Hedera P, Wu D, Lewin J S, Miller D, Lerner A J, Friedland R P
Department of Neurology, University Hospitals of Cleveland, Ohio, USA.
Invest Radiol. 1995 Nov;30(11):625-33. doi: 10.1097/00004424-199511000-00001.
Brain activation can be detected by functional magnetic resonance imaging. Termination of stimulation can cause drop of postactivation signal below preactivation baseline; the nature of this "undershoot" remains controversial. The authors investigated postactivation signal after stimulation of the visual cortex with photic stimuli of different duration.
Activation of visual cortex in 11 healthy subjects was studied. The authors underwent short and long stimulation. The relation between activation and postactivation signal was investigated after both durations of stimulation.
Average postactivation signal after short stimulation was -1.71 +/- 2.66% and after long stimulation 0.82 +/- 1.59% (P < or = 0.01). Significant "undershoot" was detected after short stimulation in the majority of cases, although this was typically absent after long stimulation.
The authors propose that "undershoot" is caused by decreased oxygen content in venous blood compared with the preactivation level and may reflect increased extraction of oxygen as a result of glycolytic metabolism. The absence of "undershoot" after longer stimulation suggests a gradual shift from uncoupling between regional cerebral blood flow and oxygen consumption toward a steady state.
脑激活可通过功能磁共振成像检测到。刺激终止可导致激活后信号降至激活前基线以下；这种“负后冲”的本质仍存在争议。作者研究了用不同持续时间的光刺激视觉皮层后的激活后信号。
对11名健康受试者的视觉皮层激活情况进行了研究。作者接受了短时间和长时间刺激。在两种刺激持续时间后，研究了激活与激活后信号之间的关系。
短时间刺激后的平均激活后信号为-1.71±2.66%，长时间刺激后为0.82±1.59%（P≤0.01）。在大多数情况下，短时间刺激后检测到显著的“负后冲”，而长时间刺激后通常不存在这种情况。
作者提出，“负后冲”是由于静脉血中氧含量与激活前水平相比降低所致，可能反映了糖酵解代谢导致的氧摄取增加。长时间刺激后不存在“负后冲”表明从局部脑血流与氧消耗之间的解偶联逐渐向稳定状态转变。
