The Ratio of CBF to CMRO2 Change with Brain Activation Increases with Increasing Stimulus Amplitude in Human Visual Cortex

Purpose: Although functional magnetic resonance imaging (fMRI) based on blood oxygenation level dependent (BOLD) signal changes provides a useful tool for mapping brain activation, a quantitative interpretation of the magnitude of the BOLD response remains problematic. The BOLD response is primarily driven by cerebral blood flow (CBF) changes, but is strongly modulated by n, the ratio of the fractional change in CBF to the change in cerebral metabolic rate of oxygen consumption (CMRO2). A number of studies have found significant regional variation of n across the brain [1,2], but variability within the same brain region for different stimuli has been relatively unexplored (with the exception of [3]). We used a calibrated BOLD method to test whether n varies with the contrast of a visual stimulus within a single brain region.