Observation of a fast response in functional MR.

A spectroscopic MR technique was used to investigate the time course of the MR signal following a single visual stimulus. Gated experiments demonstrate there is an early response (500 ms after stimulus) leading to a reduction of the MR signal by -0.25% (P = 0.02), whereas a slower response (> 1500 ms after stimulus) results in a signal increase of +0.59% (P = 0.01). The fast negative response may be attributed to increased oxygen consumption, followed by a slower vascular response with overcompensation in blood oxygenation. This explanation would be in agreement with the blood oxygenation level dependent (BOLD) contrast mechanism considered the basis of functional MR. However, other physiological events might also be responsible for the signal drop observed.