Buckner et al.:The Brain's Default Network uctures tha (Raichle 1987) ed tha cluded ma sk and co ed to iso.By the mid-vdomgin nonmemory control.In addition.to better understand were completed that examined perception,language, the cognitive processes associated with the rest state attention,and memory.Sca ns of rest-state brain ac they informally asked their participants to subjectively tivity were often acquired across these studies for a describe their mental experiences and re ongiated from this work tha what a the tim in fact at nd c onsists of a mixt The term "deactivation"was used because analyses of freely wandering past recollection,future plans,and and image visualization were referenced to the target, other personal thoughts and experiences."Second,the experimental task.Within this nomenclature,region more active in the target condi ton (c.g..rca onsas well a a distin post g d"n ation nia h stcm at isc nsistently activated in humans durin csent and often the most robust efTect in many carly ndirected mental states Broad awareness of the common regions that be interest emerged was activity reductions in unattended come active during passive task states emerged with of its a pair of meta-analyses that pooled extensive data to 1994.Buc (e.g 19 unction 1g0 sis oftask-induced de active as to explicitly determine if there were com on hrain re pared to passive task conditions.There was no initial rions active during undirected (passive)mental states They pooled data from 132 normal adults for which an r et al.19 part was cor ctc.)coul lirectly compar I to a pa pres he sa e state for an autobic phical men ctal.2001) ted data a ory task Andreasen and colleagues (1995)expl mal adults that included both visually and aurally cued the possibility that spontancous cognition makes an active tasks as compared to passive rest conditions important contribution to rest states.Much like other These two analyses revealed a remarkably consis studies at the time,the researchers included a rest con tent set of brain regions that were more actrve during n to their target co con ditio h rimental targct of the stud ns)The sults of the Shulman t al (1997)me ternally directed cognition much like the ntaneous analysis are shown in This image displays cognition that occurs during"rest"states.For this rea the full cortical extent of the brain's default network son.Andreasen and colleagues explored both the rest The broad generality of the rest activity pattern across any diverse studies reinforced the intriguing pos ty that a c this idea.ag the tes.Mo ed by ct2001) tor in the b thought by asking pants to describe their musings following the scanned y rest periods.Paralleling the informal observations byBuckner et al.: The Brain’s Default Network 3 sensitivity to deep-brain structures than earlier meth￾ods and, owing to the development of isotopes with short half-lives (Raichle 1987), typical PET studies in￾cluded many task and control conditions for compar￾ison. By the mid-1990s several dozen imaging studies were completed that examined perception, language, attention, and memory. Scans of rest-state brain ac￾tivitya were often acquired across these studies for a control comparison, and researchers began routinely noticing brain regions more active in the passive con￾trol conditions than the active target tasks—what at the time was referred to as “deactivation.” The term “deactivation” was used because analyses and image visualization were referenced to the target, experimental task. Within this nomenclature, regions relatively more active in the target condition (e.g., read￾ing, classifying pictures) compared to the control task (e.g., passive fixation, rest) were labeled “activations”; regions less active in the target condition than the con￾trol were labeled “deactivations.” Deactivations were present and often the most robust effect in many early PET studies. One form of deactivation for which early interest emerged was activity reductions in unattended sensory modalities because of its theoretical relevance to mechanisms of attention (e.g., Haxby et al. 1994, Kawashima et al. 1994, Buckner et al. 1996). A second form of commonly observed deactivationwas along the frontal and posterior midline during active, as com￾pared to passive, task conditions. There was no initial explanation for these mysterious midline deactivations (e.g., Ghatan et al. 1995, Baker et al. 1996). A particularly informative early study was con￾ducted while exploring brain regions supporting episodic memory. Confronted with the difficult issue of defining a baseline state for an autobiographical mem￾ory task, Andreasen and colleagues (1995) explored the possibility that spontaneous cognition makes an important contribution to rest states. Much like other studies at the time, the researchers included a rest con￾dition as a baseline for comparison to their target con￾ditions. However, unlike other contemporary studies, they hypothesized that autobiographical memory (the experimental target of the study) inherently involves in￾ternally directed cognition, much like the spontaneous cognition that occurs during “rest” states. For this rea￾son, Andreasen and colleagues explored both the rest aPET and functional MRI (fMRI) both measure neural activity indi￾rectly through local vascular (blood flow) changes that accompany neu￾ronal activity. PET is sensitive to changes in blood flow directly (Raichle 1987). fMRI is sensitive to changes in oxygen concentration in the blood which tracks blood flow (Heeger and Ress 2002). For simplicity, we refer to these methods as measuring brain activity in this review. and memory tasks referenced to a third control con￾dition that involved neither rest nor episodic memory. Their results showed that similar brain regions were engaged during rest and memory as compared to the nonmemory control. In addition, to better understand the cognitive processes associated with the rest state, they informally asked their participants to subjectively describe their mental experiences. Two insights originated from this work that fore￾shadow much of the present review’s content. First, Andreasen et al. (1995) noted that the resting state “is in fact quite vigorous and consists of a mixture of freely wandering past recollection, future plans, and other personal thoughts and experiences.” Second, the analysis of brain activity during the rest state revealed prefrontal midline regions as well as a distinct poste￾rior pattern that included the posterior cingulate and retrosplenial cortex. As later studies would confirm, these regions are central components of the core brain system that is consistently activated in humans during undirected mental states. Broad awareness of the common regions that be￾come active during passive task states emerged with a pair of meta-analyses that pooled extensive data to reveal the functional anatomy of unconstrained cogni￾tion. In the first study, Shulman and colleagues (1997) conducted meta-analysis of task-induced deactivations to explicitly determine if there were common brain re￾gions active during undirected (passive) mental states. They pooled data from 132 normal adults for which an active task (word reading, active stimulus classification, etc.) could be directly compared to a passive task that presented the same visual words or pictures but con￾tained no directed task goals. Using a similar approach, Mazoyer et al. (2001) aggregated data across 63 nor￾mal adults that included both visually and aurally cued active tasks as compared to passive rest conditions. These two analyses revealed a remarkably consis￾tent set of brain regions that were more active during passive task conditions than during numerous goal￾directed task conditions (spanning both verbal and nonverbal domains and visual and auditory condi￾tions). The results of the Shulman et al. (1997) meta￾analysis are shown in FIGURE 2. This image displays the full cortical extent of the brain’s default network. The broad generality of the rest activity pattern across so many diverse studies reinforced the intriguing pos￾sibility that a common set of cognitive processes was used spontaneously during the passive-task states. Mo￾tivated by this idea, Mazoyer et al. (2001) explored the content of spontaneous thought by asking partici￾pants to describe their musings following the scanned rest periods. Paralleling the informal observations by