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FIG 1. Difference correlation plots for visual and auditory conditions. These graphs were generated by subtracting CBF and BOLD measurements made in the placebo condition from those made in the caffeine condition. All values on the perfusion difference axes (
CBF) are negative because resting cerebral perfusion decreased for all study participants when the caffeine condition was compared with the placebo condition. The values on the BOLD signal difference axes ( BOLD) are positive if BOLD signal increased when perfusion decreased and are negative if BOLD signal decreased when perfusion decreased. It is evident that half of the population experienced an increase and the other half a decrease in BOLD signal when resting perfusion decreased. Data are shown separately for the high (squares) and low (triangles) caffeine users. The regression lines were calculated for the population as a whole and show the poor correlation between the magnitude of the perfusion decrease and the BOLD signal change in visual and auditory cortex. The units are mL/100 g of brain tissue/min for CBF and total percent change (mean percent change x cluster volume) for BOLD.
