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

Abstract

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.

Keywords: Animal model; Electrophysiology; Event-related potential; Evoked potential; Macaque; Marmoset; Topographic mapping.

Fig. 1

Schematics of the (A) conventional and (B) novel “dip-in electrode” methods for EEG electrode application. The new method requires substantially less space per electrode and thus enables a high-density recording of scalp EEG. The diagrams are drawn approximately to scale. EEG: electroencephalogram.

Fig. 2

(A) A schematic drawing of the primate chair and the mask that were used to immobilize the animal. (B) The electrode layout. The positions of the midline electrodes are exact with respect to the 10-10 system (Nuwer et al., 1998). The other electrode positions are approximate.

Fig. 3

A representative recording of continuous EEG obtained using the current method. EEG: electroencephalogram.

Fig. 4

The waveforms of the CAEP obtained from a marmoset monkey. The two traces (solid and dotted) represent different recording sessions. The inset shows the CAEP from the same animal that was recorded over 1 year ago, as reported by Itoh et al. (2021). CAEP: cortical auditory evoked potential.

Fig. 5

Topographic maps of the CAEP components. CAEP: cortical auditory evoked potential.