Functional Significance of High Baseline Activity

使用正电子发射断层扫描(PET)和功能磁共振成像(FMRI)的现代脑功能成像研究经常显示任务诱导的脑活动增加和减少。这种增加(通常被称为“激活”)通常被认为反映了大脑局部细胞活动的增加。减少)有时被称为“停用”)对许多人来说仍然是个谜。一些减少仅仅与成像实验的进行方式有关。因此，在控制状态下局部存在的活动和在与之相比较的任务状态中不存在的活动很自然地表现为减少。对许多人来说，这个解释已经足够了。然而，在广泛的实验范式中，反复出现的一组下降似乎在其位置上几乎没有变化，因此不能这么早地解释。在本计划项目目前获得资助的授权期内，我们已经确定，在这组经常性减少中，有一些领域的活动确实比基线活动有所减少(有关详细信息，请参阅本项目)。在这个项目中，我们研究它们可能的功能意义。我们集中在两个中线区域：后扣带回皮质和相邻的楔前叶；以及内侧和眼眶额叶皮质。它们的独特之处在于，它们的基线活动远远超过大脑皮层的其他区域，这表明存在一个默认系统，旨在集合和评估其情绪和动机意义。任何类型的集中注意力都会立即导致该系统和杏仁核活动的减少(84)。这与通常的观察一致，即集中的认知活动会减弱情绪唤醒，并抑制目标导向的活动。然而，在任务执行过程中，这个系统内的活动被暂停的程度体现了任务需求和我们的情绪状态之间的平衡。这也与我们的情绪和动机状态反过来影响认知表现的普遍观察是一致的。这个项目中实验的目的是更清楚地定义认知与情绪和动机的关系，这是通过这个默认系统的活动关于其基线活动水平的变化来表达的。

Modern functional brain imaging studies with both positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) routinely demonstrated task-induced increases as well as decreases in brain activity. The increases (usually referred to as "activations") are generally thought to reflect increases in the local cellular activity of the brain. Decreases )sometimes called "deactivations") have remained an enigma for many. Some decreases related, simply, to the manner in which the imaging experiment was conducted. Thus, activity present locally in a control state and absent in a task state to which it is compared would, quite naturally, appear as a decrease. For many, this explanation is sufficient. However, a recurring group of decreases that appear to vary little in their locations across a broad range of experimental paradigm cannot so early be explained. During the presently funded grant period of this Program Project we have determined that among this group of recurring decreases are areas where true decreases from baseline activity occur (see this Project for further details). In this project we examine their possible functional significance. We focus on two midline areas: the posterior cingulate cortex and adjacent precuneus; and, the medial and the orbital frontal cortices. They are unique in that their baseline activity greatly excee3ds other areas of the cerebral cortex suggesting the presence of a default system designed to assemble and evaluate its emotional and motivational significance. Focused attention of any sort causes an immediate reduction in the activity of this system and in the amygdala (84). This is consistent with the common observation that focused cognitive activity attenuates emotional arousal and constrains goal directed activity. However, the degree to which activity within this system is suspended during task performance presents a balance between task demands and our emotional state. This is also consistent with the common observation that our emotional and motivation state, in turn, influence cognitive performance. It is the purpose of the experiments in this project to more clearly define the relationship of cognitive to emotion and motivation as expressed through changes in the activity of this default system about its baseline level of activity.