Smells can reinvigorate old memories in a way that doctors could use to treat memory-related mood disorders, suggests a study in mice from Boston University.
The nature of memory is one of the biggest mysteries of neuroscience. Memories are at the basis of who we are, and how the brain processes and consolidates them is the subject of work around the globe.
A recent study from Boston University’s Center for Systems Neuroscience has shown the power of smell in recalling memories, challenging a decades-old theory in the process.
The new research appears in the journal Learning and Memory.
The power of the hippocampus
The hippocampus — a small brain structure shaped like (and named after) a seahorse — is critical in the process of memory formation.
Experts think that when memories first form, the hippocampus processes them and captures rich, contextual details in the memory.
Over time, and particularly during sleep, the prefrontal cortex, or front of the brain, processes the memory, where many of the details embedded by the hippocampus become lost.
Scientists call this the systems consolidation theory, and it helps to explain why memories become ‘fuzzier’ over time.
Putting theory to test
According to this theory, recalling a memory shortly after it happens involves activation of the hippocampus, whereas remembering an old memory involves activation of the prefrontal cortex.
This is because the same brain cells that are active when a memory forms reactivate when a person recalls that memory.
However, there are some inconsistencies with this theory. Some people remember old memories vividly, something familiar to people who have post-traumatic stress disorder (PTSD).
And smells, which the short-term memory area of the hippocampus process, can trigger memories from years gone by.
To test this theory, the new research tied memory to smell in mice.
The researchers created powerful memories in the mice by giving them a series of harmless but jolting electric shocks while inside a container. While receiving the shocks, the scientists exposed half of the mice to the smell of almonds.
Smell can ‘reinvigorate’ old memories
The following day, the researchers returned the mice to the container and exposed the same mice to the almond scent again.
In both groups of mice, the region of the brain associated with short-term memory, the hippocampus, was active. This suggests that all the mice remembered the shocks from the day before. This is in line with the systems consolidation theory.
However, when the researchers performed the same recall experiment 21 days after the shock, all the mice showed some activity in the prefrontal cortex. However, the hippocampus showed significantly more activity in the mice who smelled the almonds than those who had the shock alone.
This was a surprise for the researchers, who were expecting to see more activity in the front part of the brain in all the mice.
“[This finding suggests] that we can bias the hippocampus to come back online at a time point when we wouldn’t expect it to be online anymore because the memory is too old,” explains Steve Ramirez, assistant professor of psychology and brain sciences and senior author of the study.
The finding suggests smell can ‘reinvigorate’ memories with detail, via the hippocampus.
Memory as a treatment
The finding has the potential to revolutionize scientists’ understanding of memory processing. Although scientists still need to work out the finer details, the study suggests that the theory of the systems consolidation does not always hold true.
Beyond the academic interest, these findings could pave the way for new tools to treat mental health conditions caused by traumatic memories, namely PTSD.
“Now that we know that odor can shift memories to become more hippocampus-dependent, we could potentially develop strategies that engage or disengage the hippocampus. And then we could integrate some behavioral or drug-based approaches to bring the hippocampus back offline if our goal is to permanently suppress a fear memory,”
– Steve Ramirez
Most current approaches to treat PTSD try to suppress or dampen the traumatic memory, but — crucially — this only works when a person can recall the memory first. Using smells to spark memories could, therefore, be a significant pathway to treatment.