The US and Japanese team found that the brain "doubles up" by simultaneously making two memories of events. One is for the here-and-now and the other for a lifetime, they found.
It had been thought that all memories start as a short-term memory and are then slowly converted into a long-term one.
. . .
They involved watching specific memories form as a cluster of connected brain cells in reaction to a shock. The results, published in the journal Science, showed that memories were formed simultaneously in the hippocampus and the cortex.

. . . do not seem to use the cortex's long-term memory in the first few days after it is formed. They forgot the shock event when scientists turned off the short-term memory in the hippocampus. However, they could then make the **** remember by manually switching the long-term memory on (so it was definitely there).

"It is immature or silent for the first several days after formation," Prof Tonegawa said.

Researchers then used light beamed into the brain to control the activity of individual neurons - they could literally switch memories on or off.

.... with Alzheimer's were still forming memories but were not able to retrieve them
http://www.bbc.co.uk/news/health-39518580