A Primer on EEG Reading

Visual EEG analysis remains an indispensable clinical skill, yet its systematic
foundations are rarely taught explicitly. This primer conveys the core principles
of EEG description and interpretation for clinicians and researchers entering
the field. The emphasis throughout is on the physical and physiological basis of
the signals. Foundational topics include the structure of an EEG report, the role
of age and vigilance, the differential amplifier as the basis of all EEG recording,
and the principles of referential and bipolar montages. Beyond the conven­
tional descriptors of amplitude and frequency, the primer introduces shape as
a third descriptive axis. Two simple symmetries — up/down and temporal —
capture it, and their breaking is often what is clinically informative. Such sym­
metry-breaking can in turn be read as the visible trace of the brain’s nonlinear
computation. A short physical detour follows. It traces EEG oscillations to the
excitation–inhibition balance from which they arise, and their propagation
across the cortex to the local connectivity imposed by evolutionary wiring
economy. The cortical sheet thus becomes a medium in which travelling waves
act as a real-time carrier of spatiotemporal context. Particular attention is then
given to phase reversal — arguably the most important concept in visual EEG
localisation — including its four subtypes and their clinical significance. The
primer concludes with an outlook toward quantitative EEG analysis. The spa­
tial organisation of cortical signals, understood through cortical eigenmodes,
carries clinically relevant information that visual analysis alone cannot access.
The text is intended as a companion to independent EEG study, not a substitute
for it, and will be updated as the field develops.