Neuroenergetic basis of functional MRI: implications for efficiency of brain work

The conventional functional MRI (fMRI) map offers information indirectly about localized changes in neural activity because it reflects changes in blood oxygenation, not the actual neuronal activity. To provide neural basis of fMRI researchers have combined electrophysiology to show correlations of fMRI and electrical signals. But quantitative interpretation of "How much has the neural activity changed by?" still cannot be made from conventional fMRI data. The fMRI signal (S) has two partitions, one that describes the correlation between oxidative metabolism (CMR_O2) and blood flow (CBF) which supports the bioelectric work to sustain neuronal excitability and the other is the requisite dilation of blood vessels (CBV) which is the mechanical response involved in removal of waste while providing nutrients. Since changes in energy metabolism is related to bioelectric work, we tested if spiking frequency of a neuronal ensemble (v) is reflected by local energy metabolism (CMR_O2) in rat brain. We used extracellular recordings to measure Δv/v and calibrated fMRI (using S, CBF, and CBV maps) to measure ACMR02/CMR02 during sensory stimulation. We found that ACMR_O2/CMR_O2=Δv/v which suggests efficient energy use during brain work. Thus calibrated OM provides data on where and by how much the neural activity has changed.