And many more people suffering dementia never have moments of clarity. Rare cases of terminal lucidity indicates you likely haven't experienced dementia firsthand. Having experienced Alzheimer's destruction on a family members mind I am absolutely confident that memories are completely lost.
I mentioned recently on here Li-Huei Tsai's 2007 paper "Recovery of learning and memory is associated with chromatin remodelling". Tsai used CK-p25 mice models where expression of p25 - a protein implicated in various neurodegenerative diseases including Alzheimer's - can be turned on and off with a special diet.
Uninduced CK-p25 mice and control mice were trained in a fear-conditioning paradigm and returned to their home cages for four weeks, followed by six weeks of p25 induction. Next, the mice were either subjected to a regime of Environmental Enrichment or kept in their home cages for an additional four weeks.
Afterwards, all mice were subjected to the memory test. Whereas non-enriched CK-p25 mice showed significantly impaired freezing behaviour, indicating the loss of long-term memories, enriched CK-p25 mice displayed much improved freezing behaviour, indicating a marked recovery of long-term memories. Importantly, enriched and non-enriched CK-p25 mice have a similar extent of neuronal loss and brain atrophy . Evidence for the recovery of long-term memories was also found by using the water maze paradigm.
The fact that long-term memories can be recovered by EE supports the idea that the apparent ‘memory loss’ is really a reflection of inaccessible memories. These findings are in line with the phenomenon known as ‘fluctuating memories’ in which demented patients experience temporary time periods of apparent clarity.