Spatial richness of neural magnetic fields

Brain implants that measure neural magnetic fields, rather than electrical potentials, are expected to confer significant clinical advantages related to implant longevity and signal fidelity due to the elimination of the electrode-tissue interface. However, the informational differences between neural electrical potentials and magnetic fields remain poorly understood. Using a mathematical formalism based on neuronal current sources, we directly establish the complementary informational content of extracellular magnetic fields and electrical potentials. This formalism also reveals that extracellular magnetic fields generated by spiking neurons inherently exhibit one order lower spatial polarity than electric fields, resulting in more favorable distance-scaling characteristics. We then use computational modeling to illustrate how dense networks of neurons are easier to distinguish and spike sort on the basis of their magnetic, rather than electrical, spike templates. Lastly, we show how the solenoidal nature of neural magnetic fields facilitates approximate morphological reconstruction, even with sparse sensor arrays. Our findings highlight the unique experimental advantages of neural magnetic field sensing, motivating the development of compact, low-noise devices capable of meeting the stringent sensitivity requirements for cortical recordings.