EEG recording data reduction and display

The EEG is conventionally recorded from a number of electrodes applied to the scalp in positions determined by measurement according to the International 10-20 System. In an intensive care unit there is inevitably considerable electrical interference from other equipment, and so EEG signals of a few microvolts can only be recorded satisfactorily if the correct electrodes are selected and applied to appropriate areas of the scalp with low contact impedance (less than 5 k at 10 Hz), and all leads between patient and preamplifier are kept as short as possible (Prior..! 985; Prior..§O.d...M§.yO§.rd...1986).

Modern EEG machines record 16 or more channels of data at a paper speed of 3 cm/s. If recording is continued for more than a few hours, an excessive quantity of data is generated, much of which is redundant, and so some form of data reduction is essential for long-term monitoring. Spatial information is usually sacrificed or reduced to a simple comparison between corresponding areas of the right and left hemispheres.

Many methods of data reduction have been developed, each having its own advantages ( Prior..and M.ayn.ard.1986). Several popular methods depend upon frequency analysis by the fast Fourier transform, with the results being displayed as a graph of amplitude (or power) against frequency, often as a compressed spectral array in which successive spectra are plotted one above the other, with suppression of those parts of later spectra which would overwrite earlier ones. This unpredictable loss of data means that the sudden disappearance or abrupt attenuation of a frequency component is not detected until several more spectra have been plotted, resulting in potentially dangerous delay. To avoid this disadvantage of the compressed spectral array, the same data can be displayed as a density-modulated spectral array, in which the darkness (density) of each point along the frequency axis is proportional to the amplitude at that frequency. Such a display is difficult to calibrate and has limited dynamic range, but abrupt attenuation is visible immediately in the density-modulated spectral array, unlike the compressed spectral array. The ultimate data reduction is achieved by extracting from the Fourier transform a single number, such as mean frequency or spectral edge frequency. Excessive data reduction of this sort is potentially dangerous and is not recommended for clinical purposes.

Methods based upon frequency analysis cannot reliably detect the onset of a burst-suppression pattern in the EEG or the occurrence of brief high-amplitude events associated with epileptic seizures. These important EEG patterns are clearly demonstrated by data reduction in the time domain to give an indication of EEG amplitude. This has the additional advantage of retaining the variability of the EEG. The simplest display of this type is a graph of amplitude integrated over a short epoch. One device of this type, the cerebral function monitor, plots the amplitude on a logarithmic scale, thereby ensuring a large dynamic range. A further development, the cerebral function analyzing monitor, also displays the percentage of activity in the conventional EEG frequency bands ( Priorand M,,ayn,a,rd 1986).

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