Think about the last time you were on a roller coaster. You probably remember the time, the place, who you were with — and might also feel either excited or terrified, depending on whether you love or hate roller coasters.
That's because our memories don't exist in isolation. Our brains attach a lot of context to any given memory — including any strong emotions that go along with it.
Now, neuroscientists at Massachusetts Institute of Technology have been able to actually change those emotions in lab mice, thanks to a technique that lets researchers turn individual neurons on or off using blue light.
Basically, they were able to make the mice change their minds about a positive or negative memory.