The brain's default mode network consists of discrete, bilateral and symmetrical cortical areas, in the medial and lateral parietal, medial prefrontal, and medial and lateral temporal cortices of the human, nonhuman primate, cat, and rodent brains. Its discovery was an unexpected consequence of brain-imaging studies first performed with positron emission tomography in which various novel, attention-demanding, and non-self-referential tasks were compared with quiet repose either with eyes closed or with simple visual fixation. The default mode network consistently decreases its activity when compared with activity during these relaxed nontask states. The discovery of the default mode network reignited a longstanding interest in the significance of the brain's ongoing or intrinsic activity. Presently, studies of the brain's intrinsic activity, popularly referred to as resting-state studies, have come to play a major role in studies of the human brain in health and disease. The brain's default mode network plays a central role in this work.
大脑的默认模式网络由离散的、双侧对称的皮层区域组成,这些区域位于人类、非人类灵长类动物、猫和啮齿动物的大脑内侧和外侧顶叶、内侧前额叶以及内侧和外侧颞叶。它的发现是脑成像研究的意外结果,这些研究最初使用正电子发射断层扫描,比较了各种新颖的、需要注意的和非自我参照的任务与安静休息状态(无论是闭眼还是简单的视觉固定)。与这些放松的非任务状态相比,默认模式网络的活动始终减少。默认模式网络的发现重新点燃了人们对大脑持续或内在活动重要性的长期兴趣。目前,大脑的内在活动研究,通常被称为静息态研究,已在健康和疾病的人脑研究中发挥了重要作用。大脑的默认模式网络在这项工作中扮演了核心角色。
- activation, 激活,
- attention, 注意,
- baseline, 基线,
- intrinsic activity, 内在活动,
- functional connectivity,
功能连接性 - resting state, 静息状态,
- self, 自我
- memory 记忆
It has been 17 years since Shulman et al. (1997) first noted that a constellation of areas in the human cerebral cortex consistently reduced its activity while performing various novel, non-self-referential, goal-directed tasks (Shulman et al. 1997) when these tasks were compared with a control state of quiet repose (i.e., a resting state of eyes closed or visual fixation). That these localized reductions in activity were occurring at all was surprising, and their consistency across a wide variety of tasks made it all the more remarkable. The immediate challenge was to prove that these activity decreases were not due to activations in the resting state caused by experimentally uncontrolled cognition.
自从 Shulman 等人(1997)首次指出人类大脑皮层中的一组区域在执行各种新颖的、非自我相关的、目标导向的任务时,其活动始终减少(Shulman et al. 1997),已经过去了 17 年。这些任务与安静休息状态(即闭眼或视觉固定的休息状态)进行比较时,这些局部活动的减少令人惊讶,而它们在各种任务中的一致性更是引人注目。眼下的挑战是证明这些活动减少并非由于实验中未控制的认知引起的休息状态激活。
In 2001 we used positron emission tomography (PET) measurements of regional blood flow and oxygen consumption to show, by established metabolic criteria for activation, that areas consistently exhibiting activity reductions during task performance were not activated in the resting state. Our article was titled, “A Default Mode of Brain Function” (Raichle et al. 2001). We concluded that the brain areas observed to decrease their activity during attention-demanding, goal-directed tasks were not activated in the resting state but, rather, were indicative of a heretofore-unrecognized organization within the brain's intrinsic or ongoing activity. Parenthetically, it had not occurred to us that others would anoint the constellation of areas exhibiting this unique behavior as the brain's default mode network. The name obviously caught on.
在 2001 年,我们使用正电子发射断层扫描(PET)测量区域血流和氧气消耗,通过已建立的代谢激活标准,显示在任务执行过程中持续表现出活动减少的区域在静息状态下并未被激活。我们的文章标题为《大脑功能的默认模式》(Raichle 等,2001)。我们得出的结论是,在需要注意和目标导向的任务中观察到活动减少的大脑区域在静息状态下并未被激活,而是表明大脑内在或持续活动中存在一种之前未被认识的组织。顺便提一下,我们没有想到其他人会将表现出这种独特行为的区域星座称为大脑的默认模式网络。这个名称显然得到了广泛认可。
Research on the brain's default mode network and the brain's intrinsic activity more generally has moved in many directions producing a literature that has become quite extensive. In preparing this review, we examined this literature to look for general trends, which we summarize in
Figure 1
for the interested reader. Rather than attempting a detailed analysis of this entire body of work, I focus on topics within it that have been of particular interest to me and that provide a sense of the future of this work.
关于大脑默认模式网络及大脑内在活动的研究已经朝多个方向发展,形成了相当庞大的文献。在准备这篇综述时,我们审视了这些文献,以寻找一般趋势,我们在图 1 中为感兴趣的读者进行了总结。与其对整个研究领域进行详细分析,我更关注其中一些特别引起我兴趣的话题,并提供对这一领域未来发展的感知。
This article begins with a review of our initial work because of its centrality in establishing the legitimacy of the brain's default mode network.
本文首先回顾了我们的初步研究,因为它在确立大脑默认模式网络的合法性方面具有重要意义。
By the early 1980s, PET began to receive serious attention as a potential functional neuroimaging device in human subjects (Raichle 2009). The study of human cognition with neuroimaging was aided greatly by the involvement of cognitive psychologists in the 1980s. Their experimental strategies for dissecting human behaviors fit well with the emerging capabilities of functional brain imaging (Posner & Raichle 1994) in which one measures the time required to complete specific mental operations isolated by the careful selection of task and control states. This approach, in various forms, has dominated the cognitive neuroscience agenda ever since, with remarkably productive results (e.g., see Price 2012).
到 1980 年代初,正电子发射断层扫描(PET)开始受到严肃关注,作为一种潜在的人体功能性神经成像设备(Raichle 2009)。在 1980 年代,认知心理学家的参与极大地促进了神经成像对人类认知的研究。他们用于剖析人类行为的实验策略与功能性脑成像的新兴能力相得益彰(Posner & Raichle 1994),该技术通过精心选择任务和控制状态来测量完成特定心理操作所需的时间。这种方法以各种形式主导了认知神经科学的议程,并取得了显著的成果(例如,见 Price 2012)。
For the better part of the decade following the introduction of subtractive methodology to neuroimaging, the vast majority of changes reported in the literature were activity increases (or activations, to use the jargon of the cognitive neuroscience field). Activity increases but not decreases are expected in subtractions of a control condition from a task condition as long as the assumption of pure insertion is not violated. To illustrate, using an example based on mental chronometry, say that one's control task requires a key press to a simple stimulus, such as the appearance of a point of light in the visual field, whereas the task state requires a decision about the color of the light prior to the key press. Assuming pure insertion, the response latency difference between conditions is interpretable as the time needed to perform the color discrimination. However, the time needed to press a key might be affected by the nature of the decision process itself, violating the assumption of pure insertion. More generally, the brain state underlying any action could be altered by introducing an additional process.
在引入减法方法用于神经成像的十年间,大多数文献中报告的变化都是活动增加(或称为激活,使用认知神经科学领域的术语)。在从任务条件中减去控制条件时,只要不违反纯插入的假设,通常会预期到活动增加而非减少。举个例子,假设一个人的控制任务要求对简单刺激(例如视觉场中出现的光点)进行按键,而任务状态则要求在按键之前对光的颜色做出判断。在假设纯插入的情况下,条件之间的反应延迟差异可以解释为进行颜色辨别所需的时间。然而,按键所需的时间可能会受到决策过程本身性质的影响,从而违反纯插入的假设。更一般地说,任何行动背后的大脑状态都可能因引入额外的过程而发生改变。
Functional neuroimaging helped address the issue of pure insertion by employing the device of reverse subtraction. Thus, in certain circumstances, subtracting task state data from control state data revealed negative responses or task-specific deactivations (Gusnard & Raichle 2001). Investigators clearly showed, just as psychologists had suspected, that processes active in a control state could be modified when paired with a particular task. However, none of this work prepared us or anyone else for the experiment in which the control state was rest (i.e., simply lying quietly but awake in a scanner with eyes closed or visually fixating on a crosshair).
功能性神经成像通过采用逆向减法的方法帮助解决了纯插入的问题。因此,在某些情况下,从控制状态数据中减去任务状态数据揭示了负响应或任务特异性去激活(Gusnard & Raichle 2001)。研究人员清楚地表明,正如心理学家所怀疑的那样,控制状态下的活跃过程在与特定任务配对时可以被修改。然而,这些研究并没有为我们或其他人准备好控制状态为休息的实验(即,仅仅在扫描仪中安静地躺着但保持清醒,眼睛闭合或视觉集中在十字准星上)。
One of the guiding principles of cognitive psychology at that time was that a control state must explicitly contain all the elements of the associated task state other than the one element of interest (e.g., seeing a word versus reading the same word). Using a control state of rest would clearly seem to violate that principle. Despite our commitment to the strategies of cognitive psychology in our experiments, we routinely obtained resting-state scans in all our experiments, a habit largely carried over from experiments involving simple sensory stimuli (Fox et al. 1986) in which the control state was simply the absence of the stimulus (i.e., a resting state). At some point in our work, and I do not recall the motivation, I began to look at the resting-state scans minus the task scans. What immediately caught my attention was the fact that regardless of the task under investigation, the activity decreases almost always included the posterior cingulate and the adjacent precuneus.
当时认知心理学的一个指导原则是,控制状态必须明确包含与相关任务状态的所有元素,除了一个感兴趣的元素(例如,看到一个单词与阅读同一个单词)。使用静息状态作为控制状态显然似乎违反了这一原则。尽管我们在实验中坚持认知心理学的策略,但我们在所有实验中都常规地获取静息态扫描,这一习惯主要源于涉及简单感官刺激的实验(Fox 等,1986),在这些实验中,控制状态仅仅是刺激的缺失(即静息状态)。在我们工作的某个时刻,我开始查看静息态扫描与任务扫描的差异,虽然我不记得具体的动机。令我立即注意到的是,无论研究的任务是什么,活动的减少几乎总是包括后扣带皮层和相邻的前扣带皮层。
The first formal characterization of task-induced activity decreases from a resting state was a meta-analysis of 9 PET studies, involving 134 subjects, by my colleague Gordon Shulman (Shulman et al. 1997). This study generated an iconic image of a network of cortical areas that decreased in activity while performing various attention-demanding, largely non-self-referential tasks (
Figure 2
). The unique identity of this network was confirmed a short time later by Jeffrey Binder and colleagues at the Medical College of Wisconsin (Binder et al. 1999) and Bernard Mazoyer and his colleagues in France (Mazoyer et al. 2001). Similar observations are now an everyday occurrence in laboratories worldwide as investigators seek to understand the role of this network in brain function (
Figure 1
).
任务诱导的活动从静息状态下降的首次正式表征是我同事戈登·舒尔曼(Shulman et al. 1997)对 9 项正电子发射断层扫描(PET)研究的荟萃分析,涉及 134 名受试者。这项研究生成了一个标志性的图像,展示了在执行各种需要注意力的、主要非自我参照的任务时,皮层区域活动减少的网络(图 2)。这一网络的独特身份在不久后得到了威斯康星医学院的杰弗里·宾德和他的同事们(Binder et al. 1999)以及法国的伯纳德·马佐耶和他的同事们(Mazoyer et al. 2001)的确认。如今,类似的观察在全球实验室中已成为日常现象,研究人员试图理解这一网络在大脑功能中的作用(图 1)。
Finding a network of brain areas frequently seen to decrease its activity during attention-demanding tasks was both surprising and challenging: surprising because the areas involved had not previously been recognized as a system in the same way that we might think of the motor or visual system, and challenging because initially it was unclear how to characterize their activity in a passive or resting condition. Were they simply activations present in the resting state? And why should they appear in both PET and functional magnetic resonance imaging (fMRI)? Two things came to mind that offered a way forward.
发现一个在注意力需求任务中经常活动减少的脑区网络既令人惊讶又具有挑战性:令人惊讶的是,这些涉及的区域之前并未被认定为一个系统,就像我们可能认为的运动系统或视觉系统一样;而具有挑战性的是,最初不清楚如何在被动或静息状态下表征它们的活动。它们是否仅仅是在静息状态下存在的激活?而它们为什么会同时出现在正电子发射断层扫描(PET)和功能性磁共振成像(fMRI)中?有两件事浮现在脑海中,为前进提供了方向。
First, the manner in which functional imaging was conducted with fMRI carried with it a physiological definition of activation that could be measured with PET. This definition arose from the quantitative circulatory and metabolic PET studies, which demonstrated that when brain activity increases transiently above a resting state, blood flow increases more than oxygen consumption (reviewed in Raichle & Mintun 2006). As a result, the amount of oxygen in blood increases locally as the ratio of oxygen consumed to oxygen delivered falls. This ratio is known as the oxygen extraction fraction (OEF). Activation can then be defined physiologically as a transient local decrease in the OEF, which results in a local increase in oxygen availability.
首先,功能成像的方式通过功能性磁共振成像(fMRI)与正电子发射断层扫描(PET)相结合,带来了可以测量的生理激活定义。这个定义源于定量循环和代谢的 PET 研究,这些研究表明,当大脑活动暂时高于静息状态时,血流的增加超过了氧气消耗(详见 Raichle & Mintun 2006)。因此,血液中的氧气量在局部增加,因为消耗的氧气与输送的氧气的比率下降。这个比率被称为氧气提取分数(OEF)。激活可以生理上定义为 OEF 的局部暂时性下降,这导致局部氧气可用性的增加。
The practical consequence of this observation was to lay the physiological basis for fMRI using blood-oxygen-level-dependent (BOLD) contrast (Bandettini et al. 1992, Frahm et al. 1992, Kwong et al. 1992, Ogawa et al. 1990, Thulborn et al. 1982). Using this quantitative definition of activation, we asked whether activation was present in a resting state. But activation must be defined relative to something. How was a comparison to be accomplished if there was no control state for eyes-closed rest or visual fixation?
这一观察的实际结果是为功能性磁共振成像(fMRI)奠定了生理基础,使用了血氧水平依赖(BOLD)对比(Bandettini 等,1992 年;Frahm 等,1992 年;Kwong 等,1992 年;Ogawa 等,1990 年;Thulborn 等,1982 年)。利用这种定量的激活定义,我们询问在静息状态下是否存在激活。然而,激活必须相对于某种状态来定义。如果没有闭眼静息或视觉固定的对照状态,如何进行比较呢?
The definition of a control state for eyes-closed rest or visual fixation arose from the second critical piece of physiological information. Researchers using PET to quantitatively measure brain oxygen consumption and blood flow had long appreciated the fact that, across the entire brain, blood flow and oxygen consumption are closely matched when resting quietly in a PET scanner (Lebrun-Grandie et al. 1983, Raichle et al. 2001). This is observed despite a nearly fourfold difference in oxygen consumption and blood flow between gray and white matter and variations of greater than 30% within gray matter. As a result of this close matching of blood flow and oxygen consumption at rest, the OEF is strikingly uniform throughout the brain. This well-established observation led us to hypothesize that if this observation (i.e., a uniform OEF at rest) was correct then activations, as defined above, were likely absent in the resting state. We decided to test this hypothesis.
眼闭休息或视觉固定的控制状态定义源于第二个关键的生理信息。研究人员使用正电子发射断层扫描(PET)定量测量大脑氧气消耗和血流,早已认识到在 PET 扫描仪中安静休息时,整个大脑的血流和氧气消耗是密切匹配的(Lebrun-Grandie 等,1983 年;Raichle 等,2001 年)。尽管灰质和白质之间的氧气消耗和血流存在近四倍的差异,以及灰质内部超过 30%的变异,这一现象依然显著。因此,由于在休息状态下血流和氧气消耗的紧密匹配,氧气提取分数(OEF)在整个大脑中表现出显著的均匀性。这一公认的观察结果使我们假设,如果这一观察(即休息时 OEF 均匀)是正确的,那么上述定义的激活在休息状态下可能是缺失的。我们决定测试这一假设。
Using PET to quantitatively assess regional OEF, we examined two groups of normal subjects in the resting state, confining our analysis initially to the regions shown in
Figure 2
. We found no evidence that these cortical areas were activated when compared with other areas (i.e., the OEF was uniform). We concluded that the regional decreases observed commonly during task performance represented the presence of an organized functionality that was ongoing in the resting state and that this functionality was attenuated in the presence of an attention-demanding, non-self-referential task. On the basis of this perspective, we titled our paper “A Default Mode of Brain Function,” not suspecting at the time that the focus of our analysis would become known as the brain's default mode network. We (Drevets et al. 1995) and others (Amedi et al. 2005, Ghatan et al. 1998, Kawashima et al. 1995, Shmuel et al. 2006, Smith et al. 2000, Somers et al. 1999) had noted other more task-specific deactivations, consistent with the idea that a default mode of brain function is broadly based across all brain systems (a hypothesis that received substantial support from functional studies of the brain's resting state).
使用正电子发射断层扫描(PET)定量评估区域氧气提取分数(OEF),我们在静息状态下研究了两组正常受试者,最初将分析限制在图 2 所示的区域。我们发现,与其他区域相比,这些皮层区域没有被激活的证据(即,OEF 是均匀的)。我们得出结论,任务执行期间观察到的区域性减少代表了一种在静息状态下持续进行的有组织功能,而在需要注意的非自我参照任务存在时,这种功能会减弱。基于这一观点,我们将论文命名为“脑功能的默认模式”,当时并未预料到我们的分析焦点会被称为大脑的默认模式网络。我们(Drevets 等,1995)和其他研究者(Amedi 等,2005;Ghatan 等,1998;Kawashima 等,1995;Shmuel 等,2006;Smith 等,2000;Somers 等)。 1999 年) 注意到其他更具任务特异性的去激活现象,这与大脑功能的默认模式在所有大脑系统中广泛存在的观点一致(这一假设得到了大脑静息状态功能研究的 substantial 支持)。
The discovery of the default mode network made apparent the need for additional ways to study the large-scale intrinsic organization of the brain. A major step forward was the discovery that this large-scale network organization, including but not limited to the default mode network, could be revealed by studying spatial coherence patterns in the spontaneous fluctuations (i.e., noise) in the fMRI BOLD signal during the resting state (for more detailed reviews, see Raichle 2010, 2011).
默认模式网络的发现显现了研究大规模内在脑组织的额外方法的必要性。一个重要的进展是发现这种大规模网络组织,包括但不限于默认模式网络,可以通过研究静息状态下 fMRI BOLD 信号自发波动(即噪声)中的空间一致性模式来揭示(有关更详细的综述,请参见 Raichle 2010, 2011)。
A prominent feature of fMRI is the noise in the raw BOLD signal. This has prompted researchers to average their data to increase the signal and reduce noise. As first shown by Bharat Biswal and colleagues (1995) in the human somatomotor system, a considerable fraction of this noise exhibits striking patterns of coherence within known brain systems.
功能性磁共振成像(fMRI)的一个显著特征是原始 BOLD 信号中的噪声。这促使研究人员对数据进行平均,以增强信号并减少噪声。正如 Bharat Biswal 及其同事(1995 年)首次在人体躯体运动系统中所展示的,这部分噪声在已知的脑系统中表现出显著的相干模式。
The significance of this observation was brought forcefully to our attention when Michael Greicius and colleagues considered the coherence patterns in the default mode network (Greicius et al. 2003) elicited by placing a region of interest in either the posterior cingulate cortex (
Figure 2a
, yellow arrow) or the ventral medial prefrontal cortex (
Figure 2a
, orange arrow). The resulting time-activity curves (
Figure 2b
) reflected a coherence pattern within the entire default mode network (
Figure 2c
). Similar patterns of resting-state coherence have now been documented in most cortical systems in the human brain (
Figure 2d
; for recent reviews, see Fox & Raichle 2007, Smith et al. 2009, Snyder & Raichle 2012) as have their subcortical connections (Zhang et al. 2008).
这一观察的重要性在于,当迈克尔·格雷修斯及其同事考虑在默认模式网络中(Greicius et al. 2003)通过将感兴趣区域放置在后扣带皮层(图 2a,黄色箭头)或腹内侧前额叶皮层(图 2a,橙色箭头)时,强烈引起了我们的注意。由此产生的时间-活动曲线(图 2b)反映了整个默认模式网络内的相干模式(图 2c)。现在,大多数人脑皮层系统中已经记录到了类似的静息态相干模式(图 2d;有关最近的综述,参见 Fox & Raichle 2007,Smith et al. 2009,Snyder & Raichle 2012),其皮下连接也同样如此(Zhang et al. 2008)。
默认模式网络的组成部分
A frequently asked question, which has an incomplete answer at this time, is, “What is the function of the default mode network?” There are several ways to approach answering the question. I begin by summarizing what we know about the behavioral functions associated with the major anatomical subdivisions of the default mode network in humans and note the themes that have emerged from this perspective (see also Andrews-Hanna et al. 2010b). I then tackle some of the more fundamental insights that have emerged from studies of the default mode network and intrinsic activity that inject a note of caution in interpreting too literally the behavioral data obtained from humans.
一个常被问到的问题,目前的答案并不完整,那就是:“默认模式网络的功能是什么?”有几种方法可以来回答这个问题。我首先总结我们对人类默认模式网络主要解剖分区相关的行为功能的了解,并指出从这个角度出现的一些主题(另见 Andrews-Hanna 等,2010b)。然后,我将探讨一些从默认模式网络和内在活动的研究中得出的更基本的见解,这些见解在解读人类获得的行为数据时提醒我们要谨慎。
The default mode network is divided into roughly three major subdivisions: the ventral medial prefrontal cortex; the dorsal medial prefrontal cortex; and the posterior cingulate cortex and adjacent precuneus plus the lateral parietal cortex (approximately Brodmann area 39). These general subdivisions are easily appreciated in
Figure 2a
. Another area that has been associated frequently with the default mode network is the entorhinal cortex.
默认模式网络大致分为三个主要子区域:腹内侧前额叶皮层、背内侧前额叶皮层,以及后扣带皮层和相邻的楔前叶加侧顶叶皮层(大约布罗德曼区 39)。这些一般的子区域在图 2a 中很容易被识别。另一个与默认模式网络经常相关联的区域是内嗅皮层。
The ventral medial prefrontal cortex (VMPC) is an area about which much is known in terms of its cytoarchitectonics and anatomical connectivity, thanks to the work of such people as Joel Price (e.g., see Ongür & Price 2000) and Helen Barbas (Barbas 2007). Although most of this work has been done in nonhuman primates, evidence suggests that most of what was learned from nonhuman primates exists in humans (Ongür & Price 2000). From this work we know that the VMPC is a critical element in a network of areas that receive sensory information from the external world and the body via the orbital frontal cortex and convey that information to structures such as the hypothalamus, the amygdala, and the periaqueductal gray matter of the midbrain. This anatomical circuitry alone has much to say about the potential role of this component of the default mode network as a sensory-visceromotor link concerned with social behavior, mood control, and motivational drive, all of which are important components of an individual's personality.
腹内侧前额叶皮层(VMPC)是一个在细胞结构和解剖连接方面有很多研究成果的区域,这得益于乔尔·普莱斯(Joel Price)和海伦·巴尔巴斯(Helen Barbas)等人的工作(例如,见 Ongür & Price 2000,Barbas 2007)。尽管大多数研究是在非人类灵长类动物中进行的,但证据表明,从非人类灵长类动物中获得的大部分知识在人体中也存在(Ongür & Price 2000)。通过这些研究,我们知道 VMPC 是一个网络中的关键组成部分,该网络接收来自外部世界和身体的感官信息,通过眶额皮层将这些信息传递给下丘脑、杏仁体和中脑的导水管周围灰质等结构。仅从这一解剖回路来看,就可以推测该默认模式网络的这一组成部分在社会行为、情绪控制和动机驱动等方面作为感官-内脏运动连接的潜在角色,这些都是个体人格的重要组成部分。
Since the publication of the paradigmatic patient Phineas Gage (for a detailed review of this fascinating case, see Damasio et al. 1994), there have been many reports of striking personality changes and deviant social behavior appearing in premorbidly normal individuals after damage to the VMPC (Bechara et al. 1997, Damasio & Van Hoesen 1983).
自从典型患者菲尼亚斯·盖奇的研究发表以来(有关这一引人入胜案例的详细回顾,请参见 Damasio 等人 1994 年),许多报告指出,在损伤腹内侧前额叶皮层(VMPC)后,原本正常的个体出现了显著的人格变化和偏离社会行为的现象(Bechara 等人 1997 年,Damasio 与 Van Hoesen 1983 年)。
Imaging studies in normal individuals have shown that the emotional state of the subject has a direct effect on the activity level in the VMPC component of the default mode network. In studies of performance anxiety induced by task difficulty, the degree to which VMPC decreased its activity in concert with other elements of the default mode network was directly proportional to the subject's anxiety level while performing the task. With high anxiety, the VMPC decreased little if at all. As anxiety decreased with practice on the task activity so too did activity in the VMPC (Simpson et al. 2001b). In a companion study, Simpson et al. (2001a) induced anticipatory anxiety in normal subjects by having them anticipate a painful shock to the fingers of one hand. Activity decreases in the VMPC were inversely correlated with anxiety self-rating, such that the least anxious subjects exhibited the largest reductions, whereas the most anxious subjects showed no significant reduction or a slight increase. Taken together, these two studies illustrate that activity in the VMPC component of the default mode network reflects a dynamic balance between focused attention and a subject's emotional state and may occur from a functionally active baseline (i.e., a default state).
1
正常个体的成像研究表明,受试者的情绪状态对默认模式网络中腹内侧前额皮层(VMPC)成分的活动水平有直接影响。在因任务难度引发的表现焦虑研究中,VMPC 与默认模式网络其他元素的活动水平下降程度与受试者在执行任务时的焦虑水平成正比。当焦虑水平较高时,VMPC 的活动几乎没有下降。随着任务活动的练习,焦虑水平降低,VMPC 的活动也随之减少(Simpson et al. 2001b)。在一项相关研究中,Simpson et al.(2001a)通过让正常受试者预期一只手指将受到疼痛电击来诱发预期焦虑。VMPC 的活动下降与自我评估的焦虑水平呈负相关,即焦虑水平最低的受试者表现出最大的活动减少,而焦虑水平最高的受试者则没有显著减少,甚至略有增加。 综合来看,这两项研究表明,默认模式网络中 VMPC 成分的活动反映了专注注意力与个体情绪状态之间的动态平衡,并可能源于一个功能上活跃的基线(即默认状态)。1
Elements of the VMPC actually increase their activity in association with disruptions in bodily homeostasis. This change is nicely illustrated in association with the autonomic responses to stepped hypoglycemia [i.e., increase in heart rate and plasma levels of epinephrine, norepinephrine, and pancreatic polypeptide (Teves et al. 2004)], even when typical symptoms of hypoglycemia are mild.
VMPC 的元素实际上在身体稳态受到干扰时会增加其活动。这一变化在与逐步低血糖的自主反应中得到了很好的说明[即心率和肾上腺素、去甲肾上腺素及胰腺多肽的血浆水平增加(Teves 等,2004)],即使低血糖的典型症状较轻。
Finally, the discovery that the agranular, subgenual portion of the VMPC had reduced blood flow and glucose consumption in addition to a reduced volume in familial bipolar and unipolar depressives (Drevets et al. 1997) has led not only to greater insight into the functional anatomy of mood disorders but also to treatment of intractable depression by stimulating this area with chronically implanted electrodes (Mayberg et al. 2005).
最后,发现家庭性双相和单相抑郁症患者的无颗粒下前额叶皮层(VMPC)部分血流和葡萄糖消耗减少,体积也减小(Drevets et al. 1997),不仅加深了对情绪障碍功能解剖的理解,还通过对该区域进行慢性植入电极刺激,为难治性抑郁症的治疗提供了新的思路(Mayberg et al. 2005)。
Although it is immediately adjacent, the dorsal medial prefrontal cortex (DMPC) can be distinguished from the VMPC by its association with self-referential judgments. An example of this comes from our earlier work (Gusnard et al. 2001) in which subjects were asked to make a self-referential judgment (i.e., pleasant or unpleasant) about emotionally valenced pictures from the International Affective Picture System (IAPS) (Lang et al. 1997). Increases in activity were observed in the DMPC and accompanied by decreases in the VMPC, consistent with the fact that attention-demanding tasks attenuate emotional processing.
尽管它与腹内侧前额叶皮层(VMPC)紧密相邻,但背内侧前额叶皮层(DMPC)因其与自我相关判断的关联而有所区别。一个例子来自我们早期的研究(Gusnard et al. 2001),在该研究中,受试者被要求对国际情感图片系统(IAPS)中的情感图片进行自我相关判断(即愉快或不愉快)。在 DMPC 中观察到活动增加,而在 VMPC 中则伴随活动减少,这与注意力需求高的任务会减弱情感处理的事实是一致的。
The posterior cingulate cortex and the medial precuneus are prominent features of the default mode network and were the first to come to our attention. These areas, along with the lateral parietal components of the default mode network, have been consistently associated with successful recollection of previously studied items (for a review of this literature, see Vincent et al. 2006). The same paper presents resting-state functional connectivity MRI, demonstrating a significant relationship between the hippocampal formation and the posterior elements of the default mode network. In a subsequent study, Shannon and colleagues (2013) demonstrated that this hippocampal-parietal memory network exhibits remarkable diurnal variation being strongly present in the evening and absent in the morning after a normal night's sleep. This finding suggests that the relationship between the hippocampal formation and the posterior elements of the default mode network are sensitive to the cumulative experiences of wake and that sleep resets this relationship each day. Parenthetically, these dramatic diurnal variations in the functional connectivity of the default mode network should be considered when planning experiments designed to study the role of the default mode network in memory and learning.
后扣带皮层和内侧楔前叶是默认模式网络的显著特征,最早引起了我们的注意。这些区域与默认模式网络的外侧顶叶成分一起,一直与成功回忆先前学习的项目密切相关(有关此文献的综述,见 Vincent 等,2006 年)。同一篇论文展示了静息态功能连接 MRI,表明海马结构与默认模式网络的后部元素之间存在显著关系。在随后的研究中,Shannon 及其同事(2013 年)证明了这一海马-顶叶记忆网络表现出显著的昼夜变化,在晚上强烈存在,而在正常睡眠后的早晨则缺失。这一发现表明,海马结构与默认模式网络后部元素之间的关系对清醒的累积经验敏感,而睡眠每天重置这种关系。 顺便提一下,在规划旨在研究默认模式网络在记忆和学习中作用的实验时,应考虑到默认模式网络功能连接的这些显著昼夜变化。
To summarize, data from humans suggest that the default mode network instantiates processes that support emotional processing (VMPC), self-referential mental activity (DMPC), and the recollection of prior experiences (posterior elements of the default mode network). These functional elements of the default mode network can be differentially affected during task performance by the nature of the task [e.g., presence or absence of an emotional component or an element of self-reference (Andrews-Hanna et al. 2010b, Gusnard et al. 2001)]. However, regardless of the details of a particular task, the default mode network always begins from a baseline of high activity, with small changes in this activity made to accommodate the requirements of a particular task. The available evidence indicates that the functions of the default mode network are never turned off but, rather, carefully enhanced or attenuated.
总而言之,人类的数据表明,默认模式网络实现了支持情感处理(VMPC)、自我参照的心理活动(DMPC)以及回忆先前经历(默认模式网络的后部元素)的过程。这些默认模式网络的功能元素在任务执行过程中会因任务的性质而受到不同程度的影响[例如,情感成分的存在或缺失,或自我参照元素(Andrews-Hanna 等,2010b;Gusnard 等,2001)]。然而,无论特定任务的细节如何,默认模式网络始终从高活动的基线开始,并根据特定任务的要求对这种活动进行小幅调整。现有证据表明,默认模式网络的功能从未关闭,而是被精心增强或减弱。
Because the default mode network was first identified with the resting state, it has been appealing to many (
Figure 1
) to associate default mode network functionality with the mental state that commonly accompanies a relaxed state of quiet repose, namely daydreaming, mind wandering, or stimulus-independent thoughts (e.g., see Andrews-Hanna et al. 2010a). Furthermore, spontaneous cognition routinely involves thoughts about one's personal past and future, which fits comfortably with identified functionality in the default mode network in humans (see above). However, several factors lead this author to believe that focusing solely on spontaneous cognition ignores the possibility of a much more fundamental role for the default mode network in brain function. Following are observations to support this viewpoint.
由于默认模式网络最初是在静息状态下被识别出来的,因此许多人(见图 1)将默认模式网络的功能与通常伴随放松状态的心理状态联系起来,这种状态包括白日梦、思维游离或与刺激无关的思维(例如,参见 Andrews-Hanna 等,2010a)。此外,自发性认知通常涉及对个人过去和未来的思考,这与人类默认模式网络中已识别的功能相吻合(见上文)。然而,几个因素使得作者认为,仅仅关注自发性认知忽视了默认模式网络在大脑功能中可能扮演的更为根本的角色。以下是支持这一观点的观察。
Spontaneous activity, in contrast with spontaneous cognition, is the major factor contributing to the high cost of brain function in humans. Although the adult human brain is only 2% of the body weight, it consumes 20% of the body's energy budget (Clarke & Sokoloff 1999). Relative to the high rate of ongoing energy consumption, which is devoted largely to functional activity (reviewed in Raichle & Mintun 2006), the additional energy consumption associated with task-evoked changes in brain activity is small, usually less than 5% locally (Raichle & Mintun 2006). There is no reason to suppose that unconstrained thoughts are more energy demanding than are constrained ones, leaving much to be explained about the nature of spontaneous activity.
自发活动与自发认知相比,是导致人类大脑功能高成本的主要因素。尽管成年人的大脑仅占体重的 2%,但它消耗了身体能量预算的 20%(Clarke & Sokoloff 1999)。相对于持续的高能量消耗,这部分能量主要用于功能活动(详见 Raichle & Mintun 2006),与任务引发的脑活动变化相关的额外能量消耗相对较小,通常在局部低于 5%(Raichle & Mintun 2006)。没有理由认为无约束的思维比有约束的思维更耗能,这使得自发活动的本质仍有许多需要解释的地方。
Additionally, the general features of the default mode network have now been identified in the monkey (Vincent et al. 2007), cat (Popa et al. 2009), rat (Lu et al. 2012), and mouse (Stafford et al. 2014). Visually comparing the topography of the default mode network in the rat, monkey, and human (
Figure 3
) evokes a sense of similarity, but the details show clear differences. For example, in the human, the lateral parietal component is approximately in Brodmann area 39. Monkeys do not have a comparable parietal area. In the rat, the lateral parietal component resides in primary sensory cortices. Similarities are most evident along the midline, but here again, differences emerge from the details. Regardless of the details of the anatomy, if one accepts the idea that a default mode network is an integral component of the mammalian brain, which remains to be firmly established, it seems clear that the mental states of rats and humans, let alone nonhuman primates, differ substantially. A future research focus on the evolution of the default mode network will likely help us to understand its functions more deeply.
此外,默认模式网络的一般特征现在已在猴子(Vincent et al. 2007)、猫(Popa et al. 2009)、老鼠(Lu et al. 2012)和小鼠(Stafford et al. 2014)中被识别出来。通过视觉比较老鼠、猴子和人类的默认模式网络的拓扑(图 3),可以感受到相似性,但细节上却显示出明显的差异。例如,在人类中,外侧顶叶成分大约位于布罗德曼区 39,而猴子没有可比的顶叶区域。在老鼠中,外侧顶叶成分位于初级感觉皮层。相似性在中线区域最为明显,但在细节上又出现了差异。无论解剖细节如何,如果接受默认模式网络是哺乳动物大脑的一个重要组成部分这一观点(这一点尚待进一步确认),那么显然,老鼠和人类的心理状态,甚至非人类灵长类动物的心理状态,存在显著差异。未来对默认模式网络演化的研究将有助于我们更深入地理解其功能。
Finally, patterns of resting-state functional connectivity appear to transcend levels of consciousness, being present under anesthesia in humans (Greicius et al. 2008), monkeys (Vincent et al. 2007), and rats (Lu et al. 2007) and also during the early stages of sleep in humans (Fukunaga et al. 2006, Larson-Prior et al. 2009). These observations make it unlikely that the coherence patterns and the intrinsic activity they represent are primarily the result of unconstrained, conscious cognition [i.e., mind wandering or daydreaming (Christoff et al. 2009)].
最后,静息态功能连接的模式似乎超越了意识水平,在人类(Greicius et al. 2008)、猴子(Vincent et al. 2007)和老鼠(Lu et al. 2007)麻醉状态下均存在,并且在早期睡眠阶段的人类中也有出现(Fukunaga et al. 2006,Larson-Prior et al. 2009)。这些观察结果使得这些一致性模式及其所代表的内在活动主要是无约束的意识认知(即,心智游荡或白日梦(Christoff et al. 2009))的结果的可能性变得不大。
I turn now to two examples from our own work that have influenced my thinking about the default mode network. Both relate to the concept of a functional balance between the default mode network and other brain systems and the resulting implications for our understanding of the function of the default mode network.
我现在转向我们自己工作的两个例子,这些例子影响了我对默认模式网络的思考。这两者都与默认模式网络与其他脑系统之间的功能平衡概念有关,以及由此对我们理解默认模式网络功能的影响。
The first example comes from a highly cited paper in our early work on the default mode network and resting-state functional connectivity (Fox et al. 2005). This paper called attention to the presence of anticorrelations in the resting state between the default mode network and what we dubbed the “task-positive network”. The latter consisted of what is more conventionally called the dorsal attention network (DAN; Corbetta & Shulman 2002, Fox et al. 2006) and elements of frontoparietal control networks (Fair et al. 2007, Seeley et al. 2007). Even though controversy surrounded the data-processing procedures that revealed this relationship (for a detailed discussion, see Fox et al. 2009), the intuitive appeal of the observation has remained strong because it captured a relationship between the default mode network and the DAN that had been well characterized by the performance of novel, attention-demanding tasks (i.e., increases in the DAN accompanied by decreases in the default mode network). An apt metaphor that often comes to mind is “losing one's self in one's work.”
第一个例子来自我们早期关于默认模式网络和静息态功能连接的高引用论文(Fox et al. 2005)。这篇论文引起了人们对静息态中默认模式网络与我们称之为“任务正向网络”之间存在反相关关系的关注。后者主要由更传统称为背侧注意网络(DAN;Corbetta & Shulman 2002,Fox et al. 2006)和前顶叶控制网络的元素组成(Fair et al. 2007,Seeley et al. 2007)。尽管围绕揭示这一关系的数据处理程序存在争议(有关详细讨论,请参见 Fox et al. 2009),但这一观察的直观吸引力依然强烈,因为它捕捉到了默认模式网络与 DAN 之间的关系,这一关系在执行新颖且需要注意力的任务时得到了很好的表征(即,DAN 的增加伴随着默认模式网络的减少)。一个恰当的比喻常常浮现在脑海中,那就是“沉浸于工作中”。
Our work on the anticorrelations not only reinforced interest in resting-state imaging studies with fMRI of the brain's intrinsic activity, but also stimulated an important neurophysiological exploration of this relationship by Pare and his colleagues (Popa et al. 2009). Instrumenting cats with chronic indwelling electrodes, investigators recorded unit activity and local field potentials in the cat homologues of the default mode network and the DAN across the sleep-wake cycle and during variations in attentional demands. Noteworthy was the observation that anticorrelations between the two networks occurred ∼20% of the time, whereas correlations were present for the remaining 80% of the time; these observations suggested a variable relationship between cooperation and antagonism. Furthermore, during increased attentional demands, firing rates within the cat default mode network actually increased despite a reduction in local field potential (LFP) power, which suggests that the default mode network may play an enhanced role during increased attentional demands.
我们的研究不仅增强了对使用功能性磁共振成像(fMRI)研究大脑内在活动的静息态成像研究的兴趣,还激发了 Pare 及其同事(Popa 等,2009)对这一关系的重要神经生理学探索。研究人员在猫身上植入慢性留置电极,记录了猫的默认模式网络和注意网络在睡眠-觉醒周期及注意需求变化期间的单元活动和局部场电位。值得注意的是,观察到这两个网络之间的反相关关系大约发生了 20%的时间,而在其余 80%的时间内则存在相关关系;这些观察结果表明合作与对抗之间存在一种可变关系。此外,在注意需求增加期间,尽管局部场电位(LFP)功率降低,猫的默认模式网络内的放电频率实际上却增加,这表明在注意需求增加时,默认模式网络可能发挥了更重要的作用。
To fully appreciate the potential significance of the work by Pare and colleagues (Popa et al. 2009), one must note a critical difference with the data manipulations employed in resting-state fMRI, which first revealed the anticorrelations between the default mode network and DAN (Fox et al. 2005; a more detailed discussion is presented in Fox et al. 2009). In the fMRI experiment, the data were prepared by removing a signal common to all areas of the brain (i.e., the so-called global signal). By doing so, correlations between the default mode network and the DAN, if hidden within this signal, would be removed. In the work of Pare and colleagues, no such maneuver was performed. If, in fact, this observation explains the difference between the neurophysiological observations in cats (i.e., anticorrelations present only part of the time) and those in humans (i.e., anticorrelations present all the time in the resting state), then our understanding of the relationship between the default mode network [i.e., a self-centered predictive model of the world (Raichle 2010)] and the DAN (i.e., detector of novel environmental features) must be reconsidered. Is attention, as conventionally defined, limited to conscious perception of environmental novelty? Or does attention also involve a nonconscious component that orients us to the predictive regularities of the environment upon which we base most of our behaviors? We must take a more nuanced approach to our understanding of the resources of the brain in which networks such as the default mode network and the DAN are always “on” but adjusting subtly their relationships. Understanding the dialogue between the default mode network and the DAN is likely a critical place to begin this work.
要充分理解 Pare 及其同事(Popa 等,2009 年)工作的潜在重要性,必须注意到与静息态 fMRI 中使用的数据处理方法之间的一个关键区别,后者首次揭示了默认模式网络与注意力网络(DAN)之间的反相关关系(Fox 等,2005 年;更详细的讨论见 Fox 等,2009 年)。在 fMRI 实验中,数据通过去除大脑所有区域共有的信号(即所谓的全局信号)进行准备。通过这样做,默认模式网络与 DAN 之间的相关性,如果隐藏在该信号中,将被去除。在 Pare 及其同事的研究中,并未进行这样的操作。如果这一观察确实解释了猫的神经生理观察(即反相关关系仅在部分时间存在)与人类观察(即反相关关系在静息状态下始终存在)之间的差异,那么我们对默认模式网络(即一个以自我为中心的世界预测模型,Raichle 2010 年)与 DAN(即新环境特征的探测器)之间关系的理解必须重新考虑。 注意力,按照传统定义,是否仅限于对环境新奇事物的意识感知?还是说注意力还涉及一个非意识成分,使我们能够适应环境中的预测规律,而这些规律是我们大多数行为的基础?我们必须对大脑资源的理解采取更细致的方式,其中像默认模式网络和注意力网络(DAN)这样的网络始终处于“开启”状态,但其关系在微妙地调整。理解默认模式网络与注意力网络之间的对话,可能是开展这项工作的关键起点。
The second example is a resting-state fMRI study we performed on 107 juvenile offenders (Shannon et al. 2011), in whom we could relate with remarkable accuracy organizational features of their brains to their levels of impulsivity. In this study, we found that in less impulsive juveniles and normal controls, motor-planning regions were correlated with brain networks associated with spatial attention and executive control. In more impulsive juveniles, these motor-planning regions were correlated with the default mode network. Our results suggested that the balance between the default mode network and the networks controlling spatial attention and executive control was critical in determining the output of cortical motor-planning areas and, ultimately, the subject's level of impulsivity. To further explore the relationship between impulsivity and neural development, we studied functional connectivity of the same motor-planning regions in 95 typically developing individuals across a wide age span. The change in functional connectivity with age mirrored that of impulsivity: Younger subjects tended to exhibit functional connectivity similar to the more impulsive incarcerated juveniles, whereas older subjects exhibited a less impulsive pattern. It seems reasonable to suggest that the default mode network is playing a critical role in the organization and expression of preplanned, reflexive behaviors that are critical to our existence in a complex world but when unconstrained by the social and physical constraints of the environment become impulsive and destructive. These opposing forces are captured nicely in the book by Daniel Kahneman, Thinking, Fast and Slow (Kahneman 2011).
第二个例子是我们对 107 名青少年罪犯进行的静息态功能性磁共振成像研究(Shannon 等,2011),在这项研究中,我们能够以显著的准确性将他们大脑的组织特征与冲动性水平相关联。在这项研究中,我们发现,在冲动性较低的青少年和正常对照组中,运动规划区域与与空间注意和执行控制相关的大脑网络存在相关性。而在冲动性较高的青少年中,这些运动规划区域则与默认模式网络相关。我们的结果表明,默认模式网络与控制空间注意和执行控制的网络之间的平衡在决定皮层运动规划区域的输出以及最终个体的冲动性水平方面至关重要。为了进一步探讨冲动性与神经发育之间的关系,我们研究了 95 名典型发育个体在广泛年龄范围内的相同运动规划区域的功能连接性。 功能连接随年龄的变化与冲动性相呼应:年轻受试者的功能连接往往与更具冲动性的被监禁青少年相似,而年长受试者则表现出较少冲动的模式。可以合理地认为,默认模式网络在组织和表达预先计划的、反射性的行为中发挥着关键作用,这些行为对我们在复杂世界中的生存至关重要,但当不受环境的社会和物理约束时,会变得冲动和具有破坏性。这些对立的力量在丹尼尔·卡尼曼的书《思考,快与慢》中得到了很好的体现(卡尼曼 2011)。
Finally, the potential importance of the default mode network to brain organization is captured in studies focused on its connectional anatomy. This is exemplified by the work of Hagmann and colleagues (2008). By using diffusion spectrum imaging, these researchers noninvasively mapped cortico-cortical axonal pathways in humans. The analysis of their data revealed a structural core within the posterior medial and parietal cerebral cortices as well as distinct temporal and frontal modules. Brain regions within this structural core constitute connector hubs that link all major structural models. The structural core they identified contained regions that form the posterior components of the default mode network. They also compared measures of structural connectivity with measures of resting-state functional connectivity and concluded that there was a substantial correspondence between the two measures. The conclusion they drew from their work was that this structural core, centered as it were on the posterior elements of the default mode network, is important for functional integration. Additional studies of this type (e.g., see van den Heuvel & Sporns 2011) complement and expand on the perspective put forth by Hagmann and colleagues and places the default mode network at the center of the brain's organization both structurally and functionally.
最后,默认模式网络对大脑组织的重要性在于其连接解剖学的研究中得到了体现。这一点在 Hagmann 及其同事(2008)的工作中得到了例证。通过使用扩散谱成像,这些研究人员非侵入性地绘制了人类皮层-皮层轴突通路。他们数据的分析揭示了后内侧和顶叶大脑皮层内的一个结构核心,以及明显的颞叶和额叶模块。这个结构核心中的大脑区域构成了连接枢纽,连接所有主要的结构模型。他们识别出的结构核心包含形成默认模式网络后部成分的区域。他们还将结构连接性测量与静息态功能连接性测量进行了比较,得出两者之间存在显著对应关系的结论。他们从研究中得出的结论是,这个以默认模式网络后部元素为中心的结构核心对于功能整合至关重要。此类研究的进一步探索(例如……)(参见 van den Heuvel & Sporns 2011)补充并扩展了 Hagmann 及其同事提出的观点,并将默认模式网络置于大脑结构和功能组织的中心。
The brain's default mode network is a relative newcomer to discussions about brain organization and function, but it invigorates discussions that go back many years regarding the basic nature of brain function. One view espoused by Sherrington (1906) posited that the brain is primarily reflexive, driven by the momentary demands of the environment. The other view was put forth by Sherrington's student T. Graham Brown (1914) that the brain's operations were mainly intrinsic, involving the acquisition and maintenance of information for interpreting, responding to, and even predicting environmental demands (for a more recent perspective, see Llinas 2001, Raichle 2010, Yuste et al. 2005). The latter view clearly seems to be on the ascendency, with a rapidly increasing interest in the ongoing activity of the brain, which must construct and maintain an operational model of the world and implement it on the basis of highly impoverished sensory information (Raichle 2010). One of the features of that activity is its large-scale functional organization, in which the default mode network appears to be playing a commanding role. Our challenge is to better define that role, which will likely force us to rethink well-established concepts such as attention and to seek the involvement of scientists at all levels of inquiry. Although this review did not consider issues from a cellular perspective, such views are commanding increasing attention, even as they relate to the default mode network in humans (e.g., see Goyal et al. 2014). Inspection of
Figure 1
demonstrates a major interest in the role of intrinsic activity, and particularly the default mode network, in disease. This is likely to be an exciting frontier in the years ahead as we take a fresh look at such diseases as Alzheimer's (Buckner et al. 2008, Vlassenko et al. 2010) and depression (Drevets et al. 1997, Greicius et al. 2004, Mayberg et al. 2005), which defy simple explanation. The default mode network is likely to figure prominently in all these areas of inquiry, the more so as we come to understand its true role in brain function.
大脑的默认模式网络在关于大脑组织和功能的讨论中相对较新,但它为许多年来关于大脑功能基本性质的讨论注入了活力。Sherrington(1906)提出的一种观点认为,大脑主要是反射性的,受环境瞬时需求的驱动。另一种观点是 Sherrington 的学生 T. Graham Brown(1914)提出的,认为大脑的运作主要是内在的,涉及获取和维持信息,以便解释、响应甚至预测环境需求(有关更近期的观点,参见 Llinas 2001,Raichle 2010,Yuste 等,2005)。后者的观点显然正在上升,随着对大脑持续活动的兴趣迅速增加,大脑必须构建和维持一个世界的操作模型,并在高度贫乏的感官信息基础上实施该模型(Raichle 2010)。这种活动的一个特征是其大规模的功能组织,其中默认模式网络似乎发挥着主导作用。 我们的挑战是更好地定义这一角色,这可能迫使我们重新思考一些已确立的概念,如注意力,并寻求各个研究层次的科学家的参与。尽管本综述没有从细胞的角度考虑问题,但这种观点正受到越来越多的关注,甚至与人类的默认模式网络相关(例如,见 Goyal 等,2014 年)。对图 1 的检查显示了对内在活动,特别是默认模式网络在疾病中作用的重大兴趣。随着我们重新审视阿尔茨海默病(Buckner 等,2008 年;Vlassenko 等,2010 年)和抑郁症(Drevets 等,1997 年;Greicius 等,2004 年;Mayberg 等,2005 年)等疾病,这可能成为未来几年的一个激动人心的前沿领域,这些疾病难以用简单的解释来说明。默认模式网络在所有这些研究领域中可能会占据重要地位,尤其是随着我们逐渐理解其在大脑功能中的真实作用。
The author is not aware of any affiliations, memberships, funding, or financial holdings that might be perceived as affecting the objectivity of this review.
作者未意识到任何可能被视为影响本综述客观性的隶属关系、会员资格、资金或财务持有。
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