PHYSIOLOGICAL CHANGES DURING BRAIN ACTIVATION

Blood Flow and Glucose Metabolism Increase with Functional Activity

In the second quote from William James that opened Part IA, he speculated that “Blood very likely may rush to each region of the cortex according as it is most active.” With the development of tomographic techniques for measuring local cerebral blood flow (CBF) and the cerebral metabolic rate for glucose (CMRGlc), we now know that he was right. This rush of blood to activated areas is the physiological basis for most of the modern techniques of functional neuroimaging. Comparisons of CBF and CMRGlc changes have consistently found good agreement in the locations of the activation (Ginsberg, Dietrich, and Prusto, 1987; Yarowsky and Ingvar, 1981). With the development of positron emission tomography (PET), these types of experiments were extended to human subjects. For example, in a sensorimotor activation task, subjects were asked to discriminate between three classes of mah-jongg tiles by touch (Ginsberg et al., 1988). Four sequential PET scans were conducted on each subject to measure glucose metabolism with F-Fluoro-deoxyglucose (FDG) and CBF using H O in the resting and activated states. On average, in the area of the sensorimotor cortex associated with the task, the glucose metabolic rate increased by 17%, and the CBF increased by 26%. In another human study with PET and a sensorimotor task, changes of 50% in CMRGlc and 50% in CBF were found (Fox et al., 1988).

There is ample evidence that both flow and glucose metabolism increase substantially in activated areas of the brain. However, the observed correlation between changes in CBF and glucose metabolism does not necessarily imply a link between the two.