How do we remember something? Neuroscientists speak of memory engrams to describe the way in which traces of memory are stored in our brains. A memory engram might respond to something external, for example, the scent of a rose. But how do engrams work? Is an engram just an intellectual concept? Or are engrams a physical network of neurons in the brain? A new study published in the journal Nature on March 23, 2012 suggests that memories really do reside in specific brain cells, at least in mice.
Researchers from MIT and Stanford used optogenetics – a combination of genetic and optical techniques in the lab, used to control specific events in targeted cells – to scrutinize the brains of special mice that had been genetically engineered for their memory study.
First, the researchers identified which cells in the mice brains were active only when a mouse was learning about a new environment. Then they determined which genes were activated in those brain cells. Then they created transgenic mice, artificially linking the activated genes they had found with the gene for a light-activated protein called channelrhodopsin-2 (ChR2), often used in optogenetics.
Afterwards, in the brains of the mice, the light-activated protein would be expressed only when the mice were learning about a new environment.
Next, the researchers released the mice into an environment, where, after a few minutes of exploration, the mice received “a mild foot shock, learning to fear the particular environment in which the shock occurred,” according to the researchers. The brain cells activated during this fear conditioning became tagged with the light-activated protein ChR2.
Later, when exposed to triggering pulses of light in a completely different environment, the neurons involved in the fear memory switched on — and the mice quickly entered a defensive, immobile crouch. According to the press release, this posture of the mice:
… suggested that the animals were actually recalling the memory of being shocked. The mice apparently perceived this replay of a fearful memory — but the memory was artificially reactivated.
The researchers say these results show that memories really do reside in very specific brain cells. What’s more, by reactivating these cells by physical means, such as light, an entire memory can be recalled.
In our world, that reactivation of brain cells might happen when we smell a cookie and remember our childhood kitchen.
This research will be useful in the study of neurodegenerative and neuropsychiatric disorders. One of the researchers commented:
The more we know about the moving pieces that make up our brains, the better equipped we are to figure out what happens when brain pieces break down.
Bottom line: Researchers at MIT and Stanford used optogenetic studies of genetically engineered mice to learn that memory engrams – which are associated with traces of memory, often aroused by external stimuli – do reside in specific brain cells.