The most widespread measures of human brain activity are the blood oxygen level dependent (BOLD) signal and surface field potential. These measures provide complementary information about human brain activity. To develop an understanding of how to interpret these signals, we developed a model of (a) neuronal population responses, and (b) transformations from neuronal responses into the fMRI BOLD signal and electrocorticographic (ECoG) field potential. Rather than seeking a transformation between signals directly, this approach interprets each measure with respect to the underlying neuronal population responses. Our model accounts for the relationship between BOLD and ECoG data from human visual cortex (V1-V3), matching in three ways: BOLD and ECoG broadband were positively correlated, BOLD and alpha (8-13Hz) were negatively correlated, and BOLD and narrowband gamma (30-80Hz) did not correlate. We infer that features of the field potential that are uncorrelated with BOLD arise largely from changes in synchrony, rather than level, of neuronal activity.