Howseman A M, Porter D A, Hutton C, Josephs O, Turner R
Wellcome Department of Cognitive Neurology, Institute of Neurology, London, UK.
Magn Reson Imaging. 1998;16(1):1-11. doi: 10.1016/s0730-725x(97)00238-5.
Previous functional magnetic resonance imaging (MRI) studies using extended visual stimulation have reported disparate results. Two studies have shown that blood oxygen level dependent (BOLD) contrast decays over time which is cited as evidence of recoupling between oxygen utilisation and cerebral blood flow during stimulus presentation. These findings have serious implications for the design of functional MRI experiments because they raise the possibility that BOLD contrast may not accurately reflect neuronal activity. Another study reported no decay of BOLD contrast. These studies used different visual stimuli and imaging techniques. We have performed a series of experiments, using different MRI techniques (echo-planar imaging and fast low angle shot) and two different visual stimuli to assess which of these factors may explain the previous results. In all of our experiments the signal time course from areas of significant activation remained largely elevated throughout the duration of stimulation and this is not affected by the imaging method used. Our data, in accordance with that of Bandettini et al., suggest that recoupling between blood flow and oxygen extraction is not a general phenomenon in the human brain when visual stimuli are presented for an extended time.
以往使用长时间视觉刺激的功能磁共振成像（MRI）研究报告了不同的结果。两项研究表明，血氧水平依赖（BOLD）对比度随时间衰减，这被引作刺激呈现期间氧利用与脑血流重新耦合的证据。这些发现对功能MRI实验的设计具有严重影响，因为它们增加了BOLD对比度可能无法准确反映神经元活动的可能性。另一项研究报告称BOLD对比度没有衰减。这些研究使用了不同的视觉刺激和成像技术。我们进行了一系列实验，使用不同的MRI技术（回波平面成像和快速低角度激发）以及两种不同的视觉刺激，以评估这些因素中的哪些可能解释先前的结果。在我们所有的实验中，在整个刺激持续时间内，来自显著激活区域的信号时间进程在很大程度上保持升高，并且这不受所使用的成像方法的影响。我们的数据与班德蒂尼等人的数据一致，表明当长时间呈现视觉刺激时，血流与氧摄取之间的重新耦合在人类大脑中不是一种普遍现象。
