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Energetics of neural populations by fMRI: Towards quantitative neuroimaging
Fahmeed Hyder
Magnetic Resonance Research Center , Yale University, USA
Conventional fMRI map offers information indirectly about localized changes in neuronal activity because it reflects changes in blood oxygenation, not the actual neuronal activity. To provide neurophysiological basis of fMRI researchers have combined electrophysiology to show correlations of fMRI and electrical signals. But quantitative interpretation of “How much has the neuronal 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 are testing if spiking frequency of a neuronal ensemble (ν) is reflected by local energy metabolism (CMR O2) in rat brain. We will demonstrate the recent progress we have towards using extracellular recordings to measure Δν/ν and calibrated fMRI (i.e., using S, CBF, and CBV maps) to measure ΔCMR O2/CMR O2 during sensory stimulation, and under a variety of other conditions in different regions of the cerebral cortex. Generally, we find that ΔCMR O2/CMR O2 ≈ Δν/ν for a lot of regions within the cerebral cortex and under most circumstances, which suggests efficient energy use during brain work. Thus calibrated fMRI provides data on where and by how much the neuronal activity has changed. Other possibilities of utilizing calibrated fMRI as a neuroimaging method are discussed.