Progress in understanding functional imaging signals
- Brain Imaging Research Center, Department of Neurobiology, University of Pittsburgh, 3025 East Carson Street, Pittsburgh, PA 15203
More than a century ago, Roy and Sherrington observed that a change in regional cerebral blood flow (CBF) could reflect neural activity (1). This concept is a basis for modern functional brain imaging technologies including functional magnetic resonance imaging (fMRI), positron emission tomography (PET), intrinsic optical imaging, and near infrared optical tomography. These methods have been extensively used to map various brain functions in humans and animals on a spatial scale of 50 micrometers to a few centimeters. To understand functional maps as “meaningful” neurophysiological parameter(s), a chain of events from behavior to functional signal recording should be understood (see Fig. 1). Task and/or stimulation induce synaptic and electric activities in localized regions, which will trigger changes in metabolic and hemodynamic responses including CBF. Then, brain mapping techniques detect a change in one or many vascular parameters, which can be displayed as a colorful functional image. However, the exact relationship between neural activity and a functional imaging signal is not well-understood. Therefore, understanding the physiological sources of the functional imaging signals is critically important as summed by Raichle, “We have at hand tools with the potential to provide unparalleled insights into some of the most important scientific, medical, and social questions facing mankind. Understanding those tools is clearly a high priority” (2).
Schematic chain of processes from behavior to functional mapping.
This critical issue was investigated by Caesar et al. (3) in this issue of PNAS using rat cerebellum as a model, where excitatory and inhibitory neural activities in Purkinje cells can be controlled by climbing fiber and parallel fiber stimulations, respectively. When excitatory and inhibitory neural activities were independently modulated, CBF was detected by laser Doppler flowmeter. One important observation by Caesar et al. (3) is that blood flow response induced by neural activity is correlated with postsynaptic field …
