Brain Rules (Updated and Expanded)

Researcher:
Can you remember any particular event that was special
—like a holiday, Christmas, birthday, Easter?
H.M.:
There I have an argument with myself about Christmastime.
Researcher:
What about Christmas?
H.M.:
Well, ’cause my daddy was from the South, and he didn’t
celebrate down there like they do up here—in the North. Like they
don’t have the trees or anything like that. And uh, but he came
North even though he was born down Louisiana. And I know the
name of the town he was born in.
H.M.’s childhood memory is intact starting about 11 years before his
surgery. How is that possible? If the hippocampus is involved in all memory
formation, removing the hippocampus should wipe the memory clean. But
it doesn’t. The hippocampus is relevant to memory formation for about 11

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years after an event is recruited for long-term storage. After that, the
memory somehow makes it to another region, one not affected by H.M.’s
brain losses. Here’s the interaction between the cortex and hippocampus
that allows us to form long-term memories, and the reason H.M. still
remembers Christmas:
1) The cortex receives sensory information and sends it to the
hippocampus. They chat about it—a lot. Long after the initial stimulus has
faded away, the hippocampus and the relevant cortical neurons are still
yapping. As you sleep, the hippocampus is busy feeding signals back to the
cortex, replaying a memory over and over again. The importance of sleep to
learning is described in the Sleep chapter.
2) While the hippocampus and cortex are actively engaged, any memory
they mediate is labile and subject to amendment.
3) After a period of time, the hippocampus will let go, effectively
terminating the relationship with the cortex. The cortex is left holding the
memory of the event. The hippocampus files for cellular separation only if
the memory has become durable and fixed (consolidated) in the cortex. This
process is at the heart of system consolidation, and it involves a complex
reorganization of the brain regions supporting a particular memory trace.
How long does it take before the hippocampus lets go of its relationship
with the cortex? In other words, how long does it take for a piece of
information, once recruited for long-term storage, to become completely
stable? Hours? Days? Months? The answer surprises nearly everybody who
hears it for the first time. It can take 
years
.
That’s what the case of H.M., and patients like him, tell us. System
consolidation, the process of transforming a short-term memory into a long-
term one, can take years to complete. During that time, the memory is not
stable.
As with short-term memories, long-term memories are stored in the
same places that initially processed the stimulus. Retrieving a long-term
memory 10 years later may simply be an attempt to reconstruct the initial
moments of learning, when the memory was only a few milliseconds old!
Forgetting

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We’ve talked about encoding, storage, and retrieval, the first three steps of
declarative memory. The last step is forgetting. Forgetting plays a vital role
in our ability to function for a deceptively simple reason. Forgetting allows
us to prioritize. Anything irrelevant to our survival will take up wasteful
cognitive space if we assign it the same priority as events critical to our
survival. So we don’t. At least, most of us don’t.
Solomon Shereshevskii, a Russian journalist born in 1886, seemed to
have a virtually unlimited memory. Scientists would give him a list of
things to memorize, usually combinations of numbers and letters, and then
test his recall. Shereshevskii needed only three or four seconds to
“visualize” (his words) each item. Then he could repeat the lists back
perfectly, forward or backward—even lists with more than 70 elements. In
one experiment, developmental psychologist Alexander Luria exposed
Shereshevskii to a complex formula of 30 letters and numbers. After a
single recall test, which Shereshevskii accomplished flawlessly, the
researcher put the list in a safe-deposit box