If there is a
problem that bedevils us call it is memory.
It is malleable, unreliable and mostly lost except for triggers that
regenerate an image of the memory for us to fill in. How it ever works so well is perplexing. That begs the important question of whether
exact memory is even possible as suggested by memory mavens.
I would sooner
simply have access to the point in time and reconstruct from there from the
same input. There is little evidence
that we can do that although that is what a minimalist nature would try.
Way more
interesting to me would be a theory of memory triggers and how the brain processor
actually taps them at all. I remember
clear scenes from books read decades ago while that same brain has dismissed
huge amounts of material into deserved oblivion.
Important New
Theory Explains Where Old Memories Go
Why some memories disappear, some remain, and others
blend with fiction
By Emilie Reas
http://www.scientificamerican.com/article.cfm?id=important-new-theory-explains-where-old-memories-go
Think back to your first childhood beach vacation.
Can you recall the color of your bathing suit, the softness of the sand, or the
excitement of your first swim in the ocean? Early memories such as this often
arise as faded snapshots, remarkably distinct from newer memories that can feel
as real as the present moment. With time, memories not only lose their rich
vividness, but they can also become distorted, as our true experiences tango
with a fictional past.
The brain’s ability to preserve or alter memories
lies at the heart of our basic human experience. The you of today is molded not
only by your personal history, but also by your mental visits to that past,
prompting you to laugh over a joke heard yesterday, reminisce about an old
friend or cringe at the thought of your awkward adolescence. When we lose those
pieces of the past we lose pieces of our identity. But just where in the brain
do those old memories go? Despite decades studying how the brain transforms
memories over time, neuroscientists remain surprisingly divided over the
answer.
Some of the best clues as to how the brain processes
memories have come from patients who can’t remember. If damage to a
particular brain area results in memory loss, researchers can be confident that
the region is important for making or recalling memories. Such studies have
reliably shown that damage to the hippocampus,
a region nestled deep inside the brain, prevents people from creating new
memories. But a key question, still open to debate, is what happens to a memory after it’s
made. Does it stay in the hippocampus or move out to other areas of the brain?
To answer this, scientists have studied old memories formed before brain
damage, only to discover a mix of inconsistent findings that have given rise to
competing theories.
One popular theory proposes
that the hippocampus is critical only for recent memories, but not older ones.
Over time, the hippocampus teaches the surrounding brain – the cortex – how to
represent a memory. As the memory matures, the hippocampus kicks it out to
reside independently in the cortex. If you lost your hippocampus today, you
could still remember your childhood vacation to Florida, but the memory from
last weekend’s dinner party would be lost. This is the exact pattern scientists
have observed in many patients, including the celebrated amnesic H.M.After
surgery to remove a large chunk of his hippocampus, H.M. could recall some very
old memories, but could no longer make new memories or remember the years
leading up to his surgery.
But researchers have seen other patients with
hippocampal damage who have memory deficits extending back through most of
their life. An alternative theory accounts
for these discrepancies by proposing that the hippocampus selectively stores
one type of memory – “episodic” – while the surrounding cortex stores another –
“semantic.” Episodic memories are usually rich in details about our past
experiences, whereas semantic memories are based on impersonal, factual
knowledge. As a memory ages, the model proposes, it is copied many times in
both the hippocampus and the cortex. All of those cortical copies generate a
new semantic memory, representing only the gist or key facts about the
experience, without all of its elaborate episodic details. Intriguingly, this
theory is also backed by patients who exhibit memory problems depending on the type of
memory, rather than its age. Without a hippocampus, such patients couldn’t
remember the experience of their childhood Florida trip – an episodic
memory – but could remember the fact that they visited Florida – a
semantic memory.
Both theories have camps of staunch advocates,
drawing support from particular amnesia cases that only their model can
explain. But as neither theory perfectly fits together all pieces of the
puzzle, the field has entered a stalemate.
Researchers from Johns Hopkins University have come
up with a new theory that just might settle some of this
controversy. Their explanation rests on the premise that memories are
transformed each time we revisit them. According to this theory, a memory is
first encoded by the coordinated activity of neurons in the hippocampus and
cortex. The hippocampus acts as the brain’s director, telling the cortex which
particular neurons to activate. Each time we recall that memory, a similar, but
not identical set of neurons are activated. Neurons that are frequently
activated become part of the permanent memory trace in the cortex, while the
rarely activated ones are lost. Every reactivation re-encodes the memory, and
depending on what cortical neurons are engaged, can strengthen, weaken or
update particular memory features.
On the surface, this new model sounds a lot like the
earlier ones. But it breaks the longstanding stalemate by proposing that what
we do with a memory, rather than its age or type, determines where it’s stored
in the brain. While the competing theories debated whether the hippocampus is
only needed for recent memories or episodic memories, the new model suggests
that what really matters is how often you revisit the memory. When a memory is
recalled often, it will more rapidly become stored in the cortex, become less
episodic and become independent of the hippocampus. But a memory that’s rarely
revisited will remain dependent on the hippocampus. Older memories might be
recalled more often, but the relationship isn’t perfect. This would explain why
one amnesic’s memory impairment extends back forty years, while another’s extends
only ten years.
The theory also nicely accounts for our subjective
sense for how our memories change over time; namely, how the hippocampus and
cortex collaborate to gradually fade or distort our memories. Say you’re
reminded of that beach vacation every summer. With each memory reactivation,
some features are reinforced while others disappear, explaining why the memory
seems to get fuzzy over time. And the more details that are lost, the less
“episodic” and the more “semantic” the memory becomes, accounting for the sense
of personal detachment often associated with very old memories. Each time you
think back to your Florida vacation, you re-encode fewer details, making the
memory feel less vivid now than it did decades ago. Today, you might still be
able to recall your striped blue bathing suit even though the smell of the
ocean air is lost.
Each mental trip back to Florida is not only an
opportunity to strengthen or weaken the memory, but also to incorporate
fictional tidbits. You used to be sure that vacation was in Fort Lauderdale,
but your sister always talks about the fun family trip to Miami. Every time you
reminisce together, the memory of Fort Lauderdale is reactivated, but so is a
competing representation of Miami. Next time you think of the vacation, the
Fort Lauderdale and Miami representations conflict, causing uncertainty over
where you actually went. Recall the beach in Miami enough times and voilĂ , a
false memory is born!
Memories fade and transform as they age. This
intriguing new theory suggests that these changes have to do less with the age
or content of a memory, and more with what we do with that memory.
Changing the past just might be easier than we thought. Chances are, you do it
every time you remember.
ABOUT THE AUTHOR(S)
Emilie Reas is a Neuroscience doctoral student in
the Brewer lab at
the University of California, San Diego. She uses fMRI to study how we create
and recall memories. You can follow her on Twitter at @etreas.
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