β reflects the dependence between BOLD signal and deoxyhemoglobin concentration.

A method to map β in vivo for the whole brain is demonstrated.

β is measured to be uniformly 0.8 within the neocortex, significantly lower than the previously assumed value of 1.5.

As resting brain activity level is varied with different anesthesia, β does not change significantly.

The impact of measured β values on calibrated fMRI is discussed.