User:Akreuzman/sandbox

This is my sandbox. Much of the original formatting material has been removed and it will now be used as a workspace to format information on the Posterior Cingulate Cortex.

The Introduction to our page was a collaborative effort and therefore I cannot take complete credit for it in my sandbox.

Anatomy

Location and Boundaries

The posterior cingulate cortex is located within the medial part of the inferior parietal lobule. It is the backmost part of the cingulate cortex, lying behind the anterior cingulate cortex. The PCC forms 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 the thalamus, in the form of a continuous strip crossing 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, which is known to be involved in associative learning and episodic memory. The human posterior cingulate cortex is also connected to areas involved in emotion and social behavior, attention (the lateral intraparietal cortex and precuneus), and 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 posterior cingulate cortex are approximately 40% higher than average across the brain. The posterior cingulate cortex also has high functional connectivity, signifying 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 posterior cingulate cortex was the only brain structure highly active in all subjects. Furthermore, the PCC but 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 posterior cingulate cortex 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 responded 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).

The relationship between these networks in and 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 PCC's relation with the DMN, since suppressed posterior cingulate activity favors low cognitive introspection and higher external attention while increased activity indicates memory retrieval and planning, it has been hypothesized that this brain region is heavily involved in noticing internal and external changes and facilitating novel behavior or thought in response. Low activity, then would indicate continued operation with the current cognitive set, while higher 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 posterior cingulate cortex 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.