Uludağ Kâmil
Max-Planck-Institute for Biological Cybernetics, High-Field Magnetic Resonance Center, Tübingen, Germany.
Magn Reson Imaging. 2008 Sep;26(7):863-9. doi: 10.1016/j.mri.2008.01.049. Epub 2008 May 13.
Examining the transients of the blood-oxygenation-level-dependent (BOLD) signal using functional magnetic resonance imaging is a tool to probe basic brain physiology. In addition to the so-called initial dip and poststimulus undershoot of the BOLD signal, occasionally, overshoot at the beginning and at the end of stimulation and stimulus onset and offset ('phasic') responses are observed. Hemifield visual stimulation was used in human subjects to study the latter transients. As expected, sustained ('tonic') stimulus-correlated contralateral activation in the visual cortex and LGN was observed. Interestingly, bilateral phasic responses were observed, which only partly overlapped with the tonic network and which would have been missed using a standard analysis. A biomechanical model of the BOLD signal ('balloon model') indicated that, in addition to phasic neuronal activity, vascular uncoupling can also give rise to phasic BOLD signals. Thus, additional physiological information (i.e., cerebral blood flow) and examination of spatial distribution of the activity might help to assess the BOLD signal transients correctly. In the current study, although vascular uncoupled responses cannot be ruled out as an explanation of the observed phasic BOLD network, the spatial distribution argues that sustained hemifield visual stimulation evokes both bilateral phasic and contralateral sustained neuronal responses. As a consequence, in rapid event-related experimental designs, both the phasic and tonic networks cannot be separated, possibly confounding the interpretation of BOLD signal data. Furthermore, a combination of phasic and tonic responses in the same region of interest might also mimic a BOLD response typically observed in adaptation experiments.
使用功能磁共振成像检查血氧水平依赖（BOLD）信号的瞬态是探究基本脑生理学的一种工具。除了BOLD信号的所谓初始下降和刺激后下冲外，偶尔还会观察到刺激开始和结束时的过冲以及刺激开始和结束时的（“相位”）反应。在人类受试者中使用半视野视觉刺激来研究后者的瞬态。正如预期的那样，在视觉皮层和外侧膝状体中观察到了与刺激相关的持续（“紧张性”）对侧激活。有趣的是，观察到双侧相位反应，其仅部分与紧张性网络重叠，并且使用标准分析会遗漏这些反应。BOLD信号的生物力学模型（“气球模型”）表明，除了相位神经元活动外，血管解耦也可产生相位BOLD信号。因此，额外的生理信息（即脑血流量）以及对活动空间分布的检查可能有助于正确评估BOLD信号瞬态。在当前研究中，尽管不能排除血管解耦反应作为观察到的相位BOLD网络的一种解释，但空间分布表明持续的半视野视觉刺激会引发双侧相位和对侧持续神经元反应。因此，在快速事件相关实验设计中，相位和紧张性网络都无法分开，这可能会混淆BOLD信号数据的解释。此外，在同一感兴趣区域中相位和紧张性反应的组合也可能模拟在适应实验中通常观察到的BOLD反应。
