McConnell Brain Imaging Centre, Positron Emission Tomography Laboratories, Montreal Neurological Institute, McGill University, Montreal, Quebec, Canada

Correspondence: Manouchehr S Vafaee, McConnell Brain Imaging Centre, Montreal Neurological Institute, 3801 University Street, Montreal, Quebec, Canada H3A 2B4.

Received 12 December 1997; Revised 22 April 1998; Accepted 24 April 1998.

Abstract

To test the hypothesis that brain oxidative metabolism is significantly increased upon adequate stimulation, we varied the presentation of a visual stimulus to determine the frequency at which the metabolic response would be at maximum. The authors measured regional CMR_O2 in 12 healthy normal volunteers with the ECAT EXACT HR+ (CTI/Siemens, Knoxville, TN, U.S.A.) three-dimensional whole-body positron emission tomograph (PET). In seven successive activating conditions, subjects viewed a yellow-blue annular checkerboard reversing its contrast at frequencies of 0, 1, 4, 8, 16, 32, and 50 Hz. Stimulation began 4 minutes before and continued throughout the 3-minute dynamic 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 ¹⁵O-O₂ in a single breath. The CMR_O2 value was calculated using a two-compartment, weighted integration method. Normalized PET images were averaged across subjects and coregistered with the subjects' magnetic resonance imaging in stereotaxic space. Mean subtracted image volumes (activation minus baseline) of CMR_O2 then were obtained and converted to z statistic volumes. The authors found a statistically significant focal change of CMR_O2 in the striate cortex (x = 9; y = -89; z = -1) that reached a maximum at 4 Hz and dropped off sharply at higher stimulus frequencies.