Montages and
Polarity
D R . R A H E E L A H M E D
N E U R O L O G I S T &
C L I N I C A L N E U R O P H Y S I O L O G IST

Designations
➢Each electrode in EEG is designated by a letter & No
➢Letter: area of the underlying brain region
➢Number: right/left
Even numbers: the right side
Odd numbers: denote the left side
“z” midline placement
(i.e., Cz = central midline, Pz, Fz)

➢In EEG, an electrode pair in an amplifier is called a derivation
➢DERIVATIONS has input 1 and input 2
Input 1
Input 2
➢Voltage difference between two electrodes is seen as EEG signal

MONTAGE
➢The Combinations of derivations is called montage
➢Modern digital EEG machines provides the facility to redesign
montages of prerecorded tracings
Objectives:
➢To compare activity from homologous electrodes between two
hemispheres
➢To localize the electrical activity/ epileptogenic focus

Types of Montages
There are two basic montages
1. Bipolar Montages:
➢ Adjacent electrodes are paired and that can be in a longitudinal,
circumferential or transverse lines
➢Input 2 becomes input 1 of the next channel
2. Referential Montages/ monopolar:
➢Input 1 that is active and the input 2 electrode is “considered the
reference”
➢Input 2 is common to all derivations/channels

Bipolar Montages
➢Each channel has input 1 and 2
➢Input 2 becomes input 1 of next channel
➢EEG signal is displayed as the potential
difference b/w 2 inputs

Types of Bipolar montage
➢Commonly used bipolar montages are:
1. Longitudinal Bipolar Montage
2. Transverse Bipolar Montage
3. Circumferential or hatband bipolar montage

Longitudinal Bipolar Montage:

Transverse Montage
➢Runs crosswise, across the head from L to R
Fp1-Fp2
F7-F3
P3-Pz
Pz-P4
F3-Fz
Fz-F4
F4-F8
T3-C3
C3-Cz
Cz-C4
C4-T4
T5-P3

➢The localization criteria for bipolar
montages is “Phase Reversal”
➢If any activity shows phase reversal
between two channels, the activity is
maximum at the electrode that is
commonto both channels

Referential/monopolar Montage
➢ Input 1 is active electrode
➢ Input 2 is a reference electrode that is common to all
derivations/channels
➢Common referential montages are:
i. Average reference Montage
ii. Ear reference montage(ipsilateral/contralateral)
iii. Cz reference montage
➢ The criteria for localization is Amplitude in referential montages

Localization criterion

Average reference montage
➢In this activity from all the electrodes are measured, summedtogether
and averaged
➢The resulting signal is than used as a reference electrode and connected
to input 2 of each amplifier

POLARITY
➢EEG is graphical representation of difference in voltage between two
scalp locations i.e electrodes
➢Every deflection indicates a change of voltage
➢Negative potential displays upward deflection
➢Positive potentials shows downward deflection
➢All the epileptiform discharges are negative and has a field

POLARITY
➢ Deflection will be up if input 1 is relatively more negative
Or if input 2 is relatively more positive
➢ Deflection will be down if input 2 is relatively more negative Or
if input 1 is relatively more positive

End of chain phenomenon
➢When all deflections move in same direction and there is no phase
reversal
➢When the potential is maximum at input 1 of the first channel or input
2 of the last channel in bipolar montage
➢To complete the phase reversal additional electrodes are needed or
montagemay be changed

Thanks