Semantic memory deficit in multiple sclerosis; impaired understanding of conceptual meanings
Introduction
It is commonly accepted that memory is vulnerable in MS 1, 2, 3. Some investigators propose that the memory impairment in MS may be a consequence of deficient encoding 4, 6, while others suggest it is a retrieval problem 3, 7. Some investigators suggest that disturbances in encoding and retrieval both contribute to memory deficits in MS [8]. Grafman et al. [3]found the distinction between effortful vs. automatic memory processes important in understanding the memory problems in MS. According to their results, the patients were significantly impaired on memory tasks requiring effortful operations, such as explicit search and strategy procedures used to encode and then recall or recognize previously presented material. However, the patients performed normally on automatic measures which demanded less sustained concentration and fewer cognitive resources. There are also other studies which confirm that MS patients perform normally on memory tasks which make only limited demands on attention 7, 8, 9.
Semantic memory has been studied much less frequently than episodic memory in MS. Some studies have found semantic knowledge to be intact 10, 11, 12. However, we suggest that problems in conceptual reasoning and abstraction 1, 13, 14, 15, 16, 17, 18, 19, 20, 21can be a sign of a semantic memory disturbance. Beatty et al. [22]were apparently the first authors who used and linked the concept of semantic memory deficit with MS and reported inefficient search in semantic memory in MS. They concluded that the deficient search arises from an inability to retrieve knowledge from semantic memory quickly or from a loss of information from the memory stores. Paul et al. [23]have assessed different components of autobiographical memory in MS and found that MS patients have difficulties in recalling the semantic components of their autobiographical memories.
The conceptualization and operationalization of semantic memory vary and, thus, the methods used to measure semantic or conceptual deficits in MS have been heterogeneous. The different methods used do not place equal demands on effortful processing. We propose one possible way to approach the problem. We have presented [24]a framework in which three different levels of semantic knowledge are distinguished. At the lexical level there is only an associative and automatic word-recognition system that may mirror semantic relations 25, 26, 27. The level of conceptual-semantic representations consists of the keys to the multimodal associations that together designate the meaning of the concept, its attributes and its relations to other concepts 28, 29, 30. The structural organization of semantic memory is realized at this level. We propose that the tasks demanding very automatic processes measure information at the two levels at which processing can take place in the absence of conscious understanding of the meanings of the concepts 31, 32, 33, 34. The highest level of semantic knowledge refers not to an associative or automatic word-recognition system nor to the structural organization of semantic memory but to the function of accessing and using the conceptual knowledge in a sensible way. This is the level of conscious understanding of the meanings of concepts. In accordance with this view, we suggest that, using only tasks tapping some of the levels of semantic knowledge, it is not appropriate to conclude that semantic memory as a whole is intact.
In the present study, semantic knowledge in MS was explored as revealed by the conscious understanding of the concept meanings (the level of conceptual-semantic representations and the level of conscious understanding). Four different tasks were used to tap the understanding of (1) concrete and (2) abstract concepts, (3) the relationships between concepts and their attributes, and (4) the hierarchical structure among concepts. Performance in these tasks reveals not only whether conscious access to different concepts is possible, but also whether conscious insight into the precise nature and meaning of the semantic relationships between the concepts can be achieved. In some tasks we measured the spontaneous productions and cued productions separately in order to obtain more information about a possible retrieval problem in semantic processing. We used forced-choice tasks and recognition tasks in order to determine if the patients need external guidance to retrieve their semantic knowledge. Cognitive and physical factors that could have influenced the results were taken into account and carefully controlled when possible. The main aim was to determine whether a cognitive decline in MS implies deficits in semantic memory, as revealed in the understanding of conceptual meanings.
Section snippets
Subjects
The study group consisted of 12 patients (six men and six women) who all met the criteria of clinically definite MS according to the Poser et al. [35]definition. Patients with drug or alcohol abuse, psychiatric history, or nervous system disorder other than MS were excluded from the study. All the study patients were clients of the Masku Neurological Rehabilitation Centre.
Simultaneously with our study, the patients participated in a neuropsychological follow-up study [36]. At their initial
Description of Concrete Concepts
The answers were independently scored by two raters. The mean of the Kappa Coefficient for the items of this task was 0.9 (range 0.6–1.0). The final scores were agreed on after discussion. The MS group was poorer at producing the superordinate category and structural and functional attributes for the concepts given (t-test: P<0.001 for the total score). The mean of the total score for the controls was 222.8 (SD 6.3, range 213–232) and for the patient group 182.3 (SD 19.7, range 131–211) (total
Discussion
The present study revealed that cognitive decline related to MS includes deficits in semantic memory. Semantic memory functions were evaluated by studying the ability to consciously understand concept meanings and conceptual relationships. The patients had difficulties in spontaneously retrieving semantic knowledge of concepts from their memory. They needed a greater number of specific guiding questions and forced-choice questions than the controls. At the level of forced-choice tasks they
Acknowledgements
The present study was financially supported by The Emil Aaltonen Foundation and The Academy of Finland (project No. 36106), which is gratefully acknowledged. We wish to thank Anne Kaljonen, statistician, for her valuable contribution to the statistical analyses of our data. And we thank our patients for their time and interest in this study.
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