While a “Men In Black”-style memory eraser may still be some way off, a team of researchers from Oxford University has managed to successfully delete associative memories in mice. Their work contributes greatly to our understanding of how the brain creates and stores memories, and opens up new possibilities for developing novel therapies to treat conditions such as drug addiction and post-traumatic stress disorder.
Associative memories refer to the process of remembering the relationship between two unrelated phenomena, such as when a particular song reminds us of a certain person, or when a smell ‘takes us back’ to a specific place. While these are normally harmless, they can play a significant role in perpetuating substance addiction, as certain environmental cues trigger a need to use a drug.
To see if these connections can be erased, scientists used a tracking molecule to determine which neurons in the brains of mice became activated as they familiarized themselves with a particular environment. Previous work has shown that neurons called place cells, which are found in a region called the hippocampus, contribute to the formation, storage and recall of spatial memories, essentially creating mental maps of our surroundings.
As the mice became increasingly familiar with their enclosure, certain place cells began to show enhanced activity, indicating that they were driving the rodents’ memory of this particular space. Once these cells had been identified, the researchers engineered the mice to produce light-sensitive proteins that allowed them to turn these particular place cells on and off by flashing a light.
Hippocampal neurons called place cells are responsible for storing spatial memories. Andrej Vodolazhskyi/Shutterstock
They then encouraged the mice to form associative memories, by placing a cocaine solution in a chamber and a saline solution in an identical adjacent chamber. Over time, the mice began to associate the relevant area with the cocaine, and showed an increased preference to head straight for that chamber while ignoring the other. In other words, they formed an associative memory between the chamber and the experience of taking cocaine.
However, when the researchers used light to deactivate the place cells driving this memory, the mice stopped showing this preference for the cocaine-containing room, as if they had forgotten the association between this chamber and the drug. This does not mean that the mice had lost their taste for cocaine, but simply that a particular environmental cue that previously contributed to their drug-taking behavior had been erased.
Reporting their findings in the journal Nature Neuroscience, the authors describe how the deactivation of the place cells coding this associative memory also sparked the activation of an entirely different set of neurons, indicating that the process had somehow generated a shift to an “alternative hippocampal map.” In other words, new neurons took over in order to allow the mice to continue navigating and remembering their environment, but without the cocaine-place memory.
While more work is required in order to refine this process and determine whether or not it can be applied to humans, the study authors say that their research provides strong evidence for the engram theory, which holds that memories are physically coded into the brain’s neural circuitry.