Physiological basis of functional MRI

Afferent neurotransmission of a given neuronal group and its subsequent dendritic activity are followed by a transient hemodynamic response such as an increase of the local blood flow and of the intravascular level of oxygen. This neurovascular regulation involves neurons, astrocytes, interneurons and various transmitter substances. The resulting changes in the magnetic susceptibility is then detected by the MRI machine Which can specifically localize regional brain activity. Blood oxygenation level dependant contrast reflects afferent inputs and their post-synaptic (dendritic) processing, rather than the ouput from the concerned cerebral zone (spiking). In other words this contrast depends on local informational treatment of an active zone without implying any effective influence upon its targets.