At a basal level, brain activity consists of action potentials, and the BOLD signal on an MRI scan is only a proxy for brain activity. As a subset of patients with COPD suffers from hypoxemia, I am wondering whether this could distort results. There is some evidence that people suffering from chronic hypoxemia can maintain a relatively normal level of functioning despite having low blood oxygenation, but I am unsure if neurons can also adapt, become more efficient and function at a 'normal' level despite low levels of oxygen.
Specifically, I have three questions:
1) Could the actual cerebral activity (action potentials) be higher than the low blood oxygenation level would lead you to believe, for instance because of adaptation? Has this ever been investigated?
2) If the BOLD signal indeed consistently underestimates actual cerebral activity (as I suspect would be the case in the scenario above), would there be any way to correct for this?
3) If there is a discrepancy between the BOLD signal and actual cerebral activity, is there a certain threshold below which the discrepancy becomes so large as to make it impossible to draw any conclusions? Or does drawing conclusions just get progressively harder as the blood oxygenation levels get lower?