Dementia and the Late Onset of Dyslexia

Searching for Utopia

This post examines the differences between  Semantic dementia (SD), a progressive neurodegenerative disorder characterized by loss of semantic memory in both the verbal and non-verbal domains AND Semantic Aphasia (AD), a progressive neurodegenerative disorder or aphasia, characterised by the loss of recognition of the meaning of words and phrases.

The NIH paper this is taken from is very 'technical' in its form and is primarily for cognitive scientists to review and discuss but, with some effort on our part, we can see through the jargon to determine that current research is able to distinguish between the onset of Dementia and its effect on our cognitive abilities.


Below is an extract from the paper:

"This NIH study examined the impact of semantic impairment on a variety of “pre-semantic tasks” – lexical decision, object decision, colour decision, reading aloud, spelling to dictation, past tense generation and delayed picture copying – in semantic dementia (SD) and semantic aphasia (SA).

Previous research indicates that all of these tasks are affected in a highly predictable way by the semantic degradation in Semantic Dementia (see Patterson et al., 2006).

SD patients have difficulty with transformations that are atypical and make “regularisation errors” in which irregular items are produced as if they were domain-typical.

In contrast, we found that comprehension impairment in Semantic Aphasia did not result in this pattern.

1. In all seven “pre-semantic” tasks that we examined, the SA patients were less sensitive to typicality than the SD patients (producing significant group by regularity interactions), even though the two groups were matched for performance on semantic tests. 
2. In several tasks, the aphasic group were also less sensitive to item frequency and/or showed a smaller frequency by typicality interaction than the patients with SD. 
3. In production tasks, the SA patients made fewer regularisation errors and many more implausible responses than the SD group – for example, unrelated, semantic and perseverative errors.

The predicted effects of semantic impairment on pre-semantic tasks have been demonstrated most frequently in patients with SD.

In contrast, patients who show a breakdown in this association – i.e., preserved reading of irregular words despite pronounced semantic memory problems – are typically individuals with AD and stroke aphasia (Gerhand, 2001; Lambon Ralph et al., 1995).

These cases are viewed as highly significant by some researchers because they suggest that semantic memory impairment and regularisation errors on “pre-semantic” tasks are dissociable.

However, our findings indicate that strong typicality effects in “pre-semantic” tasks follow degradation of ATL semantic representations and not poor executive control of semantic cognition.

As the semantic deficit in SA and to some extent AD is likely to be related to poor executive control, individuals with these conditions do not disconfirm the hypothesis that semantic representations play an important role in “pre-semantic” cognition.

Read the full paper here at NIH website