RAPID COMMUNICATION

Reflective Self-Awareness and Conscious States: PET Evidencefor a Common Midline Parietofrontal Core

Troels W. Kjaer,*,1 Markus Nowak,†,2 and Hans C. Lou*,3

*John F. Kennedy Institute, DK-2600 Glostrup, Denmark; and †PET and Cyclotron Unit,Rigshospitalet University Hospital, DK-2100 Denmark

Received January 17, 2002

A recent meta-analysis has shown precuneus, angulargyri, anterior cingulate gyri, and adjacent structuresto be highly metabolically active in support of restingconsciousness. We hypothesize that these regions consti-tute a functional network of reflective self-awarenessthought to be a core function of consciousness. Sevennormal volunteers were asked to think intensely on howthey would describe the personality traits and physicalappearance of themselves and a neutral reference per-son known to all the subjects (the Danish Queen). Dur-ing each of the four conditions cerebral blood flow dis-tribution was measured by the intravenous H2

15O PETscanning technique. During scanning, no sensory ormotor activity was intended. After each scan, the sub-jects reported the contents of their thoughts during thescan to ascertain that the instructions had been fol-lowed. The results confirmed our hypothesis: Statisti-cal parametric mapping showed differential activityin precuneus and angular gyri during reflection onown personality traits and in anterior cingulate gyriduring reflection on own physical traits. Connectivityanalysis of synchrony showed these regions to be func-tionally connected during reflective self-awareness.The commonality between the neural networks of theresting conscious state and self-awareness reflects thephenomenological concept of a fundamental contribu-tion of reflective self-awareness to the contents andcoherence of the conscious state. © 2002 Elsevier Science (USA)

INTRODUCTION

We have previously presented evidence for a parieto-frontal core in two relaxed conscious states: normal

1 Present address: Department of Clinical Neurophysiology,NF3063, Rigshospitalet University Hospital, DK-2100 Denmark.

2 Present address: Department of Clinical Physiology, BispebjergHospital, DK-2400 Copenhagen, Denmark.

3 Present address: PET-Center, Aarhus University Hospital, DK-8000 Aarhus, Denmark.

© 2002 Elsevier Science (USA)All rights reserved.

resting consciousness and the relaxation meditation inYoga Nidra. This evidence was obtained by principalcomponent analysis of the contribution of cerebral re-gions to the neural activity pattern of the two states,identifying precuneus and frontopolar regions as themajor contributors to both states (Lou et al., 1999,Kjaer and Lou, 2000). It was hypothesized that thejoint activities of these regions were contributing to theunity of consciousness by ensuring a continuing activ-ity of high-level posterior association regions in inter-action with anterior prefrontal regions.

Subsequently, Mazoyer et al., (2001) and Raichle etal., (2001) have drawn attention to the high activity ofthese medial parietal and prefrontal regions in theresting conscious states, apparent from meta-analysisof a great number of investigations. Of these regions,the precuneus has repeatedly been shown to be activein retrieval of episodic memory, the content of whichhas varied from study to study (Andreasen et al., 1995;Krause et al., 1999; Kircher et al., 1999).

Explicit studies of the activity of precuneus and lateralparietal regions in self-referential agency have more re-cently been performed by Ruby and Decety (2001) andVogeley et al. (2001). The former demonstrated precu-neus activation in both first and third person perspec-tives utilizing various pronouns. The latter employedthinking of first person activities in a number of shortstories. Here the activation of precuneus was, however,modest in comparison with a number of coactivated re-gions, including the anterior cingulate gyrus.

Thus, there are indications of a role for precuneus inthe notion of the self. The same is true for anteriorcingulate gyrus with adjacent structures, which re-peatedly has been implicated to be activated by volun-tary action and self-regulation (Frith et al., 1991; Pos-ner and Rothbart, 1998). From a phenomenologicalperspective, the prevailing concept is that reflectiveself-awareness rather than just agency is the commondenominator of conscious experience (Tart, 1994). We

NeuroImage 17, 1080–1086 (2002)doi:10.1006/nimg.2002.1230

10801053-8119/02 $35.00

therefore hypothesize that reflective self-awarenessand conscious states have a common neural founda-tion, which includes the precuneus, the angular gyri,and the anterior cingulate gyrus. Hence, according tothe hypothesis, the precuneus, the angular gyri, andthe anterior cingulate cortex constitute a core of reflec-tive self-awareness, ensuring the unity of consciousexperience. We here present PET evidence for such acomplementary dichotomy of the neural foundation ofreflective self-awareness and show that it is identicalto the core of conscious states described above.

MATERIALS AND METHODS

To test whether precuneus and prefrontal cortex areactive in reflective self-awareness we performed H2

15OPET scans in four conditions, differing by the subject ofreflection only:

(1) Reflection on one’s own personality traits(2) Reflection on one’s own physical appearance(3) Reflection on the personality traits of the Danish

Queen(4) Reflection on the physical appearance of the Dan-

ish Queen

The Danish Queen was chosen as a neutral thirdperson control, since all our subjects were likely toknow of her physical appearance and personalitytraits, while on the other hand having no first personrelation to her and no strong feelings about her.

Seven normal healthy volunteers (five males, agerange 22–27 years, median 24 years) were scannedthree times in each of the four conditions. The orderwas counterbalanced. Participants performed each re-flection task for 2 min, starting 15–25 s before thetracer reached cerebral circulation. During the scans,there were no intended stimuli (subdued light, eyepads, earplugs, etc.) nor any intended motor responses(silent reflections). After cessation of each scan, partic-ipants gave an oral report on their thoughts in a freeformat interview, which was taped. Thus, sensory in-put and motor activity were minimized during thescans.

PET scans were carried out with subjects in supineposition using an Advance PET scanner (GE, Milwau-

kee, WI) operating in 3D mode with collimating septaretracted, producing 35 image slices with a distance of4.25 mm. The total axial field of view was 15 cm withan approximate in-plane resolution of 5 mm; each sub-ject had an initial 10-min transmission scan followedby 12 intravenous injections of 300 MBq of H2

15O(Holm et al., 1996). The tracer was administered by anautomatic water injection system via the left brachialvein over 30 s followed by 10 ml of isotonic saline forflushing. Data acquisition was triggered by total countrate buildup and began about 40 s after injection startand lasting for 90 s. There was an interval of at least 10min between repeated emission scans to allow for iso-tope decay. The subject’s eyes were covered with padsto prevent visual stimulation and to minimize eyemovement. The examination room was quiet, with sub-dued light. Earphones were plugged into both ears.

The data were analyzed using SPM99 (Frackowiaket al., 1997). All scans of each subject were realigned,coregistered to individual MRIs, and subsequently pro-jected into a standard stereotactic three-dimensionalspace. The spatially normalized images of all subjectswere smoothed with an isotropic Gaussian filter (fullwidth half-maximum 13 mm).

For detection of activated areas, Z scores were cal-culated comparing the first person and the third personreflections concerning personality traits and physicalappearance (Friston et al., 1991). Results from thisanalysis, namely the spatial extent of the structuresidentified as the precuneus and the anterior cingulategyrus, were used to extract average rCBF values fromthe two respective regions. Thus, a functional defini-tion of the precuneus and the anterior cingulate wasused.

Then, a second SPM analysis was performed. Here,the above-mentioned rCBF values were used as a co-variate in a “covariate-only” analysis to reveal otherareas in the brain with similar rCBF responses duringthe first person perspective. The analysis was based onthe responses in the precuneus and the anterior cingu-late during the first person perspective, i.e., in thephysical appearance and personality traits conditions,as this is the core of our frontalparietal connectivityhypothesis.

TABLE 1

Examples of Comments

I am extrovert and talkative, while on the other hand I have sides only my friends know. I am rather loving to my girl friends and myfamily and that like. And considerate—on the other hand I can be quite cold sometimes.

She seems open and forthcoming. She seems like a person with authority—a good representative of Denmark. She appears to be a queenwe can be proud of. She is creative and interested in art and in meeting people.

I have dark hair and brown eyes. And a quite large nose and big ears. I thought about my lips and how my body is shaped. . . . Er . . . andthen there is my belly . . . I have relatively large hands.

I thought she must be long-haired even though you never really see it. The hair is always tied in a knot. She has a rather long neck, anda long face, and normal ears.

1081RAPID COMMUNICATION

RESULTS

Before each PET scan, subjects were assigned one ofthe four tasks mentioned above. After each scan, sub-jects gave an oral report in free format about theirthoughts. Examples are given in Table 1.

The results generally confirmed our hypothesis. Re-flection on personality traits of oneself showed prefer-ential activity in the precuneus, bilateral temporopa-rietal regions, and left orbitofrontal cortex, comparedto reflections on the neutral third person, the DanishQueen (Table 2A, Fig. 1A). Reflection on the physicalappearance of oneself mainly activated the anteriorcingulate gyrus, compared to reflections on the neutralthird person (Table 2B, Fig. 1B). The reported Z scoresare significant without correction for multiple compar-isons. The majority of the identified clusters were pre-dicted by the hypothesis (marked with an asterisk in

Table 2). On the other hand, none of the identifiedclusters in the control comparison (“less active” in Ta-ble 2) were in the hypothesized brain region.

The major parietal and frontal regions identified bythe activation analysis, i.e., the precuneus and theanterior cingulate gyrus, were used as the explainingvariables for the covariate analysis (Table 3). Cross-correlation to whole brain produced the maps pre-sented in Figs. 2 and 3. The data are presented as Zscores. It is seen that precuneus and angular gyrus(posterior and anterior frontal regions) are functionallyconnected just as predicted.

DISCUSSION

We here present the novel hypothesis that the neuralcorrelate of reflective self-awareness is similar to thedistribution of high metabolic activity in resting con-

TABLE 2

(A) rCBF Clusters Which Differ during Reflections on Own Personality Traits vs Control

More active Less active

Z score x y z Region Z score x y z Region

4.62 0 �56 56 Precuneus (7)* 4.02 50 �64 �20 Cerebellar lobe4.27 �54 �36 32 Inf parietal lobe (40)* 3.77 26 �58 �14 Cerebellar lobe4.13 22 28 28 Sup front gyr (8)* 3.68 38 �20 �34 Fusiform gyr (20)3.78 �26 46 �14 Middle front gyr (11)* 3.66 28 �36 �24 Fusiform gyr (20)3.69 56 �36 44 Inf par lobe (40)* 3.60 �16 �76 �30 Cerebellar lobe3.49 0 �14 76 Suppl motor area (6)* 3.58 �30 8 �28 Sup temp gyr (36)

3.53 2 �72 2 Ling gyr (18)3.49 28 �16 74 Front sup gyr (4)3.36 64 �4 �20 Middle temp gyr (21)3.29 38 20 �30 Fusiform gyr (47)

(B) rCBF Clusters Which Differ during Reflections on Own Physical Appearance vs Control

More active Less active

Z score x y z Region Z score x y z Region

3.86 10 �50 �24 Cerebellar lobe 4.28 38 �20 �24 Fusiform gyr (20)3.65 2 42 12 Ant cing gyr (32)* 4.00 38 �84 38 Sup occ gyr (19)3.64 �22 22 �16 Middle front gyr (11)* 3.46 22 �30 16 Sup temp gyr (42)3.49 14 22 34 Ant cing gyr (31)*3.47 �38 32 4 Inf front gyr (45–47)*3.33 �14 60 4 Sup front gyr (10)*

Note. Coordinates from the Talairach and Tournoux (1988) atlas. Positive values refer to regions to the right of (x), anterior to (y), andsuperior to (z) the anterior commissure. Numbers in parentheses refer to the Brodmann areas. Clusters with Z-score peaks �3.09 wereincluded (corresponding to P � 0.001 uncorr.). Only the highest peak is included in the case of several confluent peaks. Regions expected tobe active based on the hypothesis are labeled with an asterisk.

FIG. 1. (A) rCBF pattern of reflection on own personality traits vs those of control person, with differential activity mainly in precuneusand bilateral temporoparietal regions. (B) rCBF pattern of reflection on own physical appearance vs that of control person, with differentialactivity mainly in anterior cingulate gyrus.

FIG. 2. Functional connectivity from precuneus to the whole brain during self-reflection. Mainly temporoparietal regions are functionallyconnected to the precuneus.

FIG. 3. Functional connectivity from anterior cingulate gyrus to the whole brain during self-reflection. Mainly adjacent frontal regionsand precuneus are functionally connected to the anterior cingulate gyrus.

1082 RAPID COMMUNICATION

sciousness, reflecting a fundamental role for self-awareness in the unity of conscious experience. A fewyears ago, we first presented the idea that precuneusand prefrontal cortex constituted a unifying core ofresting conscious state: A “self” (Kjaer and Lou, 2000).A detailed meta-analysis of baseline activity of thebrain led Gusnard and Raichle (2001) recently to pro-pose a similar function. We here show, for the firsttime, that the activity pattern of reflective self-aware-ness is indistinguishable from the metabolically highlyactive structures thought to constitute the baseline ofconscious states. We also show that this pattern isformed of interconnected structures mainly in the me-dial parts of posterior and anterior brain. There was noright-hemisphere bias such as was seen in self facerecognition (Keenan et al., 1999). As absolute flow wasnot measured, we cannot be certain whether the dif-ference in activation pattern between self-referentialthought and the resting conscious state constituted anactual increase or a less pronounced decrease fromrest. The similarity between active self-reference andthe resting state patterns is, however, an indication ofthe phenomenological concept of a core of self-aware-ness in the resting state, ensuring its unity by func-tionally integrating posterior and anterior brain.

Conscious experiences have often been ascribed tothe executive regions of the frontal lobe, especiallyprefrontal areas, and the cingulate gyrus, while themore posterior sensory regions have been suggested to

be a mere supplier of information to be processed con-sciously (Mazoyer et al., 2001; Quintana and Fuster,1999). The executive and sensory regions have verydistinct functions at the level of the primary cortices,but they get more functionally integrated at the level ofassociation cortices. The concept behind our work hasbeen that consciousness is complementary, being afunction of sensory as well as executive integrativeareas. One may conceive it as a summit between ante-rior and posterior brain. In a previously publishedstudy, we identified regions activated and deactivatedby meditation compared to resting normal conscious-ness (Lou et al., 1999) and did a subsequent principalcomponent analysis showing that anterior midline pre-frontal cortex and precuneus contributed to both (Kjaerand Lou, 2000). As self-awareness is part of the phe-nomenology of both states as judged by self-report(Kjaer et al., 2002) and is accompanied by activation ofthese high-level integrative regions of anterior andposterior brain, their interaction in self-awarenessmay now be considered to be a core in consciousness.

The anatomical connections between the lateral pa-rietal region and frontal cortex are well described. Pet-rides and Pandya (1984) used autoradiography to dem-onstrate abundant single-cell connections betweenprecuneus and other regions of the posterior parietallobe on one hand and frontal areas on the other (Ja-cobsen and Troganowski, 1977). In contrast, anatomicconnections between the medial aspects of posteriorand anterior brain have not been studied extensively.However, PET and MR studies have shown functionalconnectivity between these regions (Schmidt et al.,2002; Krause et al., 1999) during integration of tempo-ral and spatial information and during retrieval ofepisodic memory. Their functional connectivity wasalso illustrated in the present work by the syncronybetween anterior cingulate gyrus and precuneus.These regions are also involved in executive functions(Quintana and Fuster, 1999). In tasks requiring selec-tion of movement, strong activation was seen, particu-larly and invariably in the precuneus, but also in thepremotor and supplementary motor and in the supe-rior parietal association cortices (Tulving, 1987). Com-plex sequential finger movements were found to beaccompanied by a proportional increase in rCBF in theright dorsal premotor cortex and the right precuneus(Sadato et al., 1996), again demonstrating the closerelationship between the frontal cortex and the precu-neus in goal-directed action.

The activation of the anterior cingulate gyrus duringphysical self-awareness illustrates that this function isclosely related to volition and self-regulation, which allare attributed to the anterior cingulate gyrus (Frith etal., 1991; Posner and Rothbart, 1998). This close sen-sory–motor interaction may be the foundation for thepredictive mechanism in motor control (Blakemore etal., 2000).

TABLE 3

(A) rCBF Clusters Functionally Connected to thePrecuneus during Self-Reflection

Z score x y z Region

7.14 2 �54 54 Precuneus (7)*4.58 �46 �58 12 Middle temp gyr (37)*4.58 60 �30 8 Sup temp gyr (22)*3.96 54 �44 32 Inf par lobe (40)*3.80 �30 �42 �46 Cerebellar lobe3.68 �56 �32 �24 Inf temp gyr (20)3.63 48 �46 �36 Cerebellar lobe3.62 �58 �34 30 Inf par lobe (40)*3.59 36 0 40 Middle front gyr (6)3.51 20 0 62 Sup front gyr (6)3.38 �52 �58 �18 Inf temp gyr (37)

(B) rCBF Clusters Functionally Connected to the AnteriorCingulate Gyrus during Self-Reflection

Z score x y z Region

6.06 2 42 10 Ant cing gyr (32)*4.14 34 �40 �30 Cerebellar lobe3.87 �4 �52 52 Precuneus (7)*3.57 �6 �34 �32 Brain stem3.50 �28 �38 �42 Cerebellar lobe

Note. Organization similar to that of Table 2. Regions expected tobe active based on the hypothesis are labeled with an asterisk.

1084 RAPID COMMUNICATION

Lesion data specifically concerning the precuneusare not available, but in frontal lobe lesions, a numberof syndromes have been described. Of particular im-portance in the present context are the deficits in bothsensory and motor attention and deficiency in perceiv-ing the spatial relationship between the self and theenvironment. One striking finding in some frontallydamaged patients is an apparent decrease in normalself-awareness and, consequently, impaired self-regu-lation (Stuss and Benson, 1986). Sarazin and col-leagues (1998) compared the cognitive and behavioralfunctions of patients with frontal lesions to the locallevels of glucose utilization determined by the 18FDGPET method. They found cognitive functions to be dis-turbed in hypometabolism of the dorsolateral frontalcortex and behavioral and social skills in ventromedialfrontal cortex. Pertinent to the present findings is theirdemonstration of an association between hypometabo-lism of the anterior pole (area 10) and global frontaldysfunction, confirming recent activation studies of acoordinated role of all prefrontal functions for this re-gion.

These anatomical studies and task-elicited activa-tions have been complemented by the examination ofcerebral activity patterns during states of conscious-ness, such as diverse sleep states, and our own study ofthe meditative state vs normal consciousness. Thesleep states are, like meditation and normal conscious-ness, characterized by distinct behavioral and electro-encephalographic patterns (Rechtschaffen et al., 1966).One characteristic common to the sleep stages is re-duced reflective self-awareness (Kahan et al., 1997;Hobson et al., 1998). In REM sleep, rCBF is reduced inprecuneus and dorsolateral prefrontal, parietal, andposterior cingulate cortices (Braun et al., 1998). In deepsleep, or slow-wave sleep, a disproportionate flow re-duction is seen in precuneus, orbital frontal cortex,basal forebrain, anterior cingulate cortex, basal gan-glia, thalami, and dorsal pons (Marquet et al., 1997).Thus, reduction in activity in the precuneus is commonto the two sleep stages as predicted if they were instru-mental in consciousness of the self in its environment.

A more recent study of rCBF during rest and threedifferent depths of anesthesia induced by propofol de-tected activity changes similar to what has been foundduring voluntary loss of consciousness (Fiset et al.,1999). In addition to a decrease in global cerebral bloodflow, they identified predominantly right-sided re-gional flow decrements in thalamus, precuneus/cu-neus, and frontal cortical regions. These results sup-port a specific hypothesis that anesthetics inducebehavioral changes via an effect on specific neuronalnetworks. The finding supports our concept of a gen-eral perception–action cycle in the conscious restingbrain, independent of motor activity, involving high-level integrative cortical regions in anterior and poste-rior brain.

The ultimate state of conscious incapacity—the veg-etative state—has been investigated assessing re-gional cerebral metabolism using fluorodeoxyglucosepositron emission tomography (Laureys et al., 1999).Also, this study identified impaired function of frontaland parietotemporal association areas, including theprecuneus.

In the present study, we have noted that not onlytelencephalic structures but also cerebellum seem to bedifferentially active in reflective self-awareness. Notbeing included in our initial hypothesis, definitive con-clusions on this point was not obtained. It would haverequired statistical confirmation, corrected for multi-ple comparisons, and the cerebellum did not pass thistest (in fact, of the regions studied here only precuneusdid, with a P value of 0.05, corrected for multiple com-parisons).

In conclusion, we have here demonstrated, for thefirst time, a midline network of posterior and anteriorbrain linking self-awareness and conscious experience.

ACKNOWLEDGMENTS

We are grateful to chief psychologist Anders Gade for discussionson how to design the experimental paradigm. The staff at the PETCenter at Rigshospitalet, Copenhagen, is acknowledged for partici-pation. The work is supported by the Lundbeck Foundation. Fur-thermore, The John and Birthe Meyer Foundation is acknowledgedfor the donation of the Cyclotron and PET scanner.

REFERENCES

Andreasen, N. C., O’Leary, D. S., Cizadlo, T., Arndt, S., Rezai, K.,Watkins, G. L., Ponto, L. L., and Hichwa, R. D. 1995. Remember-ing the past: Two facets of episodic memory explored with positronemission tomography. Am. J. Psychiatry 152: 1576–1585.

Blakemore, S. J., Wolpert, D., and Frith, C. 2000. Why can’t youtickle yourself? NeuroReport 11: R11–16.

Braun, A. R., Balkin, T. J., Wesensten, N. J., Gwadry, F., Carson,R. E., Varga, M., et al. 1998. Dissociated pattern of activity invisual cortices and their projections during human rapid eye move-ment sleep. Science 279: 91–95.

Fiset, P., Paus, T., Daloze, T., Plourde, G., Meuret, P., Bonhomme,V., et al. 1999. Brain mechanisms of propofol-induced loss of con-sciousness in humans: A positron emission tomographic study.J. Neurosci. 19: 5506–5513.

Frackowiak, R. S. J., Friston, K. J., Frith, C. D., Dolan, R. J., andMazziotta, J. C., Eds. 1997. Human Brain Function. AcademicPress, San Diego.

Friston, K. J., Frith, C. D., Liddle, P. F., and Frackowiak, R. S. J.1991. Comparing functional (PET) images: The assessment of sig-nificant change. J. Cereb. Blood Flow Metab. 11: 690–699.

Frith, C. D., Friston, K. J., Liddle, P. F., and Frackowiack, R. S. J.1991. Willed action and the prefrontal cortex in man: A study withPET. Proc. R. Soc. London B 244: 241–246.

Gusnard, D. A., and Raichle, M. E. 2001. Searching for a baseline:Functional imaging and the resting human brain. Nat. Rev. Neu-rosci. 2: 685–694.

Hobson, J. A., Stickgold, R., and Pace-Schott, E. F. 1998. The neu-ropsychology of REM sleep dreaming. NeuroReport 9: R1–14.

1085RAPID COMMUNICATION

Holm, S., Law, I., and Paulson, O. B. 1996. 3D PET activation studieswith an H2

15O bolus injection. In Quantification of Brain FunctionUsing PET. (M. Rea, Ed.), pp. 93–97. Academic Press, San Diego.

Jacobsen, S., and Troganowski, J. Q. 1977. Prefrontal granular cor-tex of the rhesus monkey. Brain Res. 132: 209–233.

Kahan, T. L., La Berge, S., Levitan, L., and Zimbardo, P. 1997.Similarities and differences between dreaming and waking cogni-tion: An exploratory study. Consciousness Cognit. 6: 132–147.

Keenan, J. P., McCutcheon, B., Freund, S., Gallup, G. G., Jr., Sand-ers, G., and Pascual-Leone, A. 1999. Left hand advantage in aself-face recognition task. Neuropsychologia 37: 1421–1425.

Kircher, T. T., Senior, C., Phillips, M. L., Benson, P. J., Bullmore,E. T., Brammer, M., Simmons, A., Williams, S. C., Bartels, M., andDavid, A. S. 2000. Toward a functional neuroanatomy of self pro-cessing: Effects of faces and words. Cognit. Brain Res. 10: 133–144.

Kjaer, T. W., and Lou, H. C. 2000. Interaction between precuneusand dorsolateral prefrontal cortex may play a unitary role inconsciousness. A principal component analysis of rCBF. Con-sciousness Cognit. 9: S59.

Kjaer, T. W., Bertelsen, C., Piccini, P., Brooks, D., Alving, J., andLou, H. C. 2002. Increased dopamine tone during meditation-induced change of consciousness. Cognit. Brain Res. 13: 255–259.

Krause, B. J., Horwitz, B., Taylor, J. G., Schmidt, D., Mottaghy,F. M., Herzog, H., Halsband, U., and Muller-Gartner, H.-W. 1999.Network analysis in episodic encoding and retrieval of word-pairassociates: A PET study. Eur. J. Neurosci. 11: 3293–3301.

Laureys, S., Goldman, S., Phillips, C., Van Bogaert, P., Aerts, J.,Luxen, A., et al., 1999. Impaired effective cortical connectivity invegetative state: Preliminary investigation using PET. NeuroIm-age 9: 377–382.

Lou, H. C., Kjaer, T. W., Friberg, L., Wildschiodtz, G., Holm, S., andNowak, M. 1999. A 15O-H2O PET study of meditation and the restingstate of normal consciousness. Hum. Brain Mapping 7: 98–105.

Marquet, P., Degueldre, C., Aerts, J., Peters, J.-M., Luxen, A., andFranck, G. 1997. Functional neuroanatomy of human slow wavesleep. J. Neurosci. 17: 2807–2812.

Mazoyer, B., Zago, L., Mellet, E., Bricogne, S., Etard, O., Houde, O.,Crivello, F., Joliot, M., Petit, L., and Tzourio-Mazoyer, N. 2001.Cortical networks for working memory and executive functionssustain the conscious resting state in man. Brain Res. Bull. 54:287–298.

Petrides, M., and Pandya, D. N. 1984. Projections to the frontalcortex from the posterior parietal region in the rhesus monkey.J. Comp. Neurol. 228: 105–116.

Posner, M. I., and Rothbart, M. K. 1998. Attention, self-regulationand consciousness. Philos. Trans. R. Soc. London B. Biol. Sci. 353:1915–1927.

Quintana, J., and Fuster, J. M. 1999. From perception to action:Temporal integrative functions of prefrontal and parietal neurons.Cereb. Cortex. 9: 213–221.

Raichle, M. E., MacLeod, A. M., Snyder, A. Z., Powers, W. J., Gus-nard, D. A., Shulman, G. L. 2001. A default mode of brain function.Proc. Natl. Acad. Sci. USA 98: 676–682.

Rechtschaffen, A., Hauri, P., and Zeitlin, M. 1966. Auditory awak-ening thresholds in REM and NREM sleep stages. Percept. Mot.Skills 22: 927–942.

Ruby, P., and Decety, J. 2001. Effect of subjective perspective takingduring simulation of action: A PET investigation of agency. Nat.Neurosci. 4: 546–550.

Sadato, N., Campbell, G., Ibanez, V., Deiber, M., and Hallett, M.1996. Complexity affects regional cerebral blood flow change dur-ing sequential finger movements. J. Neurosci. 16: 2691–2700.

Sarazin, M., Pillon, B., Giannakopoulos, P., Rancurel, G., Sanison,Y., and Dubois, B. 1998. Clinico metabolic dissociation of cognitivefunctions and social behavior in frontal lobe lesions. Neurology 51:142–148.

Schmidt, D., Krause, B. J., Mottaghy, F. M., Halsband, U., Herzog,H., Tellmann, L., and Muller-Gartner, H.-W.2002. Brain systemsengaged in encoding and retrieval of word-pair associates indepen-dent of their imagery content or presentation modalities. Neuro-psychologia 20: 457–470.

Stuss, D. T., and Benson, D. F. 1986. The frontal lobes. In TheFrontal Lobes, pp. 246–247. Raven Press, New York.

Talairach, J., and Tournoux, P. 1988. Co-planar Stereotaxic Atlas ofthe Human Brain. Thieme, New York.

Tart, C. T. 1994. Living the Mindful Life. Shambhala, Boston.Tulving, E. 1987. Elements of Episodic Memory. Oxford Univ. Press,

Oxford.Vogeley, K., Bussfeld, P., Newen, A., Herrmann, S., Happe, F., Fal-

kai, P., Maier, W., Shah, N. J., Fink, G. R., and Zilles, K. 2001.Mind reading: Neural mechanisms of theory of mind and self-perspective. NeuroImage 14: 170–181.

1086 RAPID COMMUNICATION