Quantitative relationships among the neurophysiologic processes that link neuronal activity to hemodynamic change is extremely important to interpret the functional magnetic resonance imaging (fMRI) signals. In this article the neurovascular coupling relationship was noninvasively studied in the human visual cortex. Graded neuronal/hemodynamic suppression conditions were generated using a paired-stimulus paradigm. Visual evoked potential (VEP) was measured to quantify neuronal activity. Hemodynamic activities were measured and quantified by perfusion changes. All quantification was normalized to the same activation condition using a single-stimulus paradigm within each experimental session. The results reveal: (i) there is a tight neurovascular coupling at graded neuronal suppression conditions; (ii) the neurovascular coupling relationship contains a subtle, but significant, nonlinear component; (iii) the linear model, nevertheless, is still a good approximation reflecting the neurovascular coupling relationship.