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Abstract:
The blood oxygenation level-dependent (BOLD) signal is an indirect
hemodynamic signal which is sensitive to cerebral blood flow (CBF), cerebral
blood volume (CBV) and cerebral metabolic rate of oxygen (CMRO2). Therefore,
the BOLD signal amplitude and dynamics cannot be interpreted unambiguously
without additional physiological measurements and, thus, there remains a need
for a functional magnetic resonance imaging (fMRI) signal which is more closely
related to the underlying neuronal activity. In this study, we measured cerebral
blood flow with continuous arterial spin labeling, cerebral blood volume with an
exogenous contrast agent and BOLD combined with intracortical
electrophysiological recording in primary visual cortex of the anesthetized
monkey. During inhalation of 6 CO2, it was observed that CBF and CBV are not
further increased by a visual stimulus, although baseline CBF for 6 CO2 is
below the maximal value of CBF. In contrast, the electrophysiological response
to the stimulation was found to be preserved during hypercapnia. As a
consequence, the simultaneously measured BOLD signal responds negatively to
a visual stimulation for 6 CO2 inhalation in the same voxels responding
positively during normocapnia. These observations suggest that the fMRI
response to a sensory stimulus for 6 CO2 inhalation occurs in the absence of a
hemodynamic response, and it therefore directly reflects the oxygen extraction
into the tissue.
