Precuneus
Encyclopedia
The precuneus is a part of the superior parietal lobule
hidden in the medial longitudinal fissure
between the two cerebral hemisphere
s. It is sometimes described as the medial area of the superior parietal cortex. It is involved with episodic memory
, visuospatial processing, reflections upon self
, and aspects of consciousness
.
The location of the precuneus makes it difficult to study. Furthermore, it is rarely subject to isolated injury due to stroke
s, or trauma such as gunshot wounds. This has resulted in it being "one of the less accurately mapped areas of the whole cortical surface". While originally described as homogeneous by Korbinian Brodmann
, it is now appreciated to contain three subdivisions.
It is also known after Achille-Louis Foville
as the quadrate lobule of Foville. The Latin form of praecuneus was first used in 1868 and the English precuneus in 1879.
s in the rear region between the somatosensory cortex and forward of the cuneus
(which contains the visual cortex
). It is above the posterior cingulate
. Following Korbinian Brodmann
it has traditionally been considered a homogeneous structure and with limited distinction between it and the neighboring posterior cingulate
area. Brodmann mapped it as the medial continuation of lateral parietal area 7
.
Axon tracing
research on macaque monkeys has established that it consists of three subdivisions which now have been confirmed by fMRI upon resting-state functional connectivity
to also exist in human
s (parallel fMRI research has also been done upon monkeys).
This occurs around the margin of the cingulate sulcus
(blue in figure) and is connected with sensorimotor areas of the cerebral cortex such as the paracentral lobule
, supplementary motor area
, premotor cortex
, somatosensory area (Brodmann area 2), parietal operculum
and insula
. fMRI Research upon humans finds a connection with the caudalmost part of parahippocampus and superior temporal gyrus
. No connections with the inferior parietal lobule, prefrontal cortex
nor primary motor cortex
.
Cognitive/Associative Central Region
This occurs around the precuneal sulcus (green in figure) and is connected with the inferior parietal lobule particularly the angular gyrus
and prefrontal areas 10
, 46
and 8
. No connections exist with premotor, motor, or somatosensory areas. The areas with which it links are involved in executive functions
, working memory
and motor planning
.
Visual Posterior Region
This occurs along the parieto-occipital fissure (yellow in figure). This connects with visual areas in the cuneus
and primary visual cortex.
, including the ventral lateral nucleus
, the central and anterior nuclei of the intralaminar nuclear group, and the lateral pulvinar
. Other connections include the claustrum
, the dorsolateral caudate nucleus
, putamen
, and the zona incerta
. It also has links with the brainstem areas such as the pretectal area, the superior colliculus
, the nucleus reticularis tegmenti pontis, and the basis pontis
.
s such as New world monkey
s "the superior parietal and precuneate regions are poorly developed". It has been noted that "the precuneus is more highly developed (i.e. comprises a larger portion of the brain volume) in human beings than in non-human primates or other animals, has the most complex columnar cortical organization and is among the last regions to myelinate".
has been located in the forward part of the precuneus with posterior areas being involved with episodic memory
. Another area has been linked to visuospatial imagery. (It is not though clear how these—and the functions noted below—link with the above three subdivisions.)
has linked the precuneus to the processes involved in self-consciousness
, such as reflective self-awareness, that involve rating ones own personality traits compared to those judged of other people.
(in which the "source" circumstances of a memory are recalled) with the left prefrontal cortex: here its role is suggested to be providing rich episodic contextual associations used by the prefrontal cortex to select the correct past memory. In the recollection of memories, it has been suggested that the precuneus 'decides' whether context information exists that can be useful for involving the aid of the hippocampus
. Alternatively it has a different involvement when judging the familiarity as it decides whether the processing of perceptual features would be more useful. In this way the precuneus gets involved in diverse processes such as attention, episodic memory retrieval, working memory and conscious perception.
that requires shifting attention to different spatial locations. It is also together with the dorsal premotor cortex
involved in visuospatial mental operations (such as in a modified form of the game of Amidakuji). It is suggested that while the premotor area engages in the mental operation, the precuneus aids monitoring the success of that operation in terms of internally represented visual images.
The precuneus role in mental imagery has been suggested to extend to that of modeling other people's views. It is activated when a person takes a third-person versus first-person visual point of view. Together with the superior frontal gyrus
and orbitofrontal cortex
is activated when people make judgments that requires understanding whether to act out of empathy
and forgiveness
.
comes from the effects of its disruption in epilepsy
, brain lesion
s and vegetative state. Also cerebral glucose metabolism has its highest levels in these two areas in wakefulness but is most reduced during anesthesia
in them. In addition, it is one of the areas of the brain most deactivated during slow-wave sleep
and rapid eye movement sleep.
Together with the prefrontal cortex, the precuneus, is more activated upon the learning of words briefly flashed when they are supraliminal (and so enter consciousness) than subliminal (and so do not enter consciousness).
. However its involvement in the default network has been questioned. Though one of the authors raising these doubts noted "our findings in this regard should be treated as preliminary."
and Ed Bullmore have proposed that its functions link to its role as a central and well connected "small-world network
" hub between parietal and prefrontal regions.
Superior parietal lobule
The superior parietal lobule is bounded in front by the upper part of the postcentral sulcus, but is usually connected with the postcentral gyrus above the end of the sulcus....
hidden in the medial longitudinal fissure
Medial longitudinal fissure
The great longitudinal fissure is the deep groove which separates the two hemispheres of the vertebrate brain....
between the two cerebral hemisphere
Cerebral hemisphere
A cerebral hemisphere is one of the two regions of the eutherian brain that are delineated by the median plane, . The brain can thus be described as being divided into left and right cerebral hemispheres. Each of these hemispheres has an outer layer of grey matter called the cerebral cortex that is...
s. It is sometimes described as the medial area of the superior parietal cortex. It is involved with episodic memory
Episodic memory
Episodic memory is the memory of autobiographical events that can be explicitly stated. Semantic and episodic memory together make up the category of declarative memory, which is one of the two major divisions in memory...
, visuospatial processing, reflections upon self
Self (psychology)
The psychology of self is the study of either the cognitive and affective representation of one's identity or the subject of experience. The earliest formulation of the self in modern psychology derived from the distinction between the self as I, the subjective knower, and the self as Me, the...
, and aspects of consciousness
Consciousness
Consciousness is a term that refers to the relationship between the mind and the world with which it interacts. It has been defined as: subjectivity, awareness, the ability to experience or to feel, wakefulness, having a sense of selfhood, and the executive control system of the mind...
.
The location of the precuneus makes it difficult to study. Furthermore, it is rarely subject to isolated injury due to stroke
Stroke
A stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...
s, or trauma such as gunshot wounds. This has resulted in it being "one of the less accurately mapped areas of the whole cortical surface". While originally described as homogeneous by Korbinian Brodmann
Korbinian Brodmann
Korbinian Brodmann was a German neurologist who became famous for his definition of the cerebral cortex into 52 distinct regions from their cytoarchitectonic characteristics.-Life:...
, it is now appreciated to contain three subdivisions.
It is also known after Achille-Louis Foville
Achille-Louis Foville
Achille-Louis Foville was a French neurologist and psychiatrist. He produced the first description of the terminal stria....
as the quadrate lobule of Foville. The Latin form of praecuneus was first used in 1868 and the English precuneus in 1879.
Anatomy
The precuneus is located on the inside between the two cerebral hemisphereCerebral hemisphere
A cerebral hemisphere is one of the two regions of the eutherian brain that are delineated by the median plane, . The brain can thus be described as being divided into left and right cerebral hemispheres. Each of these hemispheres has an outer layer of grey matter called the cerebral cortex that is...
s in the rear region between the somatosensory cortex and forward of the cuneus
Cuneus
The cuneus is a portion of the human brain in the occipital lobe.The cuneus receives visual information from the contralateral superior retina representing the inferior visual field. It is most known for its involvement in basic visual processing. Pyramidal cells in the cuneus project to...
(which contains the visual cortex
Visual cortex
The visual cortex of the brain is the part of the cerebral cortex responsible for processing visual information. It is located in the occipital lobe, in the back of the brain....
). It is above the posterior cingulate
Posterior cingulate
The posterior cingulate cortex is the backmost part of the cingulate cortex, lying behind the anterior cingulate cortex.This is the upper part of the "limbic lobe". The cingulate cortex is made up of an area around the midline of the brain...
. Following Korbinian Brodmann
Korbinian Brodmann
Korbinian Brodmann was a German neurologist who became famous for his definition of the cerebral cortex into 52 distinct regions from their cytoarchitectonic characteristics.-Life:...
it has traditionally been considered a homogeneous structure and with limited distinction between it and the neighboring posterior cingulate
Posterior cingulate
The posterior cingulate cortex is the backmost part of the cingulate cortex, lying behind the anterior cingulate cortex.This is the upper part of the "limbic lobe". The cingulate cortex is made up of an area around the midline of the brain...
area. Brodmann mapped it as the medial continuation of lateral parietal area 7
Brodmann area 7
Brodmann area 7 is one of Brodmann's cytologically defined regions of the brain. It is involved in locating objects in space. It serves as a point of convergence between vision and proprioception to determine where objects are in relation to parts of the body....
.
Axon tracing
Anterograde tracing
In neuroscience, anterograde tracing is a research method which is used to trace axonal projections from their source In neuroscience, anterograde tracing is a research method which is used to trace axonal projections from their source In neuroscience, anterograde tracing is a research method which...
research on macaque monkeys has established that it consists of three subdivisions which now have been confirmed by fMRI upon resting-state functional connectivity
Functional neuroimaging
Functional neuroimaging is the use of neuroimaging technology to measure an aspect of brain function, often with a view to understanding the relationship between activity in certain brain areas and specific mental functions...
to also exist in human
Human
Humans are the only living species in the Homo genus...
s (parallel fMRI research has also been done upon monkeys).
Three subdivisions
Sensorimotor Anterior RegionThis occurs around the margin of the cingulate sulcus
Cingulate sulcus
The cingulate sulcus is a sulcus on the medial wall of the cerebral cortex. The frontal and parietal lobes are separated by the cingulate sulcus from the cingulate gyrus.-External links:* via the Neuroscience Information Framework...
(blue in figure) and is connected with sensorimotor areas of the cerebral cortex such as the paracentral lobule
Paracentral lobule
The more medial part of the superior frontal gyrus is distinguished as the paracentral lobule . It is continuous with the precentral and postcentral gyri of the lateral surface....
, supplementary motor area
Supplementary motor area
The supplementary motor area is a part of the sensorimotor cerebral cortex . It was included, on purely cytoarchitectonic arguments, in area 6 of Brodmann and the Vogts...
, premotor cortex
Premotor cortex
The premotor cortex is an area of motor cortex lying within the frontal lobe of the brain. It extends 3 mm anterior to the primary motor cortex, near the Sylvian fissure, before narrowing to approximately 1 mm near the medial longitudinal fissure, which serves as the posterior border for...
, somatosensory area (Brodmann area 2), parietal operculum
Parietal operculum
The parietal operculum is the portion of the parietal lobe on the outside surface of the brain bordering the lateral sulcus, and the extension of this cortex which forms the posterior ceiling of the lateral sulcus. It contains the secondary somatosensory cortex .-Secondary somatosensory cortex:In...
and insula
Insular cortex
In each hemisphere of the mammalian brain the insular cortex is a portion of the cerebral cortex folded deep within the lateral sulcus between the temporal lobe and the frontal lobe. The cortical area overlying it towards the lateral surface of the brain is the operculum...
. fMRI Research upon humans finds a connection with the caudalmost part of parahippocampus and superior temporal gyrus
Superior temporal gyrus
The superior temporal gyrus is one of three gyri in the temporal lobe of the human brain, which is located laterally to the head, situated somewhat above the external ear.The superior temporal gyrus is bounded by:* the lateral sulcus above;...
. No connections with the inferior parietal lobule, prefrontal cortex
Prefrontal cortex
The prefrontal cortex is the anterior part of the frontal lobes of the brain, lying in front of the motor and premotor areas.This brain region has been implicated in planning complex cognitive behaviors, personality expression, decision making and moderating correct social behavior...
nor primary motor cortex
Primary motor cortex
The primary motor cortex is a brain region that in humans is located in the posterior portion of the frontal lobe. Itworks in association with pre-motor areas to plan and execute movements. M1 contains large neurons known as Betz cells, which send long axons down the spinal cord to synapse onto...
.
Cognitive/Associative Central Region
This occurs around the precuneal sulcus (green in figure) and is connected with the inferior parietal lobule particularly the angular gyrus
Angular gyrus
The angular gyrus is a region of the brain in the parietal lobe, that lies near the superior edge of the temporal lobe, and immediately posterior to the supramarginal gyrus; it is involved in a number of processes related to language, mathematics and cognition...
and prefrontal areas 10
Brodmann area 10
Brodmann area 10, or BA10 is the frontopolar part of the frontal cortex in the human brain. BA10 was originally defined in terms of cytoarchitectonic traits in autopsy brains; modern functional imaging research cannot directly identify these boundaries and the terms anterior prefrontal, rostral...
, 46
Brodmann area 46
Brodmann area 46, or BA46, is part of the frontal cortex in the human brain. It is between BA10 and BA45.BA46 is known as middle frontal area 46. In the human brain it occupies approximately the middle third of the middle frontal gyrus and the most rostral portion of the inferior frontal gyrus...
and 8
Brodmann area 8
Brodmann area 8 is one of Brodmann's cytologically defined regions of the brain. It is involved in planning complex movements.-Human:Brodmann area 8, or BA8, is part of the frontal cortex in the human brain...
. No connections exist with premotor, motor, or somatosensory areas. The areas with which it links are involved in executive functions
Executive functions
The executive system is a theorized cognitive system in psychology that controls and manages other cognitive processes. It is responsible for processes that are sometimes referred to as the executive function, executive functions, supervisory attentional system, or cognitive control...
, working memory
Working memory
Working memory has been defined as the system which actively holds information in the mind to do verbal and nonverbal tasks such as reasoning and comprehension, and to make it available for further information processing...
and motor planning
Motor control
Motor control are information processing related activities carried out by the central nervous system that organize the musculoskeletal system to create coordinated movements and skilled actions...
.
Visual Posterior Region
This occurs along the parieto-occipital fissure (yellow in figure). This connects with visual areas in the cuneus
Cuneus
The cuneus is a portion of the human brain in the occipital lobe.The cuneus receives visual information from the contralateral superior retina representing the inferior visual field. It is most known for its involvement in basic visual processing. Pyramidal cells in the cuneus project to...
and primary visual cortex.
Subcortical connections
Below the cerebral cortex, the precuneus is connected with the dorsalmost nuclei of the thalamusThalamus
The thalamus is a midline paired symmetrical structure within the brains of vertebrates, including humans. It is situated between the cerebral cortex and midbrain, both in terms of location and neurological connections...
, including the ventral lateral nucleus
Ventral lateral nucleus
-Inputs and outputs:It receives neuronal inputs from the basal ganglia which includes the substantia nigra and the globus pallidus . It also has inputs from the cerebellum ....
, the central and anterior nuclei of the intralaminar nuclear group, and the lateral pulvinar
Pulvinar
The pulvinar nuclei are a collection of nuclei located in the pulvinar thalamus. The pulvinar part is the most posterior region of the thalamus....
. Other connections include the claustrum
Claustrum
The claustrum, which is suspected to be present in all mammals, is a fairly thin vertical curved sheet of subcortical gray matter...
, the dorsolateral caudate nucleus
Caudate nucleus
The caudate nucleus is a nucleus located within the basal ganglia of the brains of many animal species. The caudate nucleus is an important part of the brain's learning and memory system.-Anatomy:...
, putamen
Putamen
The putamen is a round structure located at the base of the forebrain . The putamen and caudate nucleus together form the dorsal striatum. It is also one of the structures that comprises the basal ganglia. Through various pathways, the putamen is connected to the substantia nigra and globus pallidus...
, and the zona incerta
Zona incerta
The zona incerta is a horizontally elongated region of gray matter cells in the subthalamus below the thalamus. Its connections project extensively over the brain from the cerebral cortex down into the spinal cord....
. It also has links with the brainstem areas such as the pretectal area, the superior colliculus
Superior colliculus
The optic tectum or simply tectum is a paired structure that forms a major component of the vertebrate midbrain. In mammals this structure is more commonly called the superior colliculus , but, even in mammals, the adjective tectal is commonly used. The tectum is a layered structure, with a...
, the nucleus reticularis tegmenti pontis, and the basis pontis
Basis pontis
-Clinical significance:Infarction in this region can impair motor functioning.The basis pontis undergoes demyelination in the condition known as central pontine myelinolysis...
.
Evolution
The precuneus seems to be a recently expanded part of the brain, as in less developed primatePrimate
A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...
s such as New world monkey
New World monkey
New World monkeys are the five families of primates that are found in Central and South America: Callitrichidae, Cebidae, Aotidae, Pitheciidae, and Atelidae. The five families are ranked together as the Platyrrhini parvorder and the Ceboidea superfamily, which are essentially synonymous since...
s "the superior parietal and precuneate regions are poorly developed". It has been noted that "the precuneus is more highly developed (i.e. comprises a larger portion of the brain volume) in human beings than in non-human primates or other animals, has the most complex columnar cortical organization and is among the last regions to myelinate".
Functions
The mental imagery concerning the selfSelf (psychology)
The psychology of self is the study of either the cognitive and affective representation of one's identity or the subject of experience. The earliest formulation of the self in modern psychology derived from the distinction between the self as I, the subjective knower, and the self as Me, the...
has been located in the forward part of the precuneus with posterior areas being involved with episodic memory
Episodic memory
Episodic memory is the memory of autobiographical events that can be explicitly stated. Semantic and episodic memory together make up the category of declarative memory, which is one of the two major divisions in memory...
. Another area has been linked to visuospatial imagery. (It is not though clear how these—and the functions noted below—link with the above three subdivisions.)
Self
Functional imagingFunctional imaging
Functional imaging , is a method of detecting or measuring changes in metabolism, blood flow, regional chemical composition, and absorption....
has linked the precuneus to the processes involved in self-consciousness
Self-consciousness
Self-consciousness is an acute sense of self-awareness. It is a preoccupation with oneself, as opposed to the philosophical state of self-awareness, which is the awareness that one exists as an individual being; although some writers use both terms interchangeably or synonymously...
, such as reflective self-awareness, that involve rating ones own personality traits compared to those judged of other people.
Memory
The precuneus is involved in memory tasks, such as when people look at images and try to respond based on what they have remembered in regard to verbal questions about their spatial details. It is involved with the left prefrontal cortex in the recall of episodic memories including past episodes related to the self. The precuneus is also involved in source memorySource amnesia
Source amnesia is an explicit memory disorder in which someone can recall certain information, but not where or how it was obtained.-Process and experimentation:...
(in which the "source" circumstances of a memory are recalled) with the left prefrontal cortex: here its role is suggested to be providing rich episodic contextual associations used by the prefrontal cortex to select the correct past memory. In the recollection of memories, it has been suggested that the precuneus 'decides' whether context information exists that can be useful for involving the aid of the hippocampus
Hippocampus
The hippocampus is a major component of the brains of humans and other vertebrates. It belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation. Humans and other mammals have two hippocampi, one in...
. Alternatively it has a different involvement when judging the familiarity as it decides whether the processing of perceptual features would be more useful. In this way the precuneus gets involved in diverse processes such as attention, episodic memory retrieval, working memory and conscious perception.
Visuospatial
The precuneus has been suggested to be involved in directing attention in space both when an individual makes movements and when imaging or preparing them. It is involved in motor imagery and shifting attention between motor targets. It is also involved in motor coordinationMotor coordination
thumb|right|Motor coordination is shown in this animated sequence by [[Eadweard Muybridge]] of himself throwing a diskMotor coordination is the combination of body movements created with the kinematic and kinetic parameters that result in intended actions. Such movements usually smoothly and...
that requires shifting attention to different spatial locations. It is also together with the dorsal premotor cortex
Premotor cortex
The premotor cortex is an area of motor cortex lying within the frontal lobe of the brain. It extends 3 mm anterior to the primary motor cortex, near the Sylvian fissure, before narrowing to approximately 1 mm near the medial longitudinal fissure, which serves as the posterior border for...
involved in visuospatial mental operations (such as in a modified form of the game of Amidakuji). It is suggested that while the premotor area engages in the mental operation, the precuneus aids monitoring the success of that operation in terms of internally represented visual images.
The precuneus role in mental imagery has been suggested to extend to that of modeling other people's views. It is activated when a person takes a third-person versus first-person visual point of view. Together with the superior frontal gyrus
Superior frontal gyrus
The superior frontal gyrus makes up about one-third of the frontal lobe of the human brain. It is bounded laterally by the superior frontal sulcus....
and orbitofrontal cortex
Orbitofrontal cortex
The orbitofrontal cortex is a prefrontal cortex region in the frontal lobes in the brain which is involved in the cognitive processing of decision-making...
is activated when people make judgments that requires understanding whether to act out of empathy
Empathy
Empathy is the capacity to recognize and, to some extent, share feelings that are being experienced by another sapient or semi-sapient being. Someone may need to have a certain amount of empathy before they are able to feel compassion. The English word was coined in 1909 by E.B...
and forgiveness
Forgiveness
Forgiveness is typically defined as the process of concluding resentment, indignation or anger as a result of a perceived offense, difference or mistake, or ceasing to demand punishment or restitution. The Oxford English Dictionary defines forgiveness as 'to grant free pardon and to give up all...
.
Consciousness
It has been suggested that together with the posterior cingulate, that the precuneus is "pivotal for conscious information processing". The evidence for this link with consciousnessConsciousness
Consciousness is a term that refers to the relationship between the mind and the world with which it interacts. It has been defined as: subjectivity, awareness, the ability to experience or to feel, wakefulness, having a sense of selfhood, and the executive control system of the mind...
comes from the effects of its disruption in epilepsy
Epilepsy
Epilepsy is a common chronic neurological disorder characterized by seizures. These seizures are transient signs and/or symptoms of abnormal, excessive or hypersynchronous neuronal activity in the brain.About 50 million people worldwide have epilepsy, and nearly two out of every three new cases...
, brain lesion
Stroke
A stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...
s and vegetative state. Also cerebral glucose metabolism has its highest levels in these two areas in wakefulness but is most reduced during anesthesia
Anesthesia
Anesthesia, or anaesthesia , traditionally meant the condition of having sensation blocked or temporarily taken away...
in them. In addition, it is one of the areas of the brain most deactivated during slow-wave sleep
Slow-wave sleep
Slow-wave sleep , often referred to as deep sleep, consists of stages 3 and 4 of non-rapid eye movement sleep, according to the Rechtschaffen & Kales standard of 1968. As of 2008, the American Academy of Sleep Medicine has discontinued the use of stage 4, such that the previous stages 3 and 4 now...
and rapid eye movement sleep.
Together with the prefrontal cortex, the precuneus, is more activated upon the learning of words briefly flashed when they are supraliminal (and so enter consciousness) than subliminal (and so do not enter consciousness).
Default network
It has been suggested to be the 'core node' or 'hub' of the default mode network that is activated during "resting consciousness" in which people do not engage intentionally in sensory or motor activity. This involvement in the default network is suggested to underlie its role in self-consciousnessSelf-consciousness
Self-consciousness is an acute sense of self-awareness. It is a preoccupation with oneself, as opposed to the philosophical state of self-awareness, which is the awareness that one exists as an individual being; although some writers use both terms interchangeably or synonymously...
. However its involvement in the default network has been questioned. Though one of the authors raising these doubts noted "our findings in this regard should be treated as preliminary."
Parietal prefrontal central hub
Olaf SpornsOlaf Sporns
Olaf Sporns is Professor and Associate Department Chair at Indiana University.Dr. Sporns received his degree from Eberhard Karls University of Tübingen in Tübingen, West Germany before going to New York to study at the Rockefeller University under Gerald Edelman...
and Ed Bullmore have proposed that its functions link to its role as a central and well connected "small-world network
Small-world network
In mathematics, physics and sociology, a small-world network is a type of mathematical graph in which most nodes are not neighbors of one another, but most nodes can be reached from every other by a small number of hops or steps...
" hub between parietal and prefrontal regions.
These clusters or modules are interlinked by specialized hub regions, ensuring that overall path lengths across the network are short. Most studies identified [such] hubs among parietal and prefrontal regions, providing a potential
explanation for their well-documented activation by many cognitive functions. Particularly notable is the prominent structural role of the precuneus, a region that is homologous to the highly connected posteromedial cortex in the macaque. The precuneus
is involved in self-referential processing, imagery and memory, and its deactivation is associated with anaesthetic-induced loss of consciousness. An intriguing hypothesis suggests that these functional aspects can be explained on the basis of its high centrality in the cortical network.