A new study focuses on the way the brain represents knowledge and facts about the world - what psychologists and linguists call semantics. Faye Corbett and colleagues compared the deficits shown by eight patients with semantic dementia - a form of neurodegenerative brain disease that affects the front region of the temporal lobes - and seven stroke patients with semantic aphasia. The stroke patients had damage either to the left, frontal part of their brains, or to the junction where the temporal and parietal cortices meet.
Superficially, the two groups of patients have remarkably similar impairments. They struggle to find the correct words to refer to things, and their factual knowledge and comprehension of words also seems affected. However, with the help of an extensive battery of tests, Corbett's team have shown that there are striking differences in the way the two patient groups are affected - a finding that helps further our understanding of the way the brain supports language and knowledge.
The patients with semantic dementia performed consistently across tests. So, if they struggled with a word on one test, such as matching a picture to the word "hammer", then they would also struggle when they were asked to mime the use of a hammer, or if they were asked to match a picture of a hammer to another object with a similar function. Moreover, the rarer a word, the more likely these patients were to have a problem. Altogether their performance suggests that they are progressively losing their core knowledge about objects.
By contrast, the performance of the patients with semantic aphasia was inconsistent. They'd perform well on a simple task, such as pointing to a picture of a hammer when prompted with the word, but they'd struggle as soon as a task was made more complicated - for example, involving pairing objects by their function, or by action. Moreover, the rarity of a word didn't predict whether these patients would have a problem.
This difference between the groups was particularly striking when it came to miming object use. The patients with semantic dementia could either do this for an object or they couldn't. By contrast, the aphasic patients would get some of the mime correct, but would then perform an inappropriate action, as if they were suddenly using a different type of object.
The researchers think the overall picture shown by the two groups of patients reveals that the semantic system of the brain is comprised of at least two components - a core representation of knowledge, and an overall control system that navigates through the corridors of the mind finding and comparing meanings. Core knowledge is subserved by the frontal area of the temproral lobes, which is the region afflicted by the disease process in semantic dementia, whilst the cognitive control component is subserved by the prefrontal cortex and the temporo-parietal junction, which are the areas, which when damaged, lead to semantic aphasia.
"We propose that semantic dementia patients have damage to core amodal semantic representations, whereas patients with semantic aphasia have a more general executive impairment that leads to difficulty controlling activation within the semantic system in a flexible, task appropriate fashion," the researchers said.
Corbett F, Jefferies E, Ehsan S, & Lambon Ralph MA (2009). Different impairments of semantic cognition in semantic dementia and semantic aphasia: evidence from the non-verbal domain. Brain : a journal of neurology, 132 (Pt 9), 2593-608 PMID: 19506072
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