Memories lost in Alzheimer's mice were switched back on

Neuroscientists at MIT studied mice bred to develop Alzheimer's-like plaques. Before any plaques had formed, the mice already failed to recall a fear memory, raising the question of whether the memory was destroyed or simply out of reach.

The team tagged the exact brain cells, called engram cells, that had encoded the fear memory, then used light to switch those cells on directly. The amnesic mice froze in fear again, showing the memory still existed and could be retrieved, just not through ordinary recall cues.

The researchers traced the block to lost synaptic connections on the engram cells, not to a lost memory trace. Restoring those connections with light brought back both the missing spine density and normal recall when the mice were returned to the chamber, with no light needed. The work was done in mice bred for early Alzheimer's pathology, years before any human therapy could follow.

The memory was never gone.