Memory Loss in the Early Stages of Alzheimer's Disease
The purpose of the study was to examine any semantic memory deficit in the early stages of Alzheimer's disease (AD) and further define the nature of any deficit found. In experiment one, 30 participants with AD and 30 age, gender and education-matched control participants completed a series of semantic memory tasks, an independent test of frontal lobe functioning (tower of Hanoi), a global screening measure, and measures of anxiety and depression. Participants with AD were in the early stages of the disease and divided into minimal and mild subgroups. In addition a further amnesic subgroup showed relatively intact global scores alongside impaired semantic memory task performance. Significant between-group effects were observed across the range of semantic memory tasks. In addition, dissociations were found between tower of Hanoi performance and performance on semantic memory tasks. These findings lend support to the view that semantic memory difficulties are due to underlying damage to the structure of semantic memory, rather than resulting from an access or organisational problem. The performance of the amnesic group raises the possibility that semantic memory loss may represent a preclinical stage of the disease process in AD.
In experiment two, eight participants with probable AD and eight age, gender and educationmatched control participants took part. The effects of imageability and age of acquisition (AoA) were examined along with attribute knowledge of animals and objects. Results showed that imageability made a strong contribution to semantic associations while the effect of AoA was small, but significant. Imageability, thought to produce stronger activation, may assist in recall of semantic memory items, while the somewhat meagre AoA effect raises questions concerning the temporal gradient of memory loss thought to occur in AD. Attribute knowledge was characterised by loss of subordinate knowledge alongside naming ability which was only minimally affected. Overall results suggest a loss of associations in semantic memory in the early stages of AD, along with a loss of subordinate knowledge and a strengthening of easily accessed responses. No differences were found between living and nonliving items, however knowledge of functional use of objects appeared comparatively well preserved.