Nature of the Measured Changes

Numerous studies have shown that local glucose consumption and rCBF in the rat brain are highly correlated and that both in turn are highly correlated with underlying neural activity. Studies in humans using positron emission tomography (PET) have also shown close coupling between local glucose consumption and rCBF. Metabolic and vascular changes associated with neural activity appear to arise from synaptic processes rather than from action potentials in the cell bodies. Indeed, most of the increase in metabolism (deoxyglucose uptake) appears to occur not in the cell bodies but in the region occupied by synapses between axons and dendrites. Thus, of the three potential neural sites that contribute to metabolism, soma, axon, and synapses, the latter is the one that is the most highly correlated with the metabolic requirements of neurons. Furthermore, studies have shown that the increase in glucose metabolism is brought about by presynaptic, rather than postsynap-tic activity (that is, activity of the "sending" neurons, not the "receiving" ones). This is an important observation because the source of EEG and MEG signals is thought to be primarily postsynaptic.

Work conducted to understand the energy-consuming processes taking place in axon terminals has shown that glucose consumption is correlated with the activity of the Na+ pump, suggesting that this is the crucial event in the coupling between neural activity and energy consumption. If increased metabolic activity reflects presynaptic activity, then one would predict that both excitatory and inhibitory synaptic activity results in similar increases in energy needs. Consistent with this prediction, stimulation of excitatory and inhibitory afferents of the lateral superior olive within the brain stem auditory system of the cat brings about similar increases in deoxyglucose uptake. Note that, although increased excitatory and inhibitory presynaptic activity may produce identical local increases in deoxyglucose uptake, they will have opposite effects on the postsynaptic neurons being driven. Thus, rCBF maps reflect a mixture of neuro-physiological processes that require energy; such processes include excitatory and inhibitory synaptic activity as well as additional processes needed to regulate the extracellular environment. This observation should remind us that what is generally referred to as ''brain activity'' in the neuroimaging literature potentially reflects a mixture of many different energy-consuming phenomena.

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