The precuneus is a part of the superior parietal lobule in front of the occipital lobe (cuneus). It is hidden in the medial longitudinal fissure between the two cerebral hemispheres. It is sometimes described as the medial area of the superior parietal cortex.
The precuneus is bounded anteriorly by the marginal branch of the cingulate sulcus, posteriorly by the parietooccipital sulcus, and inferiorly by the subparietal sulcus. 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 strokes, 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.
The precuneus is located on the inside between the two cerebral hemispheres 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 humans (parallel fMRI research has also been done upon monkeys).
Sensorimotor Anterior Region
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.
Below the cerebral cortex, the precuneus is connected with the dorsalmost nuclei of the thalamus, 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.
The mental imagery concerning the self 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 clear how these — and the functions noted below — link with the above three subdivisions.
Functional imaging has linked the precuneus to the processes involved in self-consciousness, such as reflective self-awareness, that involve rating one’s own personality traits compared to those judged of other people.
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 memory (in which the “source” circumstances of a memory are recalled) with the left inferior 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.
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 coordination 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, the precuneus is activated when people make judgments that requires understanding whether to act out of empathy and forgiveness.
It has been suggested that together with the posterior cingulate, the precuneus is “pivotal for conscious information processing“. The evidence for this link with consciousness comes from the effects of its disruption in epilepsy, brain lesions and vegetative state.
Also, cerebral glucose metabolism is at its highest in these two areas during wakefulness but is most reduced in them during anesthesia. 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).
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-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.” A recent study showed that only ventral precuneus is involved in the default network.
Top Image: Boundaries of precuneus are defined by the three sulci. (shown in red) marginal branch of the cingulate sulcus parieto-occipital sulcus subparietal sulcus The area colored blue represents parietal lobe. Credit: Sebastian023, CC BY-SA 3.0