Retrosplenial cortex

There is large variation in the region's size across different species. In humans it comprises roughly 0.3% of the entire cortical surface whereas in rabbits it is at least 10%[5] and in rats it extends for more than half the cerebrum dorso-ventrally, making it one of the largest cortical regions.[2]

On the basis of its microscopic cellular structure it is divided into dysgranular (area 30) and granular (area 29) regions.[1] It has dense reciprocal projections with the visual cortex, postsubiculum (also known as dorsal presubiculum) and with anterior thalamic nuclei and the hippocampus.[6]

Neurophysiological studies of retrosplenial cortex have mainly been done in rats. Early work showed that around 8.5% of neurons in the retrosplenial cortex are head direction cells while other neurons have correlated with movement parameters such as spatial location and running speed.[7][8] Recent studies have shown that retrosplenial neuronal activity reflects multiple parameters simultaneously including which environment the animal is currently in,[9] its spatial position in the environment,[10][9] its current head direction and running speed,[9] as well as whether the animal is turning[10] or plans to turn in the future.[11] Many of these features of retrosplenial neurophysiology develop slowly as the animal learns to navigate within an environment,[11] consistent with the idea that the retrosplenial cortex participates in the long-term storage of spatial memory.[12]

In humans, fMRI studies implicate the retrosplenial cortex in a wide range of cognitive functions including episodic memory, navigation, imagining future events and processing scenes more generally.[2][13] Rodent studies suggest the region is important for using surrounding visual cues to carry out these tasks.[12][14][15][16] Retrosplenial cortex is particularly responsive to permanent, non-moving environmental landmarks[17][18] and is also implicated in using them to make spatial judgements.[19][20]

It has also been suggested that retrosplenial cortex may translate between egocentric (self-centred) and allocentric (world-centred) spatial information, based upon its anatomical location between the hippocampus (where there are allocentric place cell representations) and the parietal lobe (which integrates egocentric sensory information).[12][21][22]

Competitors in the World Memory Championships are able to perform outstanding feats of memory and show increased fMRI activation in their retrosplenial cortex than normal controls when doing so.[23] This is thought to be due to their use of a spatial learning strategy or mnemonic device known as the method of loci.

The region also displays slow-wave theta rhythmicity[24] and when people retrieve autobiographical memories, there is theta band interaction between the retrosplenial cortex and the medial temporal lobe.[25]

The retrosplenial cortex is one of several brain areas that produces both an anterograde and retrograde amnesia when damaged.[26] People with lesions involving the retrosplenial cortex also display a form of topographical disorientation whereby they can recognise and identify environmental landmarks, but are unable to use them to orientate themselves.[2]

The retrosplenial cortex is one of the first regions to undergo pathological changes in Alzheimer's disease and its prodromal phase of mild cognitive impairment.[27][28][29] There are also experimental findings showing that layer 5 of the retrosplenial cortex is likely responsible for dissociative states of consciousness in mammals.[30]

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  Brodmann area 29
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  Brodmann area 30