Scientists' Experiments Confirm Theory Central to Memory Research
Sept. 20, 2002
Scientists have experimentally supported a central tenet of a theory
regarding how new memories are converted from a short-term, unstable form,
into stable, long-term memory. Experimental evidence at the University of
Arizona in Tucson shows that during quiet time or sleep, some brain cells
replay recent events, possibly to consolidate memory collected from various
sources.
For some time, neuroscientists have believed that new, or "short-term
memories" consist of separate parts, which are stored in different regions
of the cortex, depending on the type of information involved, and that
consolidation into "long-term memory" involves formation of direct
associative links among these different parts.
According to this theory, these individual parts of the memory are
initially linked, indirectly, by virtue of the common connections they share
between each cortical region to a higher region called the hippocampus. As
the memory is replayed during rest, scientists believe that direct links
form among the various parts, thus making the memory independent of the
links with the hippocampus, which then become reused for linking new
memories.
This theory tries to explain why old memories are resistant to disruption
by damage to the hippocampus, such as might occur as a result of brain
injury or degenerative diseases like Alzheimer's disease, as opposed to new
memories which simply tend to be lost.
Kari L. Hoffman and Bruce L. McNaughton report ("Science," Sept. 20) new
evidence supporting this theory. Hoffman and McNaughton found that when one
part of a recent memory is replayed in its cortical location, the other
parts are, in fact, concurrently replayed in their cortical locations. This
"concurrent reactivation" of brain cells is thought to be essential if the
correct pieces of memory are to be linked together into a coherent whole.
How the UA researchers studied this phenomenon is as interesting as the
subject matter itself.
Memory is a shorthand term for the process the brain uses to encode,
store and retrieve information. Most organisms, including humans, depend on
remembering learned behavior and past events, and the breakdown of any of
these processes reflects memory failure.
Humans are especially able to recall memories, even from the distant
past, often in vivid detail, suggesting that we have a very high-capacity
storage system. But recent memories are susceptible to disruption,
especially in the first minutes to days after an event. The researchers say
this period of instability "may be a consequence of the way memory traces
are stored throughout the cortex." How these fragile new memories become
stabile, long-term memories has long been a puzzle.
Researchers have theorized that brain cells in the hippocampus that were
active while an event occurred will reactivate, sometimes repeatedly, as a
way of assembling information for long-term memory storage. This activity
may elicit related activity from lower level brain cells in the cortex that
also were active at the time of the event.
"Through repeated coactivation, these lower-level ensembles may create
the connections necessary to encode the memory trace efficiently and to
sustain it, or some approximation of it, independently of top-down input,"
says McNaughton, a professor of psychology at the UA.
McNaughton says two critical predictions follow the theory. One is that
"Patterns of neural ensemble activity expressed during an experience should
be spontaneously reactivated during subsequent periods of behavioral
inactivity." The second holds that "The distributed components of the
reactivated memory trace should appear concurrently within the relevant
cortical sites."
Using a sophisticated array of micro electrodes to monitor brain activity
during the administration of various tasks, Hoffman, a graduate student at
the UA, and McNaughton were partly able to address the question. They
theorized that if reactivation occurs, "cells that were active together
during the task should tend to be coactive afterward, and cells active at
different times during the task should not be coactive afterward."
The results of their study demonstrated that memory trace reactivation
occurs in a "coherent, distributed manner across much of the neocortex." The
results, they say, indicate that "the observed effect is not simply an
uninterrupted persistence of a previously expressed activity state, but
rather reflects the reemergence of recent patterns."
Also significant is the technology used to study this and other ongoing
memory research at the University of Arizona. McNaughton has been conducting
this and related research for several years, using microelectrode arrays and
computers he and his colleagues have developed specifically for this kind of
inquiry.
The experimental results reported in "Science" was made possible by an
array of 144 independently advanceable microelectrodes that were able to
measure input from four regions of the cortex. Each array consisted of a 12
x 12 lattice of electrodes spaced about 6 and a half millimeters apart. A
total of 800 cells were recorded from over nine recording sessions,
producing a total of more than 21,000 cell pairs for study. The ability
McNaughton and his collaborators have to measure such large collections of
individual brain cell activity is unmatched in any other laboratory, and is
a necessary prerequisite to performing the sort of analysis they undertook.
While Hoffman and McNaughton were able to demonstrate that memory trace
reactivation is temporally ordered and concurrent across large areas of the
neocortex, what remains to be shown are the actual mechanisms leading to
this phenomenon, and that this coherent memory trace reactivation is
actually involved in memory consolidation. Their findings, however, are an
important step in the process of confirming the trace-reactivation theory of
memory.
University of Arizona in Tucson
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