Where the signal comes from
Scalp EEG records the summed postsynaptic potentials of cortical pyramidal neurons, aligned side by side and perpendicular to the surface. It does not record action potentials. For a discharge to reach the scalp, a patch of cortex has to fire in synchrony: roughly 10 cm² in simultaneous intracranial and scalp recordings. 15
EEG sees the cortical convexity best. Deep and mesial sources (mesial temporal, orbitofrontal, interhemispheric, insular cortex) can generate seizures with little or nothing on the scalp. A normal EEG does not exclude epilepsy.
The 10-20 system
Electrodes sit at 10% and 20% intervals of two measured distances: nasion to inion, and one pre-auricular point to the other. 13
- Letters name the region: Fp frontopolar, F frontal, C central, T temporal, P parietal, O occipital.
- Odd numbers are left, even numbers are right, and z marks the midline (Fz, Cz, Pz).
- The current nomenclature uses T7/T8 and P7/P8 in place of the older T3/T4 and T5/T6. You will meet both.
Montages: the same brain, viewed differently
Every channel is the difference between two inputs. By convention, if input 1 is more negative than input 2, the trace goes up. A montage is simply the list of pairs. 14
Bipolar montages link neighbouring electrodes in chains. The longitudinal bipolar montage ("double banana") runs front to back. A focal discharge is located by phase reversal: the two channels that share the electrode nearest the source deflect towards each other.
Referential montages compare each electrode with a common reference (the average of all electrodes, Cz, or linked ears). The largest deflection is at the electrode nearest the source, and amplitude reads more faithfully, but anything that contaminates the reference appears in every channel.
A bipolar channel is flat when its two electrodes see the same potential. Flat does not mean nothing is happening: it may mean you are sitting on the middle of a wide field. Change montage before deciding.
Filters and display
These are the usual starting settings; ACNS minimum standards ask that the low-frequency filter not exceed 1 Hz and the high-frequency filter not be below 70 Hz for routine recording. 12
- Lowering the high-frequency filter to 35 or 15 Hz rounds off spikes and turns muscle artefact into something that resembles beta or spikes.
- Raising the low-frequency filter to 5 Hz removes sweat and movement drifts, and with them real delta activity.
- A higher sensitivity number makes the trace smaller: 7 µV/mm means 7 µV moves the trace one millimetre.
Open the muscle artefact and mains traces in the viewer and move the filters. Seeing a filter manufacture a clean-looking trace teaches more than a page of warnings.