Selection of the control channels in a method for rapid determination of channel positions in high-density EEG studies

We have developed a practical method for rapid determination of electrode positions in high-density EEG studies. For each electrode cap, a reference data set is first established by placing the cap on a reference head and measuring the 3-D position of each channel. A set of control channels are preselected that can simply be the standard 19 channels or their closest substitutes. During the experiment, only the 3-D positions of these control channels need to be measured and the position of each of the remaining channels is calculated from the position of the same channel in the reference data set using a local transformation determined by a subset (3 or 4) of nearest control channels. Six BioSemi ActiveTwo caps of different sizes and channel numbers were used to study the effects of the choice of the control channels on prediction errors. Our results show that by including the most peripheral channels in the set of control channels and by using 4 nearest control channels to determine the local transformation, the method produces a mean prediction error of about 2 mm and a maximum prediction error less than 7 mm.