[Longitudinal study of procedural memory in patients with Alzheimer-type dementia]

We never forget how to ride a bicycle, and it is thought that procedural memories are retained for a long time. Recently, it was reported that patients with dementia of the Alzheimer type(DAT) could not only acquire, but also retain, long-lasting procedural memories. Previous group studies had shown procedural memory retention times of only 1 month in DAT patients, while amnesic patients and healthy people could retain such memories for 12 months. The relationship between the ability to retain procedural memory and the stage of the disease is not clear, as to date there has been no longitudinal study of procedural memory retention in DAT patients. Thus, we examined DAT patients' ability to retain long-term procedural memories (after 1, 5 and 20 months), and analyzed the relationship between procedural memory ability and the progress of disease. Motor-type procedural memory was examined using the mirror tracing task and the bi-manual coordinated tracing task. All three of the DAT patients showed improvement in their performance. The time required for the tracing was reduced between trials, and the improvement did not disappear between sessions, or rather, their times further decreased in subsequent sessions. Even the most severe DAT patient (Mini Mental State Examination(MMSE) score of 4) was able to acquire the procedural memory and retain it for at least 3 months. Furthermore, one of the subjects showed retention of the procedural memory at 20 months. Our results suggest that DAT patients can retain procedural memories for extended periods, with no relationship between retention ability and disease progression. It is possible for even severely demented patients to acquire and retain motor-type procedural memories. Cognitive rehabilitation in DAT appears to be effective, and it is possible for DAT patients to learn new things. It may be that DAT patients can ameliorate their quality of life by using retained procedural memory.