The five percent electrode system for high-resolution EEG and ERP measurements

Abstract

Objective: A system for electrode placement is described. It is designed for studies on topography and source analysis of spontaneous and evoked EEG activity.

Method: The proposed system is based on the extended International 10–20 system which contains 74 electrodes, and extends this system up to 345 electrode locations.

Results: The positioning and nomenclature of the electrode system is described, and a subset of locations is proposed as especially useful for modern EEG/ERP systems, often having 128 channels available.

Conclusion: Similar to the extension of the 10–20 system to the 10–10 system (‘10% system’), proposed in 1985, the goal of this new extension to a 10–5 system is to further promote standardization in high-resolution EEG studies.

Introduction

As early as the first International EEG congress, held in London in 1947, it was recognized that a standard method of placement of electrodes used in electroencephalography (EEG) was needed. Possible methods to standardize electrode placement were studied by H.H. Jasper, which resulted in the definition of the 10–20 electrode system (Jasper, 1958). Since then, the 10–20 electrode system has become the de facto standard for clinical EEG. It has also proven to be very useful in the study of event-related potentials (ERPs) in non-clinical settings. However, the advancement of topographic methods to study spontaneous and evoked potentials, and the advancement of multi-channel EEG hardware systems necessitated the standardization of a larger number of channels. Therefore, in 1985 an extension to the original 10–20 system was proposed which involved an increase of the number of electrodes from 21 up to 74 (Chatrian et al., 1985). This extended 10–20 system of electrode placement, also known as the ‘10% system’ and referred to as 10–10 system hereafter, has been accepted and is currently endorsed as the standard of the American Electroencephalographic Society (Klem et al., 1999, American Electroencephalographic Society, 1994) and the International Federation of Societies for Electroencephalography and Clinical Neurophysiology (Nuwer et al., 1998).

Laboratories pursuing EEG and ERP studies of brain activity nowadays have the possibility of using a greater number of channels than the original 21. Measuring ERPs with 64 channels has become quite common. This number of channels can be accommodated within the 74 locations in the 10–10 system. Manufacturers of EEG supplies have recognized this, and electrode caps which enable easy placement of the electrodes according to the 10–10 standard are available. Currently, more and more researchers are moving to an even higher number of channels, and EEG acquisition systems with 128 channels are not uncommon any more. Even 256 channel EEG systems are commercially available now (Pflieger and Sands, 1996, Suarez et al., 2000).

Although measurements of high-resolution EEG and ERP scalp distributions with 128 channel systems are being carried out already, there is at present no standard for the placement of this number of electrodes. For the purpose of mapping the potential distribution, or performing source analysis on this distribution, the individual electrode system of a laboratory and even any electrode placement on an individual subject is suitable as long as the positions are known accurately, e.g. by measuring them using a 3D tracker. However, to compare the EEG recordings on individual channels between laboratories for meta-analysis and to enable comparison with older studies using a smaller number of electrodes, standardized electrode locations are needed. This need is recognized by many laboratories, and in measurements with 128 EEG channels or more, a subset of these electrodes usually will correspond to the standard 10–20 system. In fact, a recommendation to identify within dense electrode arrays a number of landmark electrodes that correspond to standard sites within the 10–20 system was recently formulated in the Guidelines for human event-related potentials studies by the Society for Psychophysiological Research (Picton et al., 2000). Nevertheless, reporting on, and comparing EEG from channels other than the standard 74 channels of the 10–10 system is not easily possible. Standardization of the location of additional measurement sites would therefore be an improvement.

Currently, projects are being pursued by the International Consortium for Brain Mapping (ICBM) and in the context of the European Computerized Human Brain Database (ECHBM) to create tools and databases for the neuroscientific community to share the results of functional and anatomical neuroimaging research. These projects rely on the exchange of well-defined neuroimaging data, which for the case of EEG would be improved by further standardization of the increasing numbers of recording sites.

To our knowledge, no standard system suitable for describing the locations of scalp potential measurements at 128 or more locations is currently available. In this paper, we would like to propose such a system. The electrode system which we describe contains the standard locations of the original 10–20 system and those of the 10–10 system. Our proposal is a logical extension of the 10–10 system, enabling the use of up to 345 electrode locations. As the system uses proportional distances of 5% of the total length along contours between skull landmarks, compared to the 20 and 10% distances used in the 10–20 and 10–10 systems, respectively, we call it the 5% system or the 10–5 system.