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Summary: Background: The International 10-20 system for EEG electrode placement is increasingly applied for the positioning of transcranial magnetic stimulation (TMS) in cognitive neuroscience and in psychiatric treatment studies. The crucial issue in TMS studies remains the reliable positioning of the coil above the skull for targeting a desired cortex region. In order to asses the precision of the 10-20 system for this purpose, we tested its projections onto the underlying cortex by using neuronavigation. Methods: In 21 subjects, the 10-20 positions F3, F4, T3, TP3, and P3, as determined by a 10-20 positioning cap, were targeted stereotactically. The corresponding individual anatomical sites were identified in the Talairach atlas. Results: The main targeted regions were: for F3 Brodmann areas (BA) 8/9 within the dorsolateral prefrontal cortex, for T3 BA 22/42 on the superior temporal gyrus, for TP3 BA 40/39 in the area of the supramarginal and angular gyrus, and for P3 BA 7/40 on the inferior parietal lobe. However, in about 10% of the measurements adjacent and possibly functionally distinct BAs were reached. The ranges were mainly below 20 mm. Conclusion: Using the 10-20 system for TMS positioning is applicable at low cost and may reach desired cortex regions reliably on a larger scale level. For finer grained positioning, possible interindividual differences, and therefore the application of neuroimaging based methods, are to be considered.
Key words: International 10-20 EEG system; Cortex anatomy; Stereotaxic neuronavigation; Transcranial magnetic stimulation.
Introduction
The International 10-20 system (Jasper 1958) is commonly used for EEG electrode placement and for correlating external skull locations to underlying cortical areas. In the last years, the 10-20 system has been increasingly applied for coil positioning in transcranial magnetic stimulation (TMS) studies. TMS has been established for studying non-invasively cortical information processing in cognitive neuroscience (Walsh et al. 2000), and its therapeutic potential in psychiatry and neurology is subject of intensive investigation (Wassermann and Lisanby 2001). These applications share the basic issue of a reliable positioning of the magnetic coil above a desired cortex region in order to influence the local neuronal activity by the induced electromagnetic field. The 10-20 system is conventionally based on the identification of anatomical landmarks like nasion, inion and preauricular points with consecutive placement of the electrodes at fixed distances...