10/20, 10/10, and 10/5 systems revisited: Their validity as relative head-surface-based positioning systems

Abstract

With the advent of multi-channel EEG hardware systems and the concurrent development of topographic and tomographic signal source localization methods, the international 10/20 system, a standard system for electrode positioning with 21 electrodes, was extended to higher density electrode settings such as 10/10 and 10/5 systems, allowing more than 300 electrode positions. However, their effectiveness as relative head-surface-based positioning systems has not been examined. We previously developed a virtual 10/20 measurement algorithm that can analyze any structural MR head and brain image. Extending this method to the virtual 10/10 and 10/5 measurement algorithms, we analyzed the MR images of 17 healthy subjects. The acquired scalp positions of the 10/10 and 10/5 systems were normalized to the Montreal Neurological Institute (MNI) stereotactic coordinates and their spatial variability was assessed. We described and examined the effects of spatial variability due to the selection of positioning systems and landmark placement strategies. As long as a detailed rule for a particular system was provided, it yielded precise landmark positions on the scalp. Moreover, we evaluated the effective spatial resolution of 329 scalp landmark positions of the 10/5 system for multi-subject studies. As long as a detailed rule for landmark setting was provided, 241 scalp positions could be set effectively when there was no overlapping of two neighboring positions. Importantly, 10/10 positions could be well separated on a scalp without overlapping. This study presents a referential framework for establishing the effective spatial resolutions of 10/20, 10/10, and 10/5 systems as relative head-surface-based positioning systems.

Introduction

The international 10/20 system has stood as the de-facto standard for electrode placement used in electroencephalography (EEG) for half a century. This system describes head surface locations via relative distances between cranial landmarks over the head surface. The primary purpose of the 10/20 system (Jasper, 1958) was to provide a reproducible method for placing a relatively small number (typically 21) of EEG electrodes over different studies, and there was little need for high spatial resolution and accurate electrode placement.

With the advent of multi-channel EEG hardware systems and the concurrent development of topographic methods and tomographic signal source localization methods, there was an increased need for extending the 10/20 system to higher density electrode settings. Therefore, the 10/10 system, an extension to the original 10/20 system with a higher channel density of 81, was proposed (Chatrian et al., 1985; see Supplementary material 2 for details). After some arguments on the nomenclature of electrode positions (Nuwer, 1987), its modified form has also been accepted as a standard of the American Clinical Neurophysiology Society (ACNS; former American Electroencephalographic Society; Klem et al., 1999, American Electroencephalographic Society, 1994) and the International Federation of Clinical Neurophysiology (IFCN; former International Federation of Societies for Electroencephalography and Clinical Neurophysiology; Nuwer et al., 1998). However, high-end users sought even higher density electrode settings. 128 channel systems are now a common commercial choice, and even 256 channel EEG systems are commercially available (Suarez et al., 2000). Thus, Oostenveld and Praamstra (2001) logically extended the 10/10 system to the 10/5 system, enabling the use of more than 300 electrode locations (320 were described explicitly).

In the meantime, the 10/20 system’s primary use began to shift from simply providing guidance for placing EEG electrodes to being used for direct positional guidance for newly developing transcranial neuroimaging techniques, near-infrared spectroscopy (NIRS; Okamoto et al., 2004a, Okamoto et al., 2004b), and transcranial magnetic stimulation (TMS; Herwig et al., 2003). Use of the 10/20 system allows reproducible probe or coil settings on scalps of multiple subjects.

Moreover, the 10/20 system serves as the standard cranial landmarks for mediating probabilistic registration (Okamoto et al., 2004a, Okamoto and Dan, 2005, Singh et al., 2005, Tsuzuki et al., 2006). In a series of previous papers, we established a method to probabilistically register any given scalp position to the corresponding scalp or cortical point in standard stereotaxic brain coordinate systems such as MNI (Montreal Neurological Institute) and Talairach systems without the use of MR images of a subject. Since these stereotaxic brain coordinates serve as the common spatial platform for data presentation of conventional tomographic neuroimaging techniques including fMRI and PET (Collins et al., 1994, Talairach and Tournoux, 1988; reviewed in Brett et al., 2002), the registration of stand-alone multi-subject fNIRS and TMS data to a brain template in the MNI standard coordinate system facilitates both intra- and inter-modal data sharing within the neuroimaging community. Therefore, the 10/20 system has been gaining importance as a standard relative head-surface-based positioning method for various transcranial brain mapping methods.

However, it is also true that the original 10/20 system has not been equipped as a versatile system to fully support such unexpected applications. In the process of developing high density settings, the 10/20-derived systems have been mainly appreciated as methods to increase spatial resolution for EEG studies, where more densely positioned electrodes are proven to be effective in increasing the spatial resolution when the three-dimensional signal source estimation is applied (Pascual-Marqui et al., 2002). Meanwhile its aspect as a relative head-surface-based positioning system has not been examined well. In particular, how effectively high-resolution derivatives of the 10/20 system can separate each cranial landmark, which is especially important for head-surface-based positional estimation in TMS and NIRS, still remains unknown. Therefore, we will evaluate the effective spatial resolution of the 10/20, 10/10, and 10/5 systems for multi-subject studies. We will focus on two sources of variability. First, definitions of landmark placement in the original 10/20 system by Jasper (1958) are ambiguous, and this results in different interpretations among experimenters and variability among studies. Second, even if a fixed definition of landmark placement is used, scalp and cortical anatomies are different among subjects and this results in inter-subject variability.

To evaluate variability, we performed virtual 10/20, 10/10, and 10/5 measurements on MR images that we described previously. Subsequently, we transformed all the scalp data to MNI space and statistically assessed the spatial variability. In so doing, we sought to assess the potential of 10/20, 10/10, and 10/5 systems as relative head-surface-based positioning systems.