Why can people with semantic dementia still remember events?

A new study from the Sydney FTD group in Australia has found that certain parts of the brain seem to be protected from the effects of the disease in semantic dementia.

Different areas of the brain are selectively affected by different forms of FTD. This is why some people with FTD initially have language problems while others have more changes in personality or behaviour. By working out which particular parts of the brain are affected by different forms of FTD and what’s special about them, researchers hope to understand more about how the disease progresses and also get better insight into how FTD affects people’s thinking.

In this study researchers examined the brains of 8 people who had died with semantic dementia (SD) and compared it with 9 people’s brains who had behavioural variant FTD and 8 people who didn’t have any form of brain disease. Amongst the brains of people who had either SD or behavioural variant FTD they found selective loss of a particular type of brain cell called von Economo neurons in the front of an area of the brain called the cingulate cortex. Interestingly, von Economo cells further back in the cingulate cortex appeared to be affected in behavioural variant FTD but not in semantic dementia.

The group also looked at a number of brain structures involved in memory for events and experiences, known as episodic memory, a process that tends not to be affected in SD. They found marked shrinkage in all of these structures in SD with the exception of two areas called the mammillary bodies and the hippocampus.

The researchers suggest the reason why people with SD retain reasonably good episodic memory is because of the preservation of links between the mammillary bodies, the back of the cingulate cortex and the hippocampus.

This study is important because not only does it help us to understand why people with SD still remember events even when they cannot name simple objects, but it also gives us clues as to how the healthy brain processes memory.

Reference

Tan HT, Wong S, Kril J, Piguet O, Hornberger M, Hodges JR, Halliday GM. Beyond the temporal pole: limbic memory circuit in the semantic variant of primary progressive aphasia. Brain. 2014 May 19th. [Epub ahead of print]

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