Frequency-dependent changes in cerebral metabolic rate of oxygen during activation of human visual cortex

To test the hypothesis that brain oxidative metabolism is significantly inc reased upon adequate stimulation, we varied the presentation of a visual-st imulus to determine the frequency at which the metabolic response would be at maximum. The authors measured regional CMRO2 in 12 healthy normal volunt eers with the ECAT EXACT HR+ (CTI/Siemens, Knoxville, TN, U.S.A.) three-dim ensional whole-body positron emission tomograph (PET). In seven successive activating conditions, subjects viewed a yellow-blue annular checkerboard r eversing its contrast at frequencies of 0, 1, 4, 8, 16, 32, and 50 Hz. Stim ulation began 4 minutes before and continued throughout the 3-minute dynami c scan. In the baseline condition, the subjects began fixating a cross hair 30 seconds before the scan and continued to do so for the duration of the 3-minute scan. At the start of each scan, the subjects inhaled 20 mCi of O- 15-O-2 in a single breath. The CMRO2 value was calculated using a two-compa rtment, weighted integration method. Normalized PET images were averaged ac ross subjects and coregistered with the subjects' magnetic resonance imagin g in stereotaxic space. Mean subtracted image volumes (activation minus bas eline) of CMRO2 then were obtained and converted to z statistic volumes. Th e authors found a statistically significant focal change of CMRO2 in the st riate cortex (x = 9; y = -89; z = -1) that reached a maximum at 4 Hz and dr opped off sharply at higher stimulus frequencies.