MR microscopy of Aplysia californica at 17.2T suggests that the diffusion fMRI signal originates from neural swelling

Diffusion fMRI (DfMRI) allows to monitor brain function without BOLD vascular confounding effects. To elucidate the origin of the DfMRI response we performed DfMRI experiments at single neuron and ganglia level upon dopamine stimulation of Aplysia californica buccal ganglia using 17.2T MR microscopy. Neural swelling, evidenced from optical microscopy, resulted in an intracellular ADC increase reflecting cytoplasm dilution and an ADC decrease at ganglia level. While the mechanism of this ADC decrease remains putative these results are consistent with the hypothesis that the ADC decrease observed with DfMRI upon neuronal activation at tissue level reflects activation induced cell swelling.