Improved spatio-temporal measurements of visually evoked fields using optically-pumped magnetometers
Recent developments in performance and practicality of optically pumped magnetometers have enabled new capabilities in non-invasive brain function mapping through magnetoencephalography. In particular the lack of need of cryogenic operating conditions allows for more flexible placement of the sensor heads closer to the brain surface, leading to improved spatial measurement resolution and increased source localisation capabilities. Through the recording of visually evoked brain fields (VEF), we demonstrate that the closer sensor proximity can be further exploited to improve the temporal resolution. We use optically pumped magnetometers (OPMs), and for reference superconducting quantum interference devices (SQUIDs), to measure brain responses to standard flash and pattern reversal stimuli. We find highly reproducible signals with consistency across multiple healthy participants, stimulus paradigms and sensor modalities. The temporal resolution advantage of OPMs is manifest in a fourfold improvement of the ratio of magnetic signal peak height to temporal width, compared to SQUIDs. The resulting capability of improved spatio-temporal signal tracing is illustrated by simultaneous vector recordings of VEFs in the primary (V1) and associative (V2) visual cortex, where a time lag on the order of 10-20 ms is consistently found. This paves the way for further studies of spatio-temporal neurophysiological signal tracking in visual stimulus processing and other brain responses with potentially far-reaching consequences for time-critical mapping of functionality in the healthy and pathological brains.