X-Message-Number: 12921
Date: Thu, 09 Dec 1999 19:52:55 -0500
From: Jan Coetzee <>
Subject: "a major memory breakthrough."

Scientists Map Brain's Primary Memory Network / Alzheimer's Breakthrough

Thursday, December 09, 1999

     For the first time, a team of Wake Forest University investigators
has
mapped the functional organization of the hippocampus, the brain's
primary
memory network, a step that other scientists are calling "a major
breakthrough."  The researchers - Sam A, Deadwyler, Ph.D., Robert E.
Hampson, Ph.D. and John D. Simeral - report in today's (Dec. 9) Nature
that
they have mapped the way that a part of the brain, the dorsal
hippocampus,
encodes information when rats perform a short-term memory task.
     The researchers, members of the Department of Physiology and
Pharmacology, mapped the actions with an array of 10-16 microelectrodes.

The
electrodes are small enough to record the electrical impulses of
individual
brain neurons during the animals' performance. Recordings from the
electrodes demonstrate that different portions or segments of the
hippocampus are active at different times during the task depending on
the
type of memory function required.
     In the "News and Views" section of the same issue of Nature, Howard

Eichenbaum, Ph.D., of the Laboratory of Cognitive Neurobiology at Boston

University termed the breakthrough in understanding memory processes
highly
significant, adding that the study revealed "a functional organization
for
the hippocampus, one of the highest cortical processing areas in the
brain."
     The rats are tested in an experimental chamber with two bars or
levers
positioned on a single wall as left or right. At the start, only one
lever
is presented. It is pressed by the animal, then retracted, followed by a

delay period in which the rat must engage in other unrelated activity.
The
delay period can be as short as one second or as long as forty seconds
--
the rat never knows. At the end of the delay, both levers appear, and
the
animal is supposed to press he lever it did not press at the outset of
the
trial. If it does, it is rewarded. If the wrong lever is pressed the
chamber
goes dark for five seconds and a new trial begins.
     "The uniqueness of this situation is that the animal's task is to
remember one piece of information in one phase of the task, that is,
which
lever it pressed before the delay, and then retain and use that
information
to make a decision about which lever to press when both levers are
available
after the delay is over," said Deadwyler, professor and vice chair of
the
department.
     The task is easy if the delay is short -- the animal will get the
answer correct most of the time. However, as the delay becomes longer it
is
more likely that animals will not remember which lever it pressed at the

start and chose the wrong lever at the end of the delay. Deadwyler noted

that is similar to the rapid decrease in retention of a new telephone
number
after it is dialed.
     Animals played the game between 100 and 150 times each day and
generated very stable performance profiles.
     While the animals were performing, the researchers were recording
which
neurons in the hippocampus were active and found different patterns,
depending on which lever the animal was supposed to choose at end of the

trial. "There's a distinct separation up and down the hippocampus with
respect to which groups of cells fire during the different phases of the

memory task," Deadwyler said.
     "The findings extend this knowledge of hippocampal encoding to an
anatomic framework of overlapping 'memory networks' in which location
within
hippocampus determines which cells are activated under which short-term
memory demands," the team reported.
     In his commentary, Eichenbaum points out that the anatomic
framework
described by Deadwyler and colleagues follows known functional anatomy
present in other brain areas that are not specialized to encode
memories,
indicating that "the coding properties of hippocampal neurons 'respect'
the
anatomical circuitry in which they reside."
     Deadwyler said the work parallels ongoing studies in people in
which
scientists are trying to determine how subjects encode different types
of
information and how retrieval of that information occurs in different
brain
regions. "There might be a similar anatomic encoding scheme in the human

hippocampus for categorizing and partitioning information used in short
term
memory as we have seen in the rat."
     Hampson is associate professor and Simeral is a third-year graduate

student.
* * *

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