Steady-state visual evoked field measurement using wearable OPM-MEG compared with EEG

Optically pumped magnetometers (OPM) can replace superconducting quantum interference devices (SQUID) as the magnetic sensors in magnetoencephalography (MEG) systems to achieve high spatiotemporal resolution for measuring neuronal activity. OPMs are compact and lightweight, enabling wearable OPM-MEG devices. However, they are susceptible to residual magnetic field gradients within a magnetically shielded room as the sensors are not fixed to the room. Herein, we examined whether a 30 Hz steady-state visual evoked field elicited by the hemifield pattern-reversal stimulation procedure could be measured using 18 channel wearable OPM-MEG system without noise cancelling in six healthy adults. Steady-state responses were observed in channels near the visual cortex on the hemifield opposite the visual field where a checkerboard pattern was presented. In the same environment, we successfully verified the cortical oscillatory response using electroencephalography (EEG). Visual evoked field components (e.g. P75m), which follow stimulus onset and are clearly recorded by EEG, were not observed with OPM-MEG. These components were masked by artifacts at 3-5 Hz, likely generated by slight body movements. Given the motion artifacts in the low-frequency bandwidth, optimal experimental designs for wearable OPM-MEG measurements without noise canceling need to consider targeting cortical oscillation in the frequency band, which is unaffected by artifacts.