The posterior cingulate cortex (PCC) is the caudal part of the cingulate cortex, located posterior to 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. Surrounding areas include the retrosplenial cortex and the precuneus.
Cytoarchitectonically the posterior cingulate cortex is associated with Brodmann areas 23 and 31.
The PCC forms a central node in the default mode network of the brain. It has been shown to communicate with various brain networks simultaneously and is involved in diverse functions. Along with the precuneus, the PCC has been implicated as a neural substrate for human awareness in numerous studies of both the anesthetized and vegetative (coma) states. Imaging studies indicate a prominent role for the PCC in pain and episodic memory retrieval. Increased size of the ventral PCC is related to a decline in working memory performance. The PCC has also been strongly implicated as a key part of several intrinsic control networks.
Anatomy
Location and boundaries The posterior cingulate cortex lies behind the anterior cingulate cortex, forming a part of the posteromedial cortex, along with the retrosplenial cortex (Brodmann areas 29 and 30) and precuneus (located posterior and superior to the PCC). The PCC, together with the retrosplenial cortex, forms the retrosplenial gyrus. The posterior cingulate cortex is bordered by the following brain regions: the marginal ramus of the cingulate sulcus (superiorly), the corpus callosum (inferiorly), the parieto-occipital sulcus (posteriorly), and Brodmann area 24 (anteriorly).
Cytoarchitectural organization The posterior cingulate cortex is considered a paralimbic cortical structure, consisting of Brodmann areas 23 and 31. As part of the paralimbic cortex, it has fewer than six layers, placing its cell architecture in between the six-layered neocortex and the more primitive allocortex of core limbic structures. It has also been associated with the hippocampocentric subdivision of the paralimbic zone. The cytoarchitecture of the PCC is not entirely uniform, instead it contains distinct anterior and dorsal subregions, which are increasingly understood as distinct in function, as well as cytoarchitectural structure.
Structural connections
Nonhuman structure In non-human primates the following structural connections of the posterior cingulate cortex are well documented: Reciprocal connection with other regions of the posteromedial cortex. High connectivity to other paralimbic and limbic structures. Reciprocal connections to the medial temporal lobe. Dense connections to the hippocampal formation, the parahippocampal cortex, the ventromedial prefrontal cortex, and subgenual parts of the anterior cingulate cortex. Prominent connections to the areas of heteromodal association in the front, temporal and parietal lobes. Strong reciprocal connections to the dorsolateral prefrontal cortex (roughly Brodmann area 46) and the frontal poles (Brodmann areas 10 and 11). Less prominent connections to Brodmann areas 9/46, 8 and 9. Connections to the dorsal parts of the anterior cingulate cortex. Dense connections to the thalamus in the form of a continuous strip that crosses numerous pulvinar nuclei and the striatum. As is true in other areas of the posteromedial cortex, the posterior cingulate cortex has no apparent connections to primary sensory or motor areas. Thus, it is unlikely to be involved in low-level sensory or motor processing.
Human structure While many of the connections in non-human primates may be present in humans, they are less well documented. Studies have shown strong reciprocal connections to medial temporal lobe memory structures, such as the entorhinal cortex and the parahippocampal gyrus, the latter being involved in associative learning and episodic memory. In humans, the PCC is also connected to areas involved in emotion and social behavior, attention (the lateral intraparietal cortex and precuneus), learning and motivation (the anterior and lateral thalamic nucleus, caudate nucleus, orbitofrontal cortex and anterior cingulate cortex).
Function The posterior cingulate cortex is highly connected and one of the most metabolically active regions in the brain, but there is no consensus as to its cognitive role. Cerebral blood flow and metabolic rate in the PCC are approximately 40% higher than average across the brain. The high functional connectivity of the PCC, signifies extensive intrinsic connectivity networks (networks of brain regions involved in a range of tasks that share common spatio-temporal patterns of activity).
Emotion and memory The posterior cingulate cortex has been linked by lesion studies to spatial memory, configural learning, and maintenance of discriminative avoidance learning. More recently the PCC was shown to display intense activity when autobiographical memories (such as those concerning friends and family) are recalled successfully. In a study involving autobiographical recollection, the caudal part of the left PCC was the only brain structure highly active in all subjects. Furthermore, the PCC does not show this same activation during attempted but unsuccessful retrieval, implying an important role in successful memory retrieval (see below: Alzheimer's disease).
The posterior cingulate cortex has also been firmly linked to emotional salience. Thus, it has been hypothesized that the emotional importance of autobiographical memories may contribute to the strength and consistency of activity in the PCC upon successful recollection of these memories. The posterior cingulate cortex is significantly bilaterally activated by emotional stimuli, independent of valence (positive or negative). This is in contrast to other structures in the limbic system, such as the amygdala, which are thought to respond disproportionately to negative stimuli, or the left frontal pole, which activated only in response to positive stimuli. These results support the hypothesis that the posterior cingulate cortex mediates interactions between emotion and memory.
Intrinsic control networks The posterior cingulate cortex exhibits connectivity with a wide range of intrinsic control networks. Its most widely known role is as a central node in the default mode network (DMN). The default mode network (and the PCC) is highly reactive and quickly deactivates during tasks with externally directed, or presently centered, attention (such as working memory or meditation). Conversely, the DMN is active when attention is internally directed (during episodic memory retrieval, planning, and daydreaming). A failure of the DMN to deactivate at proper times is associated with poor cognitive function, thereby indicating its importance in attention.
In addition to the default mode network, the posterior cingulate cortex is also involved in the dorsal attention network (a top-down control of visual attention and eye movement) and the frontoparietal control network (involved in executive motor control). Furthermore, fMRI studies have shown that the posterior cingulate cortex activates during visual tasks when some form of monetary incentive is involved, essentially functioning as a neural interface between motivation-related areas and top-down control of visual attention.
The relationship between these networks within the PCC is not clearly understood. When activity increases in the dorsal attention network and the frontoparietal control network, it must simultaneously decrease in the DMN in a closely correlated way. This anti-correlated pattern is indicative of the various differences and importance of subregions in the posterior cingulate cortex.
Considering the relation of the PCC with the DMN, with suppressed posterior cingulate activity favoring low cognitive introspection and higher external attention and increased activity indicating memory retrieval and planning, it has been hypothesized that this brain region is heavily involved in noticing internal and external changes and in facilitating novel behavior or thought in response. High activity, then, would indicate continued operation with the current cognitive set, while lower activity would indicate exploration, flexibility and renewed learning.
An alternative hypothesis is focused more on the difference between the dorsal and ventral subregions and takes into consideration their functional separation. In this model, the PCC is hypothesized to take a chief regulatory role in focusing internal and external attention. Mounting evidence that the PCC is involved in both integrating memories of experiences and initiating a signal to change behavioral strategies supports this hypothesis. Under this model, the PCC plays a crucial role in controlling state of arousal, the breadth of focus and the internal or external focus of attention. This hypothesis emphasizes the PCC as a dynamic network, rather than a fixed and unchanging structure.
While both of the hypotheses are the result of scientific studies, the role of the PCC is still not well understood and there remains much work to be done to investigate the extent of their veracity.
Meditation From neuroimaging studies and subjective descriptions, the PCC has been found to be activated during self-related thinking and deactivated during meditation. Using generative topographic mapping, it was further found that undistracted, effortless mind wandering corresponds with PCC deactivation, whereas distracted and controlled awareness corresponds with PCC activation. These results track closely with findings about the role of the PCC in the DMN.
Disorders Structural and functional abnormalities in the PCC result in a range of neurological and psychiatric disorders. The PCC likely integrates and mediates information in the brain. Therefore, functional abnormalities of the PCC might be an accumulation of remote and widespread damage in the brain.
Alzheimer's disease The PCC is commonly affected by neurodegenerative disease. In fact, reduced metabolism in the PCC has been identified as an early sign of Alzheimer's disease, and is frequently present before a clinical diagnosis. The reduced metabolism in the PCC is typically one part in a diffuse pattern of metabolic dysfunction in the brain that includes medial temporal lobe structures and the anterior thalamus, abnormalities that may be the result of damage in isolated but connected regions. For instance, Meguro et al. (1999) show that experimental damage of the rhinal cortex results in hypometabolism of the PCC. In Alzheimer's disease, metabolic abnormality is linked to amyloid deposition and brain atrophy with a spatial distribution that resembles the nodes of the default mode network. In early Alzheimer's, functional connectivity within the DMN is reduced, affecting the connection between the PCC and the hippocampus, and these altered patterns can reflect ApoE genetic status (a risk factor associated with the disease). It has been found that neurodegenerative diseases spread 'prion-like' through the brain. For example, when the proteins amyloid-b and TDP-43 are in their abnormal form, they spread across synapses and are associated with neurodegeneration. This transmission of abnormal protein would be constrained by the organization of white matter connections and could potentially explain the spatial distribution of pathology within the DMN, in Alzheimer's . In Alzheimer's disease, the topology of white matter connectivity helps in predicting atrophic patterns, possibly explaining why the PCC is affected in the early stages of the disease.
Autism spectrum disorder