INTERPRETING FUNCTIONAL IMAGING STUDIES IN TERMS OF NEUROTRANSMITTER CYCLING

Functional imaging experiments, in particular positron-emission tomogr aphy and functional magnetic resonance imaging, can be analyzed either in psychological terms or on the basis of neuroscience, In the usual psychological interpretation, stimulations are designed to activate sp ecific mental processes identified by cognitive psychology, which are then Localized by the signals in functional imaging experiments. An al ternate approach would be to analyze experiments in terms of the neuro biological processes responsible for the signals. Recent in vivo C-13 NMR measurements of the glutamate-to-glutamine neurotransmitter cyclin g in rat and human brains facilitate a neuroscientific interpretation of functional imaging data in terms of neurobiological processes since incremental neurotransmitter flux showed a 1:1 stoichiometry with the incremental rate of glucose oxidation. Because functional imaging sig nals depend on brain energy consumption, a quantitative relationship c an be established between the signal (S) and the specific neurochemica l cerebral neurotransmitter activity (N) of glutamate-to-glutamine neu rotransmitter cycling. The quantitation of neuronal activity proposed has implications for the psychological design and interpretation of fu nctional imaging experiments. Measurements of the neurotransmitter cyc ling flux at rest in functional imaging experiments suggest that perfo rming cognitive tasks and sensory stimulations increases neurotransmit ter cycling by only 10-20%, Therefore it cannot be assumed that refere nce state activities are negligible, nor that they are constant during stimulation.