Magnetoencephalography measurement of somatosensory responses in optogenetic rats using optically pumped magnetometer

The aim of this study was to evaluate the effectiveness of optical stimulation in optogenetically modified rats for magnetoencephalographic (MEG) measurements in small animals. Inducing such responses through external stimulation and recording them with MEG is important for elucidating the fundamental brain mechanisms. Various stimulation methods, including electrical stimulation, have been proposed, but obtaining somatosensory responses in small animals with MEG remains difficult due to mechanical noise and electromagnetic interference. In this work, we employed optical stimulation of whiskers in channelrhodopsin-2 (ChR2)-expressing rats. Optical stimulation provides precise control, localized delivery via optical fibers, and nonmagnetic compatibility, making it well suited for MEG measurements. We used optically pumped magnetometers (OPMs), which offer the advantage of operating without a helium-based cryogenic cooling system, unlike conventional superconducting quantum interference device SQUID sensors. A custom jig was developed to construct a gradiometric measurement system with five OPM sensors inside a magnetic shield room. Whiskers of ChR2 rats were stimulated with blue light (473 nm), and brain magnetic fields were recorded at 64 locations with 6.25 mm spacing. Averaged responses revealed somatosensory-evoked magnetic fields of several hundred fT at 50 ms latency, yielding reliable dipolar field maps over a 5 × 5 cm2 area.