Language, an essential cognitive function we develop during childhood, becomes a vulnerable asset in many neurological diseases. When language processing is affected, the diagnosis is that of aphasia. It is a remarkably frequent occurrence, especially in patients who have suffered a stroke or another form of brain injury [XNUMX].
Nevertheless, due to its complexity and involvement of many brain regions, language can also be afflicted by neurodegenerative diseases. One clear example is dementia– the progressive loss of higher cognitive faculties. One subtype of dementia affects language in particular. It is called primary progressive aphasia (PPA) and arises when brain regions involved in speech processing begin to degenerate [XNUMX].
PPA can be further subdivided, depending on what particular speech difficulties patients face. Patients with the semantic variant of PPA (svPPA), for example, experience progressive trouble naming objects, places, or people. Over time, they may find it harder and harder to understand the meaning of certain words or have trouble maintaining a conversation as their vocabulary becomes severely limited [XNUMX].
This pattern is somewhat reminiscent of another neurodegenerative disease in which speech is progressively impaired – Alzheimer’s. In early stages, Alzheimer’s patients may also experience difficulties recalling words and lose their fluency. As the disease progresses, they begin slurring, stammering or using incorrect words, until they ultimately lose the ability to form coherent sentences [XNUMX].
A useful question to ask is the following: are the mechanisms that cause the language deficit in the two disorders described the same?
This is the question De Vaughn and colleagues tried to answer with research published in the Journal of Neuropsychology.
The authors' intent was to evaluate and compare verbal episodic memory (using a word list learning test) in 68 patients with svPPA and 415 with Alzheimer's disease.
Participants took part in various neuropsychological tests targeting attention, language, memory, or executive function. Of particular interest were:
- A semantic knowledge test:
- Repeating a list of 9 words aloud 4 times to the participants, then asking them to recall the words after 30 seconds and 10 minutes (verbal recall) as well as to select the words from a list (verbal recognition).
- Presenting participants with a figure, which they had to memorize and re-draw after 10 minutes (visual episodic memory).
- A semantic knowledge test:
- identifying which of four pictures best matches a certain word.
The results showed that patients with svPPA scored better on verbal learning tests than those with Alzheimer's disease. Furthermore, they exhibited better visual memory skills while people with Alzheimer's exhibited better skills related to semantic knowledge.
On the other hand, there were no differences in the recognition memory (recognition of words heard).
In Alzheimer’s patients, verbal recall seemed to be influenced by several parameters, including age, sex, their neuropsychological assessments, but also how well they performed on the visual episodic memory task.
In patients with the semantic variant of PPA, verbal recall was also influenced by similar factors, but notably more by their performance in the semantic knowledge task.
Authors conclude that a dissociation exists between the semantic variant of PPA and Alzheimer’s, in regard to their impaired verbal memory: While visual memory can predict Alzheimer’s patients’ verbal episodic memory, svPPA’s patients’ performance depends more on their semantic knowledge.
Limitations, such as the remarkably different number of participants in each group, need to be taken into account and overcome in future studies in order to reach certain conclusions.
Ultimately, this study suggests that memory and vocabulary are interconnected constructs, which become differently altered in different neurodegenerative diseases, even if they may appear similar at first glance. Such insights are valuable not just for properly understanding these afflictions, but also for designing therapeutic options that properly address patients’ needs and abilities.
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- DeVaughn, S., Casaletto, KB, Staffaroni, AM, Wolf, AA, Marx, G., & Kramer, JH (2020). Differential cognitive substrates of verbal episodic memory performance in semantic variant primary progressive aphasia and Alzheimer's disease. Journal of Neuropsychology.