Time-related increase of oxygen utilization in continuously activated human visual cortex

Oxygen utilization increase is fractionally much less than that seen in glucose metabolism and blood flow soon after onset of neuronal activation, however its behavior during continued activation is less certain. We evaluated the effects of 25 min of visual stimulation on CBF, CMRO₂, and OEF using [¹⁵O] water and [¹⁵O] oxygen PET. Seven healthy volunteers underwent a PET session consisting of serial [¹⁵O] water and [¹⁵O] oxygen scans at the fixation-only baseline visual state and after 1, 13, and 25 min of the continuous visual stimulation using a black-white vertical grating. CBF, CMRO₂, and OEF values were calculated for the entire brain and for regions of interest in visual cortex centered over the area of activation. After 1 min of stimulation, CMRO₂ increased only 4.7% compared to baseline and CBF increased 40.7%. However, after 25 min of stimulation the increase in CMRO₂ compared to baseline was 15.0%, having tripled from that measured at 1 min (P < 0.05). CBF did not significantly change during this time. OEF was 48.3% at baseline. It decreased to 37.1% after 1 min of visual stimulation (P < 0.01) and then returned almost to baseline values after 25 min of activation OEF (45.7%). There were no significant variations in whole-brain values during the study. We suggest that in the activated brain, the increased energy demands initially are not fully met with oxidative metabolism and must predominantly be supported by increased glycolysis. With continued activation, oxygen utilization increases reducing the need for excess glycolysis.