Second edition

01.qxd 3/10/08 9:33 AM Page 27
illustrated in Figure 1.5, there are five channels to consider:
F p1�A 1, F 3�A 1, C 3�A 1, P 3�A 1, and O 1�A 1. Channel
F p1�A 1, with both electrodes ‘seeing’ the same potential,
would register no difference; channel F 3�A 1 would see a
difference of 25 μV (i.e., looking up from a depth of �50 to
the surface, which is at �25); channel C 3�A 1 would see a
difference of 75 μV (looking up from a depth of �100 to
the surface at �25); channel P 3�A 1 a difference of 25 μV
(looking up from �50 to �25), and the last channel,
O 1�A 1, with both electrodes seeing �25 μV, would register
0. As seen in Figure 1.4, the greatest pen deflection is
seen in the channel containing the electrode, in this example
C 3, which lies over the deepest part of the electrical
chasm. Thus, with referential recordings, it is the channel
showing the greatest amplitude that serves to localize the
focus of the electrical paroxysm.
It may be noted that the pen deflections in channels
F 3�A 1, C 3�A 1, and P 3�A 1 are all positive, and this is
according to the convention (Knott 1985) that whenever,
in going from the first to second lead of any channel, one is
A 1
T 3
F p1 –A 1
F 3 –A 1
C 3 –A 1
P 3 –A 1
O1–A1
F 7
T 5
F 3
F p1
F z
F 4
F 8
C3 Cz C4 T4 P3
O 1
Figure 1.5 Referential recording of the epileptiform discharge
shown in Figure 1.4 (see text for details).
P z
F p2
O 2
P 4
T 6
A 2
1.5 Electroencephalography 27
‘looking’ up, the pen goes up, but if one is ‘looking’ down
the pen likewise goes down.
The situation with bipolar recordings is quite different:
here, it is not amplitude that is important but a phenomenon
known as phase reversal (Knott 1985; Lesser 1985).
Take the same example of an electrical paroxysm as used
above, but this time cover it, as illustrated in Figure 1.6,
with a longitudinal chain of electrodes, starting at F p1 and
including, sequentially, F 3, C 3, P 3, and O 1. Then construct
the following channels: F p1�F 3, F 3�C 3, C 3�P 3, and,
finally, P 3�O 1. Now consider what each channel will
record. For channel F p1�F 3, one looks down from F p1 at
�25 to F 3 at �50, for a difference of �25 μV. For the next
channel, F 3� C 3, one continues to look down into the electrical
chasm, now looking down from �50 to �100, for a
difference of �50 μV. At the next channel, C 3�P 3, however,
something very different happens; here, standing at
the nadir of the chasm at �100, one is looking ‘up’ to �50,
for a difference of �50 μV. Similarly, for the next channel,
P 3�O 1, one continues to look up, but here from �50 to
�25, for a difference of �25 μV.
Figure 1.6 shows the various pen tracings seen for each
channel. As may be noted, both channels F p1�F 3 and
A 1
T 3
F p1 –F 3
F 3 –C 3
C 3 –P 3
P 3 –O 1
F 7
T 5
F 3
F p1
F z
F 4
F 8
C3 Cz C4 T4 P3
O 1
Figure 1.6 Bipolar recording of the epileptiform discharge
shown in Figure 1.4 (see text for details).
P z
F p2
O 2
P 4
T 6
A 2