Functional Significance of High Baseline Activity

用正电子发射断层扫描（PET）和功能性磁共振成像（fMRI）进行的现代功能性脑成像研究常规地证明了任务诱发的脑活动的增加和减少。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“失活”）对许多人来说仍然是一个谜。一些减少，简单地说，与进行成像实验的方式有关。因此，活动在局部处于控制状态，而在与之比较的任务状态中不存在，这将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，在广泛的实验范式范围内，似乎在其位置变化不大的反复出现的减少组不能如此早地解释。在本项目目前资助的资助期内，我们已经确定，在这组经常性减少中，发生基线活动真正减少的领域（详见本项目）。在这个项目中，我们考察了它们可能的功能意义。我们关注两个中线区域：后扣带皮层和相邻的楔前叶；内侧和眶额皮质。它们的独特之处在于，它们的基线活动大大超过了大脑皮层的其他区域，这表明存在一个默认系统，旨在组装和评估其情感和动机意义。任何形式的集中注意力都会导致该系统和杏仁核的活动立即减少（84）。这与通常的观察一致，即集中的认知活动减弱了情绪唤醒并限制了目标导向的活动。然而，在任务执行过程中，该系统活动暂停的程度体现了任务需求和我们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相一致。这个项目中实验的目的是更清楚地定义认知与情感和动机的关系，通过这个默认系统的活动变化来表达它的基线活动水平。

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.