Evidence for the default network's role in spontaneous cognition

Identification of neural targets for the treatment of psychiatric disorders: the role of functional neuroimaging

Neurosurgical treatment of psychiatric disorders has been influenced by evolving neurobiological models of symptom generation. The advent of functional neuroimaging and advances in the neurosciences have revolutionized understanding of the functional neuroanatomy of psychiatric disorders. This article reviews neuroimaging studies of depression from the last 3 decades and describes an emerging neurocircuitry model of mood disorders, focusing on critical circuits of cognition and emotion, particularly those networks involved in the regulation of evaluative, expressive and experiential aspects of emotion. The relevance of this model for neurotherapeutics is discussed, as well as the role of functional neuroimaging of psychiatric disorders.

Strength of default mode resting-state connectivity relates to white matter integrity in children

A default mode network of brain regions is known to demonstrate coordinated activity during the resting state. While the default mode network is well characterized in adults, few investigations have focused upon its development. We scanned 9-13-year-old children with diffusion tensor imaging and resting-state functional magnetic resonance imaging. We identified resting-state networks using Independent Component Analysis and tested whether the functional connectivity between the medial prefrontal cortex (mPFC) and posterior cingulate cortex (PCC) depends upon the maturation of the underlying cingulum white matter tract. To determine the generalizability of this relationship, we also tested whether functional connectivity depends on white matter maturity between bilateral lateral prefrontal cortex (lateral PFC) within the executive control network. We found a positive relationship between mPFC-PCC connectivity and fractional anisotropy of the cingulum bundle; this positive relationship was moderated by the age of the subjects such that it was stronger in older children. By contrast, no such structure-function relationship emerged between right and left lateral PFC. However, functional and structural connectivity of this tract related positively with cognitive speed, fluency, and set-switching neuropsychological measures.

Meta-awareness, perceptual decoupling and the wandering mind

Mind wandering (i.e. engaging in cognitions unrelated to the current demands of the external environment) reflects the cyclic activity of two core processes: the capacity to disengage attention from perception (known as perceptual decoupling) and the ability to take explicit note of the current contents of consciousness (known as meta-awareness). Research on perceptual decoupling demonstrates that mental events that arise without any external precedent (known as stimulus independent thoughts) often interfere with the online processing of sensory information. Findings regarding meta-awareness reveal that the mind is only intermittently aware of engaging in mind wandering. These basic aspects of mind wandering are considered with respect to the activity of the default network, the role of executive processes, the contributions of meta-awareness and the functionality of mind wandering.

The self: from philosophy to cognitive neuroscience

Neuroscientists have recently begun to explore topics, such as the nature of the self, that were previously considered problems for philosophy rather than for science. This article aims to provide a starting point for interdisciplinary exchange by reviewing three philosophical debates about the nature of the self in light of contemporary work in cognitive neuroscience. Continental rationalist and British empiricist approaches to the unity of the self are discussed in relation to earlier work on split-brain patients, and to more recent work on "mental time travel" and the default mode network; the phenomenological movement, and the central concept of intentionality, are discussed in relation to interoceptive accounts of emotion and to the mirror neuron system; and ongoing philosophical debates about agency and autonomy are discussed in relation to recent work on action awareness and on insight in clinical populations such as addicts and patients with frontotemporal dementia.

Spontaneous resting-state BOLD fluctuations reveal persistent domain-specific neural networks

Resting-state functional connectivity MRI (rs-fcMRI) analyses have identified intrinsic neural networks supporting domain-general cognitive functions including language, attention, executive control and memory. The brain, however, also has a domain-specific organization, including regions that contribute to perceiving and knowing about others (the 'social' system) or manipulable objects designed to perform specific functions (the 'tool' system). These 'social' and 'tool' systems, however, might not constitute intrinsic neural networks per se, but rather only come online as needed to support retrieval of domain-specific information during social- or tool-related cognitive tasks. To address this issue, we functionally localized two regions in lateral temporal cortex activated when subjects perform social- and tool conceptual tasks. We then compared the strength of the correlations with these seed regions during rs-fcMRI. Here, we show that the 'social' and 'tool' neural networks are maintained even when subjects are not engaged in social- and tool-related information processing, and so constitute intrinsic domain-specific neural networks.

Solving future problems: default network and executive activity associated with goal-directed mental simulations

Mental simulations are often focused on a goal in the future or a problem to be solved. Recent neuroimaging studies have associated mental simulations of the future with default network activity, but the simulations in these studies were not typically directed toward achieving a particular goal. Goal-directed simulation requires cognitive control to maintain information, make decisions, and coordinate abstract action sequences. Therefore, it should recruit not only the default network, but also executive regions. To investigate whether default network and executive regions can be coactive in the context of goal-directed simulation, we designed a problem-solving task in which participants simulated solving several specific problems in imaginary scenarios while in the MRI scanner. We analyzed brain activity during simulation relative to a semantic elaboration task and found that goal-directed simulation engaged core regions of the default network and executive dorsolateral prefrontal cortex. A functional connectivity analysis with posterior cingulate and dorsolateral prefrontal cortex seeds revealed that activity in these regions was coupled throughout the goal-directed simulation period and associated with a distributed network of other default and executive regions, including medial prefrontal cortex, medial temporal, and parietal regions.

Cooperation between the default mode network and the frontal-parietal network in the production of an internal train of thought

The ability to generate and sustain an internal train of thought unrelated to external reality frees an agent from the constraints of only acting on immediate, environmentally triggered events. The current paper proposes that such thought is produced through cooperation between autobiographical information provided by the default mode network and a frontal-parietal control network which helps sustain and buffer internal trains of thought against disruption by the external world. This hypothesis explains at least two features of the literature on internally guided thought. First, access to the top-down control system is a generally accepted prerequisite of conscious experience; this explains why activation of this system and default mode activity is often observed together during periods of internally guided thought. Second, because the top-down attentional control system has a limited capacity, internally and externally driven streams can come into conflict, with the result that perceptual information must be denied attentional amplification if the internal stream is to be maintained. This explains why internal thought is routinely associated with a state of perceptual decoupling, reflected in both measured anticorrelations between the default mode network and sensory areas and the manner in which task unrelated thoughts compromise task performance. This paper offers a hypothesis that should help to constrain and guide interpretations, investigations, and analyses of the neural processes involved in internally driven cognition. This article is part of a Special Issue entitled Special Issue The Cognitive Neuroscience.

The neural substrate for dreaming: is it a subsystem of the default network?

Building on the content, developmental, and neurological evidence that there are numerous parallels between waking cognition and dreaming, this article argues that the likely neural substrate that supports dreaming, which was discovered through converging lesion and neuroimaging studies, may be a subsystem of the waking default network, which is active during mind wandering, daydreaming, and simulation. Support for this hypothesis would strengthen the case for a more general neurocognitive theory of dreaming that starts with established findings and concepts derived from studies of waking cognition and neurocognition. If this theory is correct, then dreaming may be the quintessential cognitive simulation because it is often highly complex, often includes a vivid sensory environment, unfolds over a duration of a few minutes to a half hour, and is usually experienced as real while it is happening.

Neural correlates of ongoing conscious experience: both task-unrelatedness and stimulus-independence are related to default network activity

The default mode network (DMN) is a set of brain regions that consistently shows higher activity at rest compared to tasks requiring sustained focused attention toward externally presented stimuli. The cognitive processes that the DMN possibly underlies remain a matter of debate. It has alternately been proposed that DMN activity reflects unfocused attention toward external stimuli or the occurrence of internally generated thoughts. The present study aimed at clarifying this issue by investigating the neural correlates of the various kinds of conscious experiences that can occur during task performance. Four classes of conscious experiences (i.e., being fully focused on the task, distractions by irrelevant sensations/perceptions, interfering thoughts related to the appraisal of the task, and mind-wandering) that varied along two dimensions ("task-relatedness" and "stimulus-dependency") were sampled using thought-probes while the participants performed a go/no-go task. Analyses performed on the intervals preceding each probe according to the reported subjective experience revealed that both dimensions are relevant to explain activity in several regions of the DMN, namely the medial prefrontal cortex, posterior cingulate cortex/precuneus, and posterior inferior parietal lobe. Notably, an additive effect of the two dimensions was demonstrated for midline DMN regions. On the other hand, lateral temporal regions (also part of the DMN) were specifically related to stimulus-independent reports. These results suggest that midline DMN regions underlie cognitive processes that are active during both internal thoughts and external unfocused attention. They also strengthen the view that the DMN can be fractionated into different subcomponents and reveal the necessity to consider both the stimulus-dependent and the task-related dimensions of conscious experiences when studying the possible functional roles of the DMN.

Modulation of the brain's functional network architecture in the transition from wake to sleep

The transition from quiet wakeful rest to sleep represents a period over which attention to the external environment fades. Neuroimaging methodologies have provided much information on the shift in neural activity patterns in sleep, but the dynamic restructuring of human brain networks in the transitional period from wake to sleep remains poorly understood. Analysis of electrophysiological measures and functional network connectivity of these early transitional states shows subtle shifts in network architecture that are consistent with reduced external attentiveness and increased internal and self-referential processing. Further, descent to sleep is accompanied by the loss of connectivity in anterior and posterior portions of the default-mode network and more locally organized global network architecture. These data clarify the complex and dynamic nature of the transitional period between wake and sleep and suggest the need for more studies investigating the dynamics of these processes.

Functional subdivisions in the left angular gyrus where the semantic system meets and diverges from the default network

The left angular gyrus (AG) is reliably activated across a wide range of semantic tasks, and is also a consistently reported component of the so-called default network that it is deactivated during all goal-directed tasks. We show here that there is only partial overlap between the semantic system and the default network in left AG and the overlap defines a reliable functional landmark that can be used to segregate functional subdivisions within AG. In 94 healthy human subjects, we collected functional magnetic resonance imaging (fMRI) data during fixation and eight goal directed tasks that involved semantic matching, perceptual matching or speech production in response to familiar or unfamiliar stimuli presented in either verbal (letters) or nonverbal (pictures) formats. Our results segregated three different left AG regions that were all activated by semantic relative to perceptual matching: (1) a midregion (mAG) that overlapped with the default network because it was deactivated during all tasks relative to fixation; (2) a dorsomesial region (dAG) that was more activated by all tasks relative to fixation; and (3) a ventrolateral region (vAG) that was only activated above fixation during semantic matching. By examining the effects of task and stimuli in each AG subdivision, we propose that mAG is involved in semantic associations regardless of the presence or absence of a stimulus; dAG is involved in searching for semantics in all visual stimuli, and vAG is involved in the conceptual identification of visual inputs. Our findings provide a framework for reporting and interpreting AG activations with greater definition.

Default network activity, coupled with the frontoparietal control network, supports goal-directed cognition

Tasks that demand externalized attention reliably suppress default network activity while activating the dorsal attention network. These networks have an intrinsic competitive relationship; activation of one suppresses activity of the other. Consequently, many assume that default network activity is suppressed during goal-directed cognition. We challenge this assumption in an fMRI study of planning. Recent studies link default network activity with internally focused cognition, such as imagining personal future events, suggesting a role in autobiographical planning. However, it is unclear how goal-directed cognition with an internal focus is mediated by these opposing networks. A third anatomically interposed 'frontoparietal control network' might mediate planning across domains, flexibly coupling with either the default or dorsal attention network in support of internally versus externally focused goal-directed cognition, respectively. We tested this hypothesis by analyzing brain activity during autobiographical versus visuospatial planning. Autobiographical planning engaged the default network, whereas visuospatial planning engaged the dorsal attention network, consistent with the anti-correlated domains of internalized and externalized cognition. Critically, both planning tasks engaged the frontoparietal control network. Task-related activation of these three networks was anatomically consistent with independently defined resting-state functional connectivity MRI maps. Task-related functional connectivity analyses demonstrate that the default network can be involved in goal-directed cognition when its activity is coupled with the frontoparietal control network. Additionally, the frontoparietal control network may flexibly couple with the default and dorsal attention networks according to task domain, serving as a cortical mediator linking the two networks in support of goal-directed cognitive processes.