Data were analyzed off-line using EEGLAB (Delorme and Makeig, 2004, http://www.sccn.ucsd.edu/eeglab), and the CSD toolbox (Kayser and Tenke, 2006; Kayser, 2009) running under Matlab 7.4 (The Mathworks, Inc.). A number of pre-processing steps were performed on the data before applying either ICA or CSD interpolation. First, the data were high-pass filtered (1 Hz) to minimize drift. Then the number of channels was pruned from 128 to 64. Pruning was necessary in order to improve the quality of ICA decomposition, given the relatively small amount of data recorded (∼5 min). Initially channels that showed noise artifacts due to poor connection to the scalp were deleted, then channels were removed if they showed high kurtosis, finally, additional channels that showed the smallest inter-electrode distance were removed until 64 relatively evenly spaced electrodes remained. In some cases a small number of additional channels were deleted by the experimenter if any noise artifacts on any particular channel were still visible. (Given the small amount of data recorded there was a bias toward rejecting channels rather than portions of data in order to facilitate ICA.)
