Ready...go: Amplitude of the FMRI signal encodes expectation of cue arrival time.
Ready...go: Amplitude of the FMRI signal encodes expectation of cue arrival time.
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DOI：
10.1371/journal.pbio.1000167
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发表时间：
2009-08
期刊：
影响因子：
9.8
通讯作者：
Eagleman DM
中科院分区：
文献类型：
--
作者：
Cui X;Stetson C;Montague PR;Eagleman DM
一项神经成像研究揭示了大脑如何对预期事件做出反应的新见解，比如发令枪或对绿灯的反应。当大脑等待一个不确定到达时间的信号时，就像短跑运动员等待发令枪一样，会发生什么？发令枪打响后会发生什么？利用功能性磁共振成像（fMRI），我们发现了几个大脑区域中时间预期的新关联，最突出的是在辅助运动区（SMA）。与预期相反，我们发现在等待期间几乎没有fMRI活动；然而，在“开始”信号之后出现一个大信号，其振幅反映了对可能等待时间分布的学习预期。具体来说，fMRI信号的振幅似乎编码了一个累积条件概率，也称为累积风险函数。fMRI信号在“倒计时”条件下失去了对等待时间的依赖，在“倒计时”条件下，围棋提示的到达时间是提前知道的，这表明信号编码的是时间概率，而不仅仅是经过的时间。信号对时间期望的依赖存在于“no-go”条件下，表明该效应不是运动输出的结果。最后，编码不依赖于模态，以与听觉或视觉信号相同的方式操作。这一发现扩展了我们对时间预期和可测量神经信号之间关系的理解。就像短跑运动员等待发令枪一样，所有的动物都会对事件何时会及时发生产生预期。我们使用功能性磁共振成像（fMRI）探索了准备和期望的神经相关性，并发现在等待期间，大脑区域的fMRI信号保持在基线，并在提示做出反应（“开始”提示）后急剧上升。引人注目的是，上升幅度反映了当时发生事件概率的函数。即使在没有运动行为的情况下（即，当go提示出现时没有按下按钮），对概率的依赖仍然存在。当围棋提示的到达时间提前已知时，依赖于预期的信号就消失了，这表明大脑的反应反映的是预期，而不仅仅是经过的时间。这些结果与之前对大脑期望的研究相吻合，但有一个重要的区别：之前的电生理学实验表明，随着等待期的进行，期望是由峰值活动的积累来编码的，而我们的功能磁共振成像数据显示，等待结束后，期望的特征变得明显。我们讨论了测量大脑中预期相关活动的这些不同技术之间明显的不匹配。
A neuroimaging study reveals novel insights into how the brain responds to an anticipated event, such as a starting gun or responding to a green light. What happens when the brain awaits a signal of uncertain arrival time, as when a sprinter waits for the starting pistol? And what happens just after the starting pistol fires? Using functional magnetic resonance imaging (fMRI), we have discovered a novel correlate of temporal expectations in several brain regions, most prominently in the supplementary motor area (SMA). Contrary to expectations, we found little fMRI activity during the waiting period; however, a large signal appears after the “go” signal, the amplitude of which reflects learned expectations about the distribution of possible waiting times. Specifically, the amplitude of the fMRI signal appears to encode a cumulative conditional probability, also known as the cumulative hazard function. The fMRI signal loses its dependence on waiting time in a “countdown” condition in which the arrival time of the go cue is known in advance, suggesting that the signal encodes temporal probabilities rather than simply elapsed time. The dependence of the signal on temporal expectation is present in “no-go” conditions, demonstrating that the effect is not a consequence of motor output. Finally, the encoding is not dependent on modality, operating in the same manner with auditory or visual signals. This finding extends our understanding of the relationship between temporal expectancy and measurable neural signals. Like the sprinter waiting for the starting pistol, all animals develop expectations about when events will occur in time. We explored the neural correlates of readiness and expectation using functional magnetic resonance imaging (fMRI), and found areas of the brain in which the fMRI signal remains at baseline during the waiting period and rises sharply after a cue to react (a “go” cue). Strikingly, the amplitude of the rise reflects a function of the probability of an event occurring at that time. The dependence on probability remains even in the absence of a motor act (that is, not pressing a button when the go cue appears). When the arrival time of the go cue is known in advance, the expectation-dependent signal disappears, indicating that this brain response reflects expectation, not simply elapsed time. These results match up with prior studies of expectation in the brain, with one important difference: previously, electrophysiology experiments showed that expectation is encoded by a build-up of spiking activity as the waiting period progresses, while our fMRI data reveal a signature of expectation that becomes apparent after the waiting concludes. We discuss the apparent mismatch between these different technologies for measuring expectation-related activity in the brain.
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