Table 4.

Proposed stages of progressive memory impairment in Alzheimer's disease, relative to healthy ageing

	Memory encoding	Early retention	Long-term retention
Alzheimer's disease dementia	Reduced	Reduced	Reduced
Mild cognitive impairment	Normal/unchanged	Reduced	Reduced
Presymptomatic Alzheimer's disease	Normal/unchanged	Normal/unchanged	Reduced
Healthy ageing	Normal/unchanged	Normal/unchanged	Normal/unchanged

Previous studies have established differences in memory function between Alzheimer's disease dementia, Alzheimer's disease mild cognitive impairment, and healthy ageing, based on tests of memory encoding and retention over a short interval. The results of our study allow us to propose a third column, for long-term retention, which shows impairment several years before onset of noticeable symptoms.