Figure 3.

The current view of the physiological basis of the BOLD response is more complex than what was modeled with the balloon model. The key difference is that current studies support a picture of CBF and CMRO₂ responses driven in parallel by different aspects of neural activity, so that we must think of a hemodynamic response and a metabolic response as independent effects. The origin of the post-stimulus undershoot is still uncertain, although the original assumption of a pure CBV effect cannot explain all of the current experimental data. Instead, a useful focus now is between a hemodynamic hypothesis, including elements of both a CBF undershoot and a slow venous CBV recovery (as shown) or a metabolic hypothesis related to slow recovery of CMRO₂.