A practical method for quickly determining electrode positions in high-density EEG studies

This report describes a simple and practical method for determining electrode positions in high-density EEG studies. This method reduces the number of electrodes for which accurate three-dimensional location must be measured, thus minimizing experimental set-up time and the possibility of digitization error. For each electrode cap, a reference data set is first established by placing the cap on a reference head and digitizing the 3-D position of each channel. A set of control channels are pre-selected that should be adequately distributed over the cap. A simple choice could be the standard 19 channels of the International 10-20 system or their closest substitutes. In a real experiment, only the 3-D positions of these control channels need to be measured and the position of each of the remaining channels are calculated from the position data of the same channels in the reference data set using a local transformation determined by the nearest three or four pairs of control channels. Six BioSemi ActiveTwo caps of different size and channel numbers were used to evaluate the method. Results show that the mean prediction error is about 2mm and is comparable with the residual uncertainty in direct position measurement using a Polhemus digitizer.