a, A ‘seed’ location was selected in sensorimotor cortex (at the cross-hairs). Four thousand random ‘test’ locations (red squares), within 3 cm of the seed, were selected randomly and probed. Shared variance was measured between electrophysiological time courses at the seed and test locations. b, Left, correlation between the seed and test time courses plotted as a function of spatial separation. Right, source separation at which shared variance dropped to below 50%. In both cases the error bars show s.d. over test locations. This serves as an absolute quantification of spatial resolution. Note that the OPM array, when static, significantly outperforms SQUIDs (P = 0.002, Wilcoxon sum-rank test). c, Quantification of the robustness of the source orientation estimation. While source power can vary between experiments, source orientation relies only on the local orientation of the cortical sheet, and should therefore be the same across equivalent experiments. Here the histograms show the source orientation difference (as an angle) across runs for 4,000 locations of interest. Note for all three experiments (static OPMs (top), moving OPMs (middle) and SQUIDs (bottom)) the probability distribution peaks at zero as would be expected. The bar chart shows the probability of observing an angular discrepancy below 5°; note that the OPM array, when static, significantly (P < 0.05, permutation test) outperformed the SQUID array in terms of orientation robustness. Moving OPMs demonstrated the lowest robustness; however, this would be expected since the execution of natural movements differed across runs and therefore brain activity in the sensorimotor strip will also differ. The improvement in spatial specificity and robustness in our OPM-MEG system compared to a cryogenic (SQUID) system is likely to be a consequence of two factors: first, the closer proximity of the OPM sensors to the scalp provides higher signal-to-noise ratio in OPMs compared to SQUIDs; second, the scanner-cast ensures that, for each run, OPM sensors are in exactly the same location with respect to the brain. Cryogenic MEG, on the other hand, is subject to co-registration errors. d, Quantification of the OPM sensor’s frequency response, which also defines its temporal resolution. An OPM sensor was placed in a Helmholtz coil and a white noise current source applied to the coil. The blue line shows the Fourier spectrum of the current source, the green line shows the spectrum of the measured field. Note that sensitivity falls by 3 dB at 130 Hz, giving a temporal resolution of 7.7 ms. See also Supplementary Information section 2.