Ances B M, Zarahn E, Greenberg J H, Detre J A
Department of Neurology, University of Pennsylvania, Philadelphia 19104, USA.
J Cereb Blood Flow Metab. 2000 Jun;20(6):921-30. doi: 10.1097/00004647-200006000-00004.
Changes in cerebral blood flow (CBF) because of functional activation are used as a surrogate for neural activity in many functional neuroimaging studies. In these studies, it is often assumed that the CBF response is a linear-time invariant (LTI) transform of the underlying neural activity. By using a previously developed animal model system of electrical forepaw stimulation in rats (n = 11), laser Doppler measurements of CBF, and somatosensory evoked potentials, measurements of neural activity were obtained when the stimulus duration and intensity were separately varied. These two sets of time series data were used to assess the LTI assumption. The CBF data were modeled as a transform of neural activity (N1-P2 amplitude of the somatosensory evoked potential) by using first-order (linear) and second-order (nonlinear) components. Although a pure LTI model explained a large amount of the variance in the data for changes in stimulus duration, our results demonstrated that the second-order kernel (i.e., a nonlinear component) contributed an explanatory component that is both statistically significant and appreciable in magnitude. For variations in stimulus intensity, a pure LTI model explained almost all of the variance in the CBF data. In particular, the shape of the CBF response did not depend on intensity of neural activity when duration was held constant (time-intensity separability). These results have important implications for the analysis and interpretation of neuroimaging data.
在许多功能神经影像学研究中，因功能激活引起的脑血流量（CBF）变化被用作神经活动的替代指标。在这些研究中，通常假定CBF反应是潜在神经活动的线性时不变（LTI）变换。通过使用先前开发的大鼠前爪电刺激动物模型系统（n = 11）、激光多普勒测量CBF以及体感诱发电位，在分别改变刺激持续时间和强度时获得神经活动测量值。这两组时间序列数据用于评估LTI假设。通过使用一阶（线性）和二阶（非线性）成分，将CBF数据建模为神经活动（体感诱发电位的N1 - P2振幅）的变换。尽管纯LTI模型解释了刺激持续时间变化数据中的大量方差，但我们的结果表明，二阶核（即非线性成分）贡献了一个在统计上显著且幅度可观的解释成分。对于刺激强度的变化，纯LTI模型解释了CBF数据中几乎所有的方差。特别是，当持续时间保持恒定时（时间 - 强度可分离性），CBF反应的形状不依赖于神经活动的强度。这些结果对神经影像学数据的分析和解释具有重要意义。
