Dependence of Bold Signal Amplitude on Baseline Venous Oxygenation and Cerebral Blood Flow

Introduction The vast majority of functional magnetic resonance imaging (fMRI) studies use measures of the blood oxygenation level dependent (BOLD) signal as a reflection of neural activity. However, the BOLD signal is a complex function of a number of physiological variables, and there is growing evidence indicating that changes in the baseline vascular and metabolic state can significantly modulate the amplitude of the BOLD signal. In a recent set of studies, Lu et al [1,2] presented a non-invasive method for measuring the oxygenation of venous blood and demonstrated a strong dependence (R= 0.44) of the BOLD signal amplitude on venous oxygenation across a sample of healthy young subjects. They also found a strong correlation (R=0.64) between baseline venous oxygenation and baseline cerebral blood flow (CBF), but did not find a significant correlation (R=0.17) between BOLD signal amplitude and baseline CBF. In contrast, another recent study [3], reported a significant correlation (R= 0.60) between BOLD signal amplitude and baseline CBF. The goal of this study was to resolve these contradictory findings and to investigate in detail the dependence of BOLD signal amplitude on both baseline venous oxygenation and CBF.