In a new
paper published in Learning and Memory, researchers from Boston
University’s Center for Systems
Neuroscience reveal just how much power scents have in
triggering the memory of past experiences–and the potential for odor to be used
as a tool to treat memory-related mood disorders.
“If odor
could be used to elicit the rich recollection of a memory–even of a traumatic
experience–we could take advantage of that [therapeutically],” says BU
neuroscientist Steve Ramirez, assistant professor of psychology
and brain sciences and senior author of the study.
Until now, the scent-memory
connection has been something of an enigma. In fact, even the mechanisms that
underlie memory formation in general have been debated in recent years. The
traditional theory–systems consolidation theory–suggests that our memories
start out being processed by a small, horseshoe-shaped brain area called the
hippocampus, which infuses them with rich details. Over time, especially when
we sleep, the set of brain cells that holds onto a particular memory
reactivates and reorganizes. The memory then becomes processed by the front of
the brain–the prefrontal cortex–instead of the hippocampus, and many of the
details become lost in the shuffle.
This theory has its merits. For
starters, it would explain why our memories tend to get a bit fuzzy as time
passes. It also helps explain why people with hippocampal damage are often
unable to form new memories while their ability to keep old, prefrontal
cortex-stored memories remains perfectly intact. In contrast, those with
prefrontal cortex damage often exhibit the flavor of amnesia we often see in
soap operas: an inability to remember the past.
However, critics of the systems
consolidation theory maintain that it doesn’t tell the whole story. If memories
slip out of the hippocampus and become stripped of their details over time,
then why do many people retain vivid recollections of an event even years
later–particularly people with post-traumatic stress disorder (PTSD)? And why
do scents, which are processed in the hippocampus, sometimes trigger seemingly
dormant memories?
To answer these questions, Ramirez
and members of his lab created fear memories in mice by giving them a series of
harmless but startling electric shocks inside a special container. During the
shocks, half of the mice were exposed to the scent of almond extract, while the
other half were not exposed to any scent. The next day, the researchers
returned the mice to the same container to prompt them to recall their newly
formed memories. Once again, the mice in the odor group got a whiff of almond
extract during their session, while the no-odor group was not exposed to any
scent. But this time, neither group received any new electric shocks.
Consistent with the systems consolidation theory, both groups exhibited
significant activation of the hippocampus during this early recall session,
indicating they remembered receiving the shocks from the day before.
However, during the next recall
session 20 days later, the researchers were in for a shock of their own. As
expected, in the no-odor group, processing of the fear memory had shifted to
the prefrontal cortex–but the odor group still had significant brain activity
in the hippocampus.
“[This finding suggests] that we can
bias the hippocampus to come back online at a timepoint when we wouldn’t expect
it to be online anymore because the memory is too old,” Ramirez says. “Odor can
act as a cue to reinvigorate or reenergize that memory with detail.”
Ramirez adds that we still aren’t
sure about odor’s exact role in memory processing. Perhaps odors delay a
memory’s shift to relying on the prefrontal cortex, thereby preserving the
details for longer. If this is the case, an odor needs only to be present
during memory formation for a memory to retain its vividness. Alternatively,
it’s possible that the prefrontal-cortex shift still occurs in an
odor-associated memory, but that if the same odor emerges again later on, the
hippocampus becomes reactivated and the memory regains the details it had lost.
Regardless of the specifics, Ramirez
says that this research provides us with a “blueprint” of memory processing in
nonhuman animals, and this information might one day lead to breakthroughs in
the treatment of mental health conditions in humans, such as PTSD.
Many psychotherapy- and drug-based
treatments for PTSD involve trying to suppress or dampen traumatic memories,
but this process can only be carried out effectively when people actively
recall the memories first.
“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,” Ramirez
says.
In other words, the scents that
spark our memories may be more powerful than we realize. Today, they serve as
the triggers for our nostalgia and our anxiety–but tomorrow, they could be our
treatments.
“We can potentially view memory as
its own kind of drug–as an antidepressant or [anxiety reducer],” Ramirez says.
“And [odor] could be an experimentally controllable factor that we could
deliver to people. It may be a very powerful tool.”
Source: https://www.bu.edu/csn/
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