EEG topogram--an aid in describing EEG to the clinician

The investigation presents a simple, reliable method to show localized EEG abnormalities in a topographical display. The aim of the study was to find a method which could be used in clinical practice, and which in topographical form could complement the written EEG report with information about degree and localization of an EEG abnormality. The EEG was recorded on a 21-channel polygraph (Siemens-Elema). The source derivation recording technique was used. Besides being displayed as a conventional EEG record, the signals were fed to a small computer that calculated an abnormality feature based on the time domain descriptors activity, mobility and complexity. Obtained values were converted into ink density in topograms by modulating the displacement velocity of the ink jet beam. The dynamic range was chosen so that maximum and minimum densities represented 1.5 and 0.5 times the average for each descriptor. The ink density in the topogram was thus related to the degree of EEG abnormality. The recording procedure followed the ordinary clinical routine except that the conventional EEG display was interrupted several times for a few seconds, on command by the EEG technician, to write out the topogram on the EEG paper. It is concluded that EEG topograms produced on line as part of the EEG examination procedure may greatly enhance EEG information conveyed from the neurophysiologist to the clinician. Furthermore, they permit direct visual comparison with other diagnostic topographic displays such as CT scan.