Department of Radiology, Michigan State University, East Lansing, MI 48824, USA.
Neuroimage. 2011 Jul 15;57(2):391-402. doi: 10.1016/j.neuroimage.2011.04.067. Epub 2011 May 7.
In event-related (ER) BOLD-fMRI brain activation studies, understanding the relationship between the elicited BOLD signal and its underlying neuronal activity is essential for any quantitative interpretation of the neural events from the BOLD measurements. This requires a better understanding of the dynamic BOLD response. Besides the neuronal activity-induced positive BOLD response, the dynamic response is also characterized by a profound post-stimulus undershoot. The relationship between the positive response and the post-stimulus undershoot, however, remains poorly understood. Earlier studies using block-design paradigms with long stimulation durations (>10s) do not suggest a quantitative relationship. Using an ER paradigm, this study revealed a linear coupling between the positive BOLD response and the post-stimulus undershoot across the human visual cortex. The voxelwise linear coupling across the visual cortex strongly supports a homogeneous hemodynamic response in ER paradigms, though the BOLD response magnitude varies substantially over a wide range across the visual cortex. Although underlying neuronal activity is responsible for a BOLD response, the blood volume fraction affects the magnitude of the BOLD response; the larger the blood volume fraction, the larger the magnitude. This effect needs to be accounted for in any quantitative interpretation of the BOLD measurements. In the absence of nonlinear neuronal activities, the nonlinear vascular response renders the estimated BOLD responses smaller in rapid presentation (RP) ER paradigms compared to that in ER paradigms, and this reduction effect also needs to be considered when interpreting the estimated BOLD responses in RP-ER paradigms. Interestingly, this nonlinear effect might be simply accounted for by a scaling factor across the visual cortex.
在事件相关（ER）BOLD-fMRI 脑激活研究中，理解诱发的 BOLD 信号与其潜在的神经元活动之间的关系对于从 BOLD 测量中对神经事件进行任何定量解释都是至关重要的。这需要更好地了解动态 BOLD 响应。除了神经元活动引起的正 BOLD 响应之外，动态响应还以深刻的刺激后欠冲为特征。然而，正响应与刺激后欠冲之间的关系仍未得到很好的理解。使用具有长刺激持续时间（> 10s）的块设计范式的早期研究并未表明存在定量关系。本研究使用 ER 范式，揭示了人类视觉皮层中正 BOLD 响应与刺激后欠冲之间的线性耦合。视觉皮层中的体素水平线性耦合强烈支持 ER 范式中的均匀血液动力学响应，尽管 BOLD 响应幅度在整个视觉皮层上的广泛范围内有很大变化。虽然潜在的神经元活动是 BOLD 响应的原因，但血容量分数会影响 BOLD 响应的幅度；血容量分数越大，幅度越大。在对 BOLD 测量进行任何定量解释时，都需要考虑到这一影响。在没有非线性神经元活动的情况下，快速呈现（RP）ER 范式中的非线性血管响应会导致估计的 BOLD 响应小于 ER 范式中的响应，在解释 RP-ER 范式中的估计 BOLD 响应时也需要考虑这种降低效应。有趣的是，这种非线性效应可以通过跨视觉皮层的缩放因子来简单地解释。
