The MIT researchers made this discovery by triggering small clusters of neurons in mice causing them to recall specific memories. When they would remove these neurons the mice would then lose those specific memories.
In order to show that a specific memory was triggered the mice were administered an electric shock to create a memory of fear. Later when the cluster of neurons were triggered the mice went into a a defensive posture showing the fear memory was triggered
What still needs to found out though is HOW memories are encoded into neurons. Like how exactly a cluster of 100,000 neurons creates the memory of an elephant. There has been some work done in this area on the Optic Nerve Psychology is primarily the science how WHY their are encoded and how they affect us.
Still a little ways off from the envisioned future of brain downloading and uploading but getting a hell of a lot closer
For further reading on Optic Nerve research go here
The article was a summary of Extremetech.com