Magnetically shielded high-resolution visual stimulation for OPM-MEG applications.

Abstract

Many magnetoencephalography (MEG) experiments require visual stimulation (VS) inside a magnetically shielded room (MSR). For conventional MEG utilizing superconducting quantum interference devices (SQUIDs), the participant's head must stay within the semi-spherical surface of a cryogenic storage Dewar. This design allows to have many SQUID sensors as close as possible to the head in order to achieve good signal quality. Because Dewars have very restricted mobility, VS is usually realized using a projector outside of the MSR, some optical elements and a back-projection screen in the line of sight of the participant. Recently, the feasibility of MEG using optically pumped magnetometers (OPMs) was demonstrated. These sensors can be attached directly to the head because they operate near room temperature. OPM-MEG therefore offers more experimental freedom including different postures, movements or hyperscanning, creating the need for a more flexible kind of VS setup. In this paper, we present a compact, high-resolution VS setup which is enclosed by a portable magnetic shield with an opening for the projection. The VS setup is based on a single-board computer which acts as experiment control device to create visual stimuli, process inputs, log participant activity and set off trigger signals. This setup supports the new possibilities of OPM-MEG and can be easily installed into any MSR. We investigate if the shielded VS inside the MSR generates distortion signals above the noise floor of the OPMs. We also show that visual cortex activity can be evoked with our setup and recorded with a custom-made OPM-MEG cap. By applying two well-established visual stimulation paradigms, we demonstrate the ability of our setup to elicit brain activity in different frequency ranges.