Directly mapping magnetic field effects of neuronal activity by magnetic resonance imaging.

Magnetic resonance imaging (MRI) of brain functional activity relies principally on changes in cerebral hemodynamics, which are more spatially and temporally distributed than the underlying neuronal activity changes. We present a novel MRI technique for mapping brain functional activity by directly detecting magnetic fields induced by neuronal firing. Using a well-established visuomotor paradigm, the locations and latencies of activations in visual, motor, and premotor cortices were imaged at a temporal resolution of 100 msec and a spatial resolution of 3 mm, and were found to be in consistent with the electrophysiological and functional MRI (fMRI) literature. Signal strength was comparable to traditional event-related fMRI methods: about 1% of the baseline signal. The magnetic-source MRI technique greatly increases the temporal accuracy in detecting neuronal activity, providing a powerful new tool for mapping brain functional organization in human and animals.