Figure 2.

Stereotypical example of a blood-oxygenation level-dependent (BOLD) hemodynamic response observed using functional magnetic resonance imaging (fMRI) subsequent to a brief (impulse-like) stimulation. Four response components are depicted following this stimulation. (A) The “initial dip” in which oxygen consumption temporarily outstrips delivery of oxygenated hemoglobin to the neural tissue. (B) A delayed, yet rapid increase of the signal level subsequent to increased local blood volume and blood flow, resulting in greater delivery of oxygenated hemoglobin than is required to offset oxygen consumption. The delay of this rise is typically observed to be 1–2 seconds, with the peak being achieved between 4–7 seconds after stimulus onset. (C) Gradual return of the signal level to “baseline” (that observed prior to presentation of the stimulus) associated with decreases in blood volume and blood flow (D) The post-stimulus undershoot during which the observed signal levels are depressed relative to those observed prior to stimulus presentation. This drop in signal level is associated with a temporal decoupling of the blood flow recovery relative to blood volume, producing decreased oxygen delivery. See Mandeville et al. (1998) for further information about the underlying physiology.