Kevin Murphy1, Ashley D. Harris1, Richard G. Wise1
1CUBRIC, School of Psychology, Cardiff University, Cardiff, Wales, United Kingdom
Hypercapnia is often used in calibrated BOLD techniques to determine the maximum possible BOLD response. The assumption is that an increase in arterial CO2 tension will not affect CMRO2, a measure of brain metabolism. This assumption has been called into question with studies showing conflicting results. A framework is presented that models changes in CMRO2 caused by hypercapnia at two different CO2 levels: +4mmHg and +8mmHg. Decreases in CMRO2 during hypercapnia were observed but only for the +8mmHg condition. This suggests that a low level of hypercapnia (~4mmHg) must be used for the assumption of calibrated BOLD to hold.