The neurophysiological correlates of neuroimaging signals Martin Lauritzen Department of Medical Physiology, Panum Institute, University of Copenhagen, and Department of Clinical Neurophysiology, Glostrup Hospital, 2600-DK Glostrup, Denmark

Detailed knowledge of the changes in cerebral blood flow (CBF) or oxygen and glucose consumption that are triggered by different types of nerve cell activity is the key to an understanding of signals used by functional neuroimaging techniques. Recent studies have raised the possibility that spike generation is neither necessary nor sufficient for basal or activity-dependent rises in CBF since increases in hemodynamic signals are easily dissociated from neuronal spiking activity. Subthreshold integrative synaptic activity as indicated by the local field potentials correlate strongly to CBF, but the relation is non-linear (sigmoid) in several neuronal networks with a clearly defined threshold and saturation characteristics. The findings imply that a defined threshold of synaptic activity must be surpassed before increases in hemodynamic signals are observed, and that no further increases in signals are observed above a certain level of neuronal activity. Still other studies under conditions of decreased activity suggest a lack of proportionality between decreases of spiking activity and CBF, and that evoked synaptic and vascular responses are context-sensitive in that preceding activity influences the vascular signal evoked by a second stimulus.   The findings suggest that the interpretation of functional neuroimaging data may be more complex than commonly believed due to the complex interaction among neuronal networks and the mechanisms that transform synaptic activity into a hemodynamic signal.