Anatomically Veridical On-Scalp Sensor Topographies

When working with sensor-level data recorded using on-scalp neuroimaging methods such as electroencephalography (EEG), it is common practice to use two-dimensional (2D) representations of sensor positions to aid interpretation. Positioning of sensors relative to anatomy, as in the classic 10-20 system of EEG electrode placement, enables the use of 2D topographies that are familiar to many researchers and clinicians. However, when using another increasingly popular on-scalp neuroimaging method, optically pumped magnetometer-based magnetoencephalography (OP-MEG), bespoke sensor arrays are much more common, and these are not prepared according to any standard principle. Consequently, polar projection is often used to produce individual sensor topographies that are not directly related to anatomy and cannot be averaged across people simply. Given the current proliferation of OP-MEG facilities globally, this issue will become an increasing hindrance when visualising OP-MEG data, particularly for group studies. To address this problem, we adapted and extended the 10-20 system to build a flexible, anatomical projection method applied to digitised head shape, fiducials and sensor positions. We demonstrate that the method maintains spatially veridical representations across individuals improving on standard polar projections at varying OPM sensor array densities. By applying our projection method, the benefits of anatomically veridical 2D topographies can now be enjoyed when visualising data, such as those from OP-MEG, regardless of variation in sensor placement as in sparse or focal arrays.