A novel 'dip-in electrode' method for electrode application to record noninvasive scalp electroencephalograms and evoked potentials in an awake common marmoset

Noninvasive electroencephalogram (EEG) in unanesthetized nonhuman primates is useful for basic science and biomedical applications but presents several technical challenges. In particular, multiple EEG electrode application to the scalp of small-sized monkeys, such as common marmosets, has been difficult, if not impossible. Here, we describe a novel approach, using the “dip-in electrode” method, which enables multichannel noninvasive EEG recording in a marmoset through short and small silicone tubes, with an internal diameter of 4 mm. First, the cut face of the tube was glued to the shaved scalp. Then, the tube was filled with the electrode gel, and a small electrode (4 mm in diameter) was dipped into the gel to contact the skin electrically while being electrically isolated from the neighboring channels. The minimum inter-electrode distance was 6 mm, which was limited by the outer diameter of the tubes. A proof of concept was provided through successful 32-channel recording of scalp EEG and topographic mapping of the cortical auditory evoked potentials in an unanesthetized marmoset. This had not been possible with the existing methods using gauze or tape for electrode fixation. Our method provides a feasible means for noninvasive scalp recording of EEG and evoked potentials in marmoset monkeys and possibly other nonhuman primates and non-primate mammals.