Detection of an Earthworm Axon Current with Simultaneous MRS

INTRODUCTION Direct detection of axonal neural magnetic fields (NMFs) by magnetic resonance imaging has met with conflicting evidence. Chow et al. reported a significant magnitude change in the human optic nerve and corpus callosum [1,2]. However, Park and Lee concluded that the modeled signal changes of axonal NMFs were negligible [3]. The objective of this study is to demonstrate the temporal signature of axonal NMFs in the free induction decay (FID), which provides the temporal resolution required to capture an axonal event. Simultaneous electrophysiology is used to time-lock earthworm action potentials to FID acquisition. The phase of the FID signal change is also simulated using an experimentally verified method for predicting the magnetic field generated by an axon [4]. Our data demonstrates clear evidence of a phase change that temporally corresponds to the electrophysiologically recorded action potential and is consistent to our theoretical predictions.