Male mice were given a negative experience, consisting of small electric shocks, in a particular room, and the neurons working to store that memory were labelled using "optogenetics".
This is a technique that effectively installs a switch in those neurons, allowing them to be turned on again at will. The trigger used to stimulate them is a beam of blue light, sent into the brain by an optical fibre.
The next day, with the mouse in a different room, stimulating the labelled neurons in this way effectively "reactivated" the original, fearful memory. When the mice were offered a choice between having the blue light on or off, they chose to leave it off.
But next, the researchers stimulated the labelled neurons while they gave the mouse a positive emotional cue (a female for company), in an attempt to "flip" the emotional association of the stored memory.
Sure enough, offered the same choice again, the mice now wanted the blue light switched on: the original memory trace had been altered, and now they liked it.
Importantly, when returned to the original room, in the absence of any brain stimulation, the mice were less afraid than after the first round of training. Their memory of that place had changed - for the better.
The whole procedure also worked in the other direction, allowing the researchers to engineer a corresponding flip from an original positive memory to an artificial negative one.