National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland 20892, USA.
J Neurosci. 2013 Mar 13;33(11):4672-82. doi: 10.1523/JNEUROSCI.2922-12.2013.
A widely held assumption is that spontaneous and task-evoked brain activity sum linearly, such that the recorded brain response in each single trial is the algebraic sum of the constantly changing ongoing activity and the stereotypical evoked activity. Using functional magnetic resonance imaging signals acquired from normal humans, we show that this assumption is invalid. Across widespread cortices, evoked activity interacts negatively with ongoing activity, such that higher prestimulus baseline results in less activation or more deactivation. As a consequence of this negative interaction, trial-to-trial variability of cortical activity decreases following stimulus onset. We further show that variability reduction follows overlapping but distinct spatial pattern from that of task-activation/deactivation and it contains behaviorally relevant information. These results favor an alternative perspective to the traditional dichotomous framework of ongoing and evoked activity. That is, to view the brain as a nonlinear dynamical system whose trajectory is tighter when performing a task. Further, incoming sensory stimuli modulate the brain's activity in a manner that depends on its initial state. We propose that across-trial variability may provide a new approach to brain mapping in the context of cognitive experiments.
人们普遍认为，自发性和任务诱发的大脑活动是线性相加的，因此在每个单独的试验中记录的大脑反应是不断变化的持续活动和典型诱发活动的代数和。我们使用从正常人类获得的功能磁共振成像信号表明，这种假设是无效的。在广泛的皮质中，诱发活动与持续活动呈负相互作用，因此较高的预刺激基线导致较少的激活或更多的去激活。由于这种负相互作用，皮质活动的trial-to-trial 可变性在刺激开始后减小。我们进一步表明，可变性减小遵循与任务激活/去激活重叠但不同的空间模式，并且包含与行为相关的信息。这些结果支持了一种替代传统的持续和诱发活动二分法框架的观点。也就是说，将大脑视为一个非线性动力系统，在执行任务时其轨迹更紧。此外，传入的感觉刺激以依赖于其初始状态的方式调节大脑的活动。我们提出，跨试验可变性可能为认知实验背景下的大脑映射提供一种新方法。
