[tri-med] FYI - Imaging studies illuminate competition between brain systems

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Public release date: 28-Nov-2001
  Contact: Susan McGreevey
617-724-2764
Massachusetts General Hospital

  Imaging studies illuminate competition between brain systems

          What areas of the brain are activated during the process of
learning and how does the  pattern of activation change as learning
proceeds? Brain imaging studies conducted by researchers at
Massachusetts General Hospital (MGH) in collaboration with
scientists at Rutgers University-Newark, are revealing that brain
systems known to be involved in learning seem to compete with each
other, with the type of learning involved determining which system is
dominant.

          In a study appearing in the Nov. 29 issue of Nature, the
researchers describe how increased activity in one brain system is
associated with decreased activity in another  system during learning
of a simple skill. The findings which suggest how the brain  mediates
between the need to store and access a wide range of information and
the need for virtually automatic responses in key situations may
eventually lead to new  strategies for dealing with learning
disorders or for diagnosing Alzheimer's disease,  Parkinson's disease
and other brain disorders.

  Previous studies have identified several brain structures that are
key to learning and  memory and have suggested associations with
particular learning tasks. The medial  temporal lobe of the brain,
which includes a structure called the hippocampus, has  been
associated with what is called declarative learning, the learning of
facts and  events. An area called the basal ganglia, deep within the
brain, has been associated  with nondeclarative learning, learning
based on experience that may not involve  conscious memory. In the
current study, conducted at the Martinos Center for  Biomedical
Imaging located at the MGH, healthy volunteers were given a simple
learning task while undergoing functional MRI scans, which reveal the
level of activity  in various areas of the brain.

          "We have been studying how the brain changes when people
acquire skills, which  brain systems support particular kinds of
learning, and how those systems interact,"  says Russ Poldrack, PhD,
of the MGH Department of Radiology and the Martinos   Center, the
paper's first author. "The idea that these systems may compete with
each  other was suggested by animal research, and we wanted to see if
this takes place in  humans as well."

          In the first phase of the current study, volunteer
participants learned to associate  certain combinations of symbols
with particular weather patterns in one of two ways:   either they
were presented with symbol-bearing cards, asked to indicate which
type of  weather the cards were associated with and then told the
correct answer, or they were  simply presented with the cards and
told which type of weather they signified. The first  version
basically a learn by trial-and-error test was designed to test
nondeclarative  memory, while the second version, in which
associations were memorized, was  designed to involve declarative
memory.

          Functional MRI images taken during these tasks revealed
increased basal ganglia  activity and reduced medial temporal lobe
(MTL) activity during the feedback-based,  trial-and-error version of
the test. The memorization version produced the opposite pattern,
with increased MTL activity and lower basal ganglia activity.

          A second experiment presented a different group of
volunteers with the  feedback-based version of the weather prediction
test in a way that could follow over  time any changes occuring in
brain activation. The overall pattern was similar to that  seen in
the first experiment ? decreased MTL activity and increased basal
ganglia   activity. But the time-based analysis showed that at the
very earliest stages of the  experiment the MTL was active and the
basal ganglia inactive, with the activation  pattern switching as
learning proceeded.

          Mark Gluck, PhD, of the Center for Molecular and Behavioral
Neuroscience at  Rutgers-Newark, the study's last author, developed
the weather prediction task with his Rutgers colleagues as a way to
study how people learn categorization rules. He  explains that
traditionally the MTL and especially the hippocampus had been
regarded  as being involved with declarative memory only. In
contrast, Gluck and Catherine  Myers, PhD, also a coauthor, have
developed an alternative theory: that the  hippocampus is involved in
all learning and is responsible for determining how new  information
is encoded by other brain regions. Gluck explains, "The current
results  provide the first functional neuroimaging data to support
our theory. As our models  predicted, the hippocampus was activated
in the earliest stages of learning, when we  expect new encodings to
be established, but not in later learning when the encodings  are
used by other brain structures, such as the basal ganglia." Gluck and
Myers?  theory is detailed in their recent book Gateway to Memory: An
Introduction to Neural  Network Modeling of the Hippocampus and
Learning.

          Gluck also notes that the hippocampus and related structures
are damaged early in the process of Alzheimer?s disease and that the
current findings may allow   development of learning-based tests that
could diagnose that disorder in its earliest  stages. Poldrack adds
that better understanding of how the brains memory systems  interact
could lead to a better understanding of brain plasticity, the ability
of brain  areas to take on functions usually accomplished by other
structures. "We're most  excited about the possibility of using
plasticity to solve problems like dyslexia."

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