EEG topography

EEG topography is a neuroimaging technique in which a large number of EEG electrodes are placed onto the head, following a geometrical array of even-spaced points. A special software inside the apparatus' computer, plots the activity on a color screen or printer, by coding the amount of activity in several tones of color (for example, black and blue might depict low EEG amplitude, while yellow and red might depict larger amplitudes). The spatial points lying between electrodes are calculated by mathematical techniques of interpolation (calculating intermediary values on the basis on the value of its neighbors), and thus a smooth gradation of colors is achieved.

This approach gives a much more accurate and representative view of the location of alterations of rhythm, amplitude, etc., in relation to the surface of the skull. Neurologists working with the EEG brain topographic system were soon able to differentiate several kinds of diagnoses (including some mental diseases whose biological origin, or etiology, was previously unknown). Pinpointing the exact location of EEG alterations was also made much easier. In addition, the use of the cinè mode (animations using several sequential pictures taken from the brain maps) made possible the dynamic study of brain function in action.

History
EEG brain topography was invented by Walter Grey Walter, who, in 1936, proved that, by using a larger number of electrodes pasted to the scalp, each one having a small size, and a triangulation algorithm, it was possible to identify abnormal electrical activity in the brain areas around a tumor, and diminished activity inside it. Impressed with the possibilities of building bidimensional maps of the EEG activity over the brain surface, Grey Walter invented the toposcope in 1957.

This was a remarkably complex device and showed Grey Walter's inventiveness (besides being a physician, he was also an engineer). It had 22 cathode ray tubes (similar to a TV tube), each of them connected to a pair of electrodes attached to the skull. The electrodes (and their corresponding tubes) were arranged in a bidimensional geometrical array, such as that each tube was able to depict the intensity of the several rhythms which compose the EEG in a particular area of the brain (the frontal, parietal and occipital lobes, etc.). This array of CRT tubes, were photographed face up, so that a kind of phosphorescent spiral display showed simultaneously which kind of rhythm was present in a particular part of the brain.

Medical applications
EEG brain topography is not performed in all cases requiring a recording of the brain activity. Initially its main indication was to determine the presence of tumors and focal disease of the brain (including arteriovenous mal-formations and stroke), but today other imaging technologies have replaced the EEG to describe changes in brain structure. There are several neurosurgical procedures that rely on topographic EEG to localize electro-functional problems, such as epilepsy, before and during surgery. It is also appropriate when disturbances in consciousness and vigilance are present, such as narcolepsy (the abrupt onset of sleep), coma, etc.. In addition, EEG brain topography is being increasingly used to monitor the effects of withdrawal of psychoactive drugs, and in infectious diseases of the brain, such as meningitis, as well as to follow up patients who where subjected to brain operations. In psychiatry, EEG brain topography has been of value in identifying disorders of biological origin, such as schizophrenia, dementias, hyperactivity and depression, brain atrophy and attention deficit disorders in children.

Today, there are many commercial EEG brain topography systems in use. They are generally installed in Windows- or Macintosh-based microcomputer platforms, and can be easily operated by technicians or physicians, due to the graphical software which comes with the package. This software is highly flexible, permitting the programming of many recording configurations and parameters, as well as to build a reference database of images, composed of typical patient cases in several pathologies. Usually, the multichannel records can be shown side by side with the reconstructed topographical brain map. In addition, several maps taken in different epochs can be displayed side by side.

There are several very-well researched databases of normal subjects, with which the physician may accurately determine whether a patient's EEG is abnormal. That research, conducted principally at Harvard, New York University, and the National Institute of Health extends from the 1970's forward and is intensely active today at a number of international sites. A recent development is three-dimensional analysis, allowing localization of an EEG source deep within the brain.

Source

 * Renato M.E. Sabbatini, Mapping the Brain. Brain & Mind Magazine, September/November 1997 (reproduced with permission).