The signals were bandpass-filtered to 0.03–330 Hz and sampled at 1 kHz. Four head-position-indicator coils were attached to the subject's scalp, and head coordinates were registered with a 3D digitizer by identifying the locations of the coils together with the locations of three anatomical landmarks (nasion and left and right preauricular points) and some additional points on the scalp.
Source: wiktionary