At the end of the nineteenth century (1875) an English medical doctor from Liverpool named Richard Caton (* 1842; † 1926) discovered that the brain of animals produced electricity. His work led German Hans Berger in Jena to discover that also human brains produced potential differences which he was able to measure in 1924. In 1929 he published his first paper on this discovery where he also cited the work of Caton. Since then “encephalography or EEG” as he named this methodology spread all over the world and there is hardly a neurologist who is not familiar with the basic methodology. But the size of these potential changes (1 – 250 mV) is very small and asks for special amplifiers to record them. The methodology of EEG recording has conquered the neurological ambulances and hospitals worldwide and is regarded as a standard diagnostic procedure since then. Nevertheless, interpretation of the electric changes is still a matter of debate. Despite highly sophisticated technical equipment and good artifact-free recordings the physiological meaning of signal changes very often remains unknown. This book aims at the description of current knowledge especially on the interpretation of EEG changes after transformation of the signals from time dependence into frequency dependence by a mathematical procedure called Fast Fourier Transformation (FFT). Already Hans Berger had recognized the usefulness of frequency analysis of his ”Enkephalogramm” , since he published a manuscript together with Dietsch in 1932, when it took him weeks to perform the mathematical calculation without computer aid. Today, FFT is performed by computers online and real time. However, the documentation of the results varies from software to software resulting in more or less easy understandable depictions of the electric brain activity. We have spent extremely much time and money to develop a hardware-software combination in order to present the data in a meaningful and easy interpretable manner. Years of recording of the EEG under a whole variety of physiological and pathological conditions using the newly developed technology “CATEEM” brought new insights into this fascinating activity of the brain which is available non-invasively. Relations of the electric activity to biochemical neurotransmitter activity as well as relations to cognitive and emotional features have been recognized. Also pathological findings can be documented in a more precise way. Finally, quantitative assessment of sleep and anaesthesia has become feasible based on the quantitative description of the EEG using source density analysis.

Classic EEG Recording

In order to measure the potential differences produced by the brain one needs at least two different locations. Preferably, one of those two places is electrically silent. For many recordings the nose or ears are used as one pole. The term “ monopolar” has been used for this type of electrode montage which is however not correct. It should be called “referenced” recording. One uses one ear or even linked ears as reference for one or more other electrode positions. Each of the different locations over the skull as recorded against the reference is called a channel. A disadvantage of this kind of recording is the fact that the distances between several locations and the reference electrode differ quite a bit, making comparisons of the channels among each other difficult. An alternative method consists in recording from two “active” electrodes in order to evaluate the difference between them. This has been called “bipolar” recording. Many different “montages” of this kind have been published with the aim of documenting more or less specific pathological features of brain activity. The disadvantage is that it takes an enormous effort to interpret the particular electric patterns or grapho-elements in relation to disease. Tons of publications and books deal with this kind of EEG analysis. A more reliable and standardized way to document electric brain activity consists in using the so-called common averaged reference like based on the electrode position Cz. In this case, firstly all potential differences from all other electrode positions are measured against Cz and then the averaged signal is taken as reference.

Historically, electrode positions are named according to the position at the skull where C stands for central, F for frontal, T for temporal, P for parietal and O for occipital (Fig. 1). Even numbers like C4 stand for the right hemisphere, uneven numbers like C3 for the left hemisphere. A standard distribution of electrode positions over the skull is called the 10/20 system according to 10 and 20% distances used between single electrodes.