Inhibition and functional magnetic resonance imaging

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This review summarizes our current knowledge on how inhibitory phenomena are reflected in the functional magnetic resonance imaging (fMRI) signal. It is well-established that activity-related changes of brain metabolism and blood flow are dominated by changes in synaptic activity. Both excitatory and inhibitory synaptic activities are associated with increased metabolic demands. The amount of energy consumption associated with inhibition vs. excitation is reflected in metabolism- and blood flow-related neuroimaging signals such as the blood oxygen level-dependent (BOLD) contrast; the relationship between the different “signals”, however, may not be linear. The influence on these signals depends on the number of active inhibiting synapses, the duration of inhibition and the degree of propagation within subsequent neuronal circuits. The relative influence of inhibition as compared to excitation on the metabolism/blood flow may also vary in different neuronal circuits. Inhibition leads to locally decreased discharge activity, which does not have a significant effect on the cerebral blood flow (CBF)/BOLD images. However, inhibition may also result in suppression of complex neuronal circuits, leading to a decrease in excitatory as well as inhibitory synaptic activity, and therewith, to a decreased metabolism and blood flow within those complex neuronal networks. The available fMRI data indicate that, depending on the abovementioned factors, inhibition may be reflected in positive, negative or no BOLD-signal at all. Thus, the BOLD-signal is ambiguous with respect to the underlying electrophysiological event. In the future, combining fMRI with electrophysiological methods will strengthen neuroimaging studies.