Ready��Go: Amplitude of the fMRI Signal Encodes Expectation of Cue Arrival Time

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.当大脑等待一个抵达时间不确定的信号时，会发生什么？正如短跑运动员等待发令枪时的状态。而在发令枪响后即刻又会发生什么？本研究利用功能磁共振成像（functional magnetic resonance imaging，fMRI）技术，在多个脑区中发现了与时间预期相关的全新神经关联特征，其中以补充运动区（supplementary motor area，SMA）最为显著。与预期相悖的是，我们在等待阶段几乎未观测到fMRI信号活动；然而在出发信号发出后，会出现一个显著的大振幅信号，其振幅反映了对潜在等待时间分布的习得性预期。具体而言，fMRI信号的振幅似乎编码了累积条件概率，这一指标也被称为累积风险函数（cumulative hazard function）。在“倒计时”范式中，出发提示的抵达时间已预先知晓，此时fMRI信号不再依赖于等待时长，这表明该信号编码的是时间概率，而非单纯的已流逝时间。在“不执行（no-go）”范式中，信号仍表现出对时间预期的依赖性，这证明该效应并非运动输出的结果。最后，这种编码并不依赖于刺激模态，在听觉或视觉信号下均以相同方式运作。本研究发现拓展了我们对时间预期与可观测神经信号之间关系的认知。