The BOLD Fmri Response

The brain is one of the most energy demanding organ in humans. Neurons are characterized by aerobic metabolism (glucose oxidation in aerobic environment) which increases when the neuron is active. In particular, according to hemodynamics response model an increase in neural activity results in different physiological events, being reflected in the BOLD fMRI response: an initial increase in oxygen consumption due to the increased metabolic demand. This increases the concentration of deoxyhaemoglobin and at the same time the concentration of oxyhaemoglobin, resulting in the so called initial ‘spectral dip’ of the BOLD fMRI response and therefore to a small decrease of image intensity. This initial phase is believed to be closely related to neural
activity. After ~2s the neuronal activity triggers a large increase of local blood flow providing a proportional increase in glucose consumption. This second phase is believed to be triggered by an increased synaptic activity since this is linked to glucose metabolism and it results in a large increase of image intensity. In fact the amount of oxygen being extracted is less than the amount available, i.e. there is an overcompensation mechanism of oxygenated blood. The oversupply of oxygenated blood is believed to be a consequence of the less efficiency at high flow rates of local oxygen extraction by passive diffusion [Buxton]. Furthermore in this phase the increased blood flow causes vasodilation of veins and consequently an increase of mostly deoxygenated blood volume in veins.