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Abstract

Hypercapnia is often used as vasodilatory challenge in clinical applications and
basic research. In functional magnetic resonance imaging (fMRI), elevated CO2
is applied to derive stimulus-induced changes in the cerebral rate of oxygen
consumption (CMRO2) by measuring cerebral blood flow (CBF) and bloodoxygenation-
level-dependent (BOLD) signal. Such methods, however, assume
that hypercapnia has no direct effect on CMRO2. In this study, we used
combined intracortical recordings and fMRI in the visual cortex of anesthetized
macaque monkeys to show that spontaneous neuronal activity is in fact
significantly reduced by moderate hypercapnia. As expected, measurement of
cerebral blood volume using an exogenous contrast agent and of BOLD signal
showed that both are increased during hypercapnia. In contrast to this,
spontaneous fluctuations of local field potentials in the beta and gamma
frequency range as well as multi-unit activity are reduced by ~15 during
inhalation of 6 CO2 (pCO2 = 56 mmHg). A strong tendency toward a reduction
of neuronal activity was also found at CO2 inhalation of 3 (pCO2 = 45 mmHg).
This suggests that CMRO2 might be reduced during hypercapnia and caution
must be exercised when hypercapnia is applied to calibrate the BOLD signal.