Cerebral energetics and spiking frequency: The neurophysiological basis of fMRI

Functional MRI (fMRI) is widely assumed to measure neuronal activity, but no satisfactory mechanism for this linkage has been identified. Here we derived the changes in the energetic component from the blood oxygenation level-dependent (BOLD) fMRI signal and related it to changes in the neuronal spiking frequency in the activated voxels. Extracellular recordings were used to measure changes in cerebral spiking frequency (Δν/ν) of a neuronal ensemble during forepaw stimulation in the α-chloralose anesthetized rat. Under the same conditions localized changes in brain energy metabolism (ΔCMR O2 /CMR O2) were obtained from BOLD fMRI data in conjunction with measured changes in cerebral blood flow (ΔCBF/CBF), cerebral blood volume (ΔCBV/CBV), and transverse relaxation rates of tissue water (T 12ptminimal amsmath wasysym amsfonts amssymb amsbsy mathrsfs -69pt document equation*₂^*equation*document and T 2) by MRI methods at 7T. On stimulation from two different depths of anesthesia ΔCMR O2 /CMR O2 ≈ Δν/ν. Previous 13 C magnetic resonance spectroscopy studies, under similar conditions, had shown that ΔCMR O2 /CMR O2 was proportional to changes in glutamatergic neurotransmitter flux (ΔV cyc /V cyc). These combined results show that ΔCMR O2 /CMR O2 ≈ ΔV cyc /V cyc ≈ Δν/ν, thereby relating the energetic basis of brain activity to neuronal spiking frequency and neurotransmitter flux. Because ΔCMR O2 /CMR O2 had the same high spatial and temporal resolutions of the fMRI signal, these results show how BOLD imaging, when converted to ΔCMR O2 /CMR O2, responds to localized changes in neuronal spike frequency.