Mental stimulation and drug treatment could help people with degenerative brain diseases such as Alzheimer's recover their memories, a study says.
The study says progress is possible even after major brain damage
Scientists found mice with a similar condition to Alzheimer's were able to regain memories of tasks they had previously been taught.
A team at the Massachusetts Institute of Technology found two methods - brain stimulation and drugs - both worked.
Their findings were published in British journal Nature.
The researchers used genetically engineered mice in which a protein linked to degenerative brain disease could be triggered.
Scientists had previously given the mice tests where they learnt to avoid an electric shock and how to find their way through a maze to reach food.
After six weeks with the brain disease, the mice were no longer able to remember how to perform these tasks.
Some of the mice were then placed in a more stimulating environment with toys, treadmills and other mice.
The playground mice were able to remember the shock test far better than the mice in other cages. They were also better at learning new things.
Scientists then tested a class of drugs called histone deacetylase, or HDAC, inhibitors on the mice.
These also improved memory and learning, similar to improvements made by environmental stimulation.
Neuroscientist Li-Huei Tsai of the Howard Hughes Medical Institute and the Massachusetts Institute of Technology said the results could offer hope to people with diseases like Alzheimer's.
"We show the first evidence that even if the brain suffered some very severe neurodegeneration and the individual exhibits very severe learning impairment and memory loss, there is still the possibility to improve learning ability and recover to a certain extent lost long-term memories."
She said the study suggested that in people with degenerative brain diseases, memories were not erased from the brain, but rather could not be accessed because of the disease.
She added that while most treatments for Alzheimer's targeted the disease's early stages, this research showed that even after major brain damage it was still possible to improve learning and memory.
Dr Susanne Sorensen, head of research, Alzheimer's Society, said: "These results cannot automatically be translated to people and a lot more has to be done to narrow the focus on the processes that are involved.
"However, by demonstrating that lost memories can be accessed again these results offer hope of a better understanding of what happens to memories as dementia develops.
"It highlights the role of both an 'enriching environment' and through its focus on biochemical processes could provide important building blocks for new treatments to alleviate the symptoms of dementia."