Investigating direct detection of axon firing in the adult human optic nerve using MRI

The aim of this study was to directly detect spectral components of the magnetic fields of ionic currents caused by firing of the axons in the optic nerve in response to visual strobe stimulation. The magnetic field parallel to the main B0 field can potentially alter the local phase and magnitude of the MR signal which can cause signal loss due to intravoxel dephasing. Measured frequency spectra showed evidence of the strobe stimulus localized to regions containing the optic nerve, not thought to be due to motion artifacts, in 30 out of 52 experiments in 5 adult human subjects. The effect was (0.15 ± 0.05)% of the mean magnitude equilibrium signal from the voxel in the frequency range 0.7–3.3 Hz, corresponding to an estimated field of (1.2 ± 0.4) nT, at an echo time of TE = 32.4 ms using a 1.5 T MRI scanner. Only 1 of 12 phase image experiments showed effects. These findings provide preliminary evidence for direct detection of axonal firing in the optic nerve.