Hemodynamics elicited by an impulse of neuronal activity as predicted by a dynamical biophysical model (see Friston et al 2000a for details). A burst of neuronal activity causes an increase in flow inducing signal that decays with first order kinetics and is down regulated by local flow. This signal increases rCBF with dilates the venous capillaries, increasing volume (v). Concurrently, venous blood is expelled from the venous pool decreasing deoxyhemoglobin content (q). The resulting fall in deoxyhemoglobin concentration leads to a transient increases in BOLD (blood oxygenation level dependent) signal and a subsequent undershoot.