/sec, it causes
electrical interference which equally contaminates input 1 and 2.
This gets cancelled out by the differential amplifier.
• Because differential amplifier is used, the absolute voltage value at
electrodes attached to either Input 1 or 2 is never known, only the
difference between them is known.

15. Differential amplifier polarity
• Current flowing through the scalp: positive or negative polarity.
• Amplifier output (Input 1- Input 2) is assigned a polarity (positive
or negative).
• Input 1 more negative than Input 2: upward deflection
• Input 1= Input 2: no deflection (flat line)
• Input 1 less negative than Input 2: downward deflection

16. -40uV
-70uV
-80uV
-40uV
-40uV
-40uV
+60uV
-40uV
-40 –(-70)=+30uV
-80 –(-40)= -40uV
-40- (-40)= 0
+60-(-40)=+100uV
+50uV
+70uV
+50-(+70)=-20uV
1
1
1
1
1
2
2
2
2
2

17. Derivations and Montages.
• An electrode pair in an amplifier: electrode derivation. Eg Fp1-F3.
• If adjacent electrodes are paired, it is called bipolar. Eg Fp1-F3
• If the pair includes a referential electrode, it is called referential
derivation. Eg Fp1-A1.
• A display that consists of a combination of derivations is referred
to as a montage.
• Accordingly montages can be referential or bipolar, to visualise
widespread vs localised fields respectively.
• Bipolar montages can be longitudinal or transverse.

18. Montages

19. Spatial filtering
Bipolar
Referential
Bipolar derivation shows the electrical field of the small cortical generator (red). Larger generator (green) is
cancelled out as both the electrodes are placed over it. In the referential derivation, the larger field is clearly
seen as one electrode is outside the two fields, but the field of the smaller generator is not well made out.

20. Referential Vs Bipolar (longitudinal vs transverse)
• Both are equally important.
• Neither is superior; they are spatial filters that emphasize different
aspects of the scalp topography of EEG voltage fields.
• Longitudinal and transverse bipolar montages are both used
because scalp voltage fields can be asymmetric along either
anterior to posterior or transverse axis.
• To assess such asymmetries, we need to use both a longitudinal
and a transverse bipolar montage, or by using one bipolar and
reference montage.

21. Spatial filters: Montages
Longitudinal bipolar montage
Transverse bipolar montage
Laplacian montage
Average reference montage
Common reference montage
Maximumfilteringofwidespreadfields
Maximumenhancementofhighlylocalisedfields

22. EEG scalp localisation
1. The identification and classification of EEG activity is always
based to some degree on localisation.
2. Localisation can help determine if an activity is cerebral in
origin or an artefact.
3. Clinical correlation of an EEG requires cortical localization of
the EEG findings.
4. Localization helps in correctly interpreting an EEG pattern the
reader has not encountered before.

23. The EEG signal
• Localization consists of 2 steps:
1. Localising EEG activity to specific electrode positions on the
scalp
2. Relating the scalp localization to the likely source in the
underlying cerebral cortex.

25. Localising current fields: Bipolar montage
• The electrode with the highest voltage potential is found by:
1. Instrumental phase reversal.
2. Instrumental phase reversal with cancellation in an intervening
derivation.
3. End of chain phenomenon or the absence of phase reversal.
4. End of chain phenomenon with cancellation.
5. True phase reversal (double phase reversal).

26. Bipolar Vs Referential
• Bipolar montage: localisation is accomplished by analysing phase
orientation and amplitude.
• Referential montage: localisation is mainly by analysing
amplitude.
• Phase refers to the upward or downward deflection of the
waveform from the baseline.
• Phase reversal has occurred if the phase has changed direction
from up to down or from down to up.

27. Instrumental phase reversal
Fp1-F3
F3-C3
C3-P3
P3-O1
The electrode common to both channels that reverse, phase localises the maximum field potential to
that common electrode (e.g. F3). Since it is the way the electrodes are arranged with the input 2
electrode becoming Input 1 electrode in the next channel that causes the phase reversal, it is called
instrumental phase reversal.

28. Bipolar longitudinal montage showing instrumental phase reversal across C3, and also frequent
spikes at P3 suggesting that C3 and P3 are closest to the field maxima.

29. Bipolar longitudinal montage showing instrumental phase reversal across F3, suggesting that F3
is closest to the field maxima.

30. Sharp
waves at
F3,C3,
Bipolar longitudinal montage showing instrumental phase reversal across F3, suggesting that F3
is closest to the field maxima.

31. Instrumental phase reversal with cancellation in an
intervening channel.
Fp1-F3
F3-C3
C3-P3
P3-O1
The intervening channel with cancellation (F3-C3) localises the field maximum between the
electrodes in that channel.

32. Bipolar longitudinal montage showing instrumental phase reversal across C3, and also phase
reversal with cancellation at T3-T5.

33. End of chain phenomenon.
Fp1-F3
F3-C3
C3-P3
P3-O1
If no phase reversal is seen, then the field maxima is closest to the electrode with the
maximum amplitude. It is localising to the electrode at the end of the electrode chain and
does not suggest the source is deep in the brain.

34. End of chain phenomenon with cancellation.
Fp1-F3
F3-C3
C3-P3
P3-O1
The field maxima is localised between the electrodes Fp1 and F3 at the end of the bipolar
chain. The rate of change of voltage (amplitude) between two electrodes is usually greatest
near the field maxima and least at a distance from it. The presence of a higher amplitude in
channel 2 than in 3 localises the field maxima to the front of the head.

35. True phase reversal
• This is rare and is usually seen in BCECTS or in conditions with
altered cortical anatomy.
• It means that two different polarities are simultaneously present in
adjacent cortical areas.
• The epileptiform discharges arise at an angle that is parallel to the
cortical surface (tangential dipole), allowing both the negativity
from the outer cortex and positivity from the deeper cortex to be
seen simultaneously.

36. Referential montage showing frequent spikes with frontal positivity and negativity in the central,
parietal and temporal channels, consistent with BCECTS (tangential dipole).

37. Referential montage of a 5 year old boy with BCETS: The positivity projects anteriorly and
negativity more posteriorly. Anterior positivity is lower in amplitude than the posterior negativity.
This would produce two instrumental phase reversals in a longitudinal bipolar chain.

38. Common reference montage
Fp1-A1
F3-A1
C3-A1
P3-A1
The channel with the highest amplitude identifies the input 1 electrode closest to the field
maxima (F3). In this example, reference electrode A1 is outside the scalp field.

39. Referential montage showing frequent spikes with maximum amplitude at P3 and C3, localising
the field maxima to these electrodes.

40. Referential montage showing frequent spikes with maximum amplitude at T6, and smaller
amplitude at O2, localising the field maxima to the T6 electrode.

41. Referential montage showing generalised polyspike and wave discharges with maximum
amplitude at F3, F4, Fp1 and Fp2.

42. Bipolar Referential montage showing generalised polyspike and wave discharges with maximum
amplitude at F3, F4, Fp1 and Fp2. Few leading focal spikes (frontal) are seen

43. Common reference montage with reference
contamination
Fp1-A1
F3-A1
C3-A1
P3-A1
The reference electrode A1 is within the positive scalp field, whereas the other scalp electrodes
are outside. Therefore, the deflection is each channel is the same, and is upward, because for
each channel, input 1 is more negative than input 2.
+

44. Common reference montage showing waveforms of similar morphology and amplitude in all the
channels suggesting reference contamination. Changing the reference to another location may
eliminate this abnormality.

45. Common reference montage with reference
contamination
Fp1-A1
F3-A1
C3-A1
P3-A1
The reference electrode A1 is contaminated with activity also detected by the Input 1
electrodes. A field with a single polarity will not produce waveforms of opposite phase in a
referential recording between adjacent channels unless the reference is contaminated. Two
phase reversals are pathognomonic of reference electrode contamination.

46. Conclusion
• EEG is largely a graphic representation of summated PSPs of
pyramidal neurons of the cerebral cortex.
• The pyramidal dipole is recorded on the scalp, mainly a radial
dipole, but sometimes it is a tangential dipole.
• Bipolar montages identify localised fields, referential montages
record widespread fields.
• Surface negativity is recorded as upward deflection in input 1.
• For source localisation, phase reversal with or without cancellation
is used in bipolar montage, and maximum amplitude is used in
referential montages.

47. THANK YOU