Model of blood-brain transfer of oxygen explains nonlinear flow-metabolismcoupling during stimulation of visual cortex

The coupling between cerebral metabolic rate of oxygen (CMRO2) and blood fl ow (CBF) in response to visual stimulation was evaluated by means of a mode l of oxygen delivery. The model predicted a nonlinear relationship between stimulus-evoked changes of oxygen consumption and blood flow. The magnitude of the CMRO2/CBF ratio index (I-O2) was used to indicate the degree of flo w-metabolism coupling prevailing in specific areas of the brain during phys iological stimulation. Therefore, the index provided a measure of the blood oxygenation level dependent (BOLD) magnetic resonance contrast. To evaluat e the changes of I-O2 in response to visual stimulation, the model was appl ied to the effect of a changing flicker rate of a visual stimulus on the ma gnitudes of CBF, CMRO2, and oxygen diffusion capacity, in the human brain. Positron emission tomography (PET) was used to measure the CBF and the CMRO 2 in 12 healthy volunteers who viewed a cross-hair (baseline) or a yellow-b lue annular checkerboard reversing at frequencies of 1, 4, or 8 Hz. The mag nitude of CBF in the primary visual cortex increased as a function of the c heckerboard reversal rate and reached a maximum at the frequency of 8 Hz (z = 16.0), while the magnitude of CMRO2 reached a maximum at 4 Hz (z = 4.0). Therefore, the calculated I-O2 was lower at 8 Hz than at 1 and 4 Hz, in co ntrast to the oxidative metabolic rate that reached its maximum at 4 Hz. Th e model explained the increase of oxygen consumption as the combined effect of increased blood flow and increased oxygen diffusion capacity in the reg ion of visual activation.