WHERE UNCONSCIOUS MEMORIES FORM
A small area deep in
the brain called the perirhinal cortex is critical for forming unconscious
conceptual memories, researchers at the UC Davis Center for Mind and Brain have
found.
The perirhinal
cortex was thought to be involved, like the neighboring hippocampus, in
"declarative" or conscious memories, but the new results show that
the picture is more complex, said lead author Wei-chun Wang, a graduate student
at UC Davis.
The results were
published Dec. 9 in the journal Neuron.
We're all familiar
with memories that rise from the unconscious mind. Imagine looking at a beach
scene, said Wang. A little later, someone mentions surfing, and the beach scene
pops back into your head.
Declarative
memories, in contrast, are those where we recall being on that beach and
watching that surf competition: "I remember being there."
Damage to a
structure called the hippocampus affects such declarative "I
remember" memories, but not conceptual memories, Wang said.
Neuroscientists had previously thought the same was true for the perirhinal
cortex, which is located immediately next to the hippocampus.
Wang and colleagues
carried out memory tests on people diagnosed with amnesia, who had known damage
to the perirhinal cortex or other brain areas. They also carried out functional
magnetic resonance imaging (fMRI) scans of healthy volunteers while they
performed memory tests.
In a typical test,
they gave the subjects a long list of words, such as chair, table or spoon, and
asked them to think about how pleasant they were.
Later, they asked
the subjects to think up words in different categories, such as
"furniture."
Amnesiacs with
damage to the perirhinal cortex performed poorly on the tests, while the same
brain area lit up in fMRI scans of the healthy control subjects.
The study helps us
understand how memories are assembled in the brain and how different types of
brain damage might impair memory, Wang said. For example, Alzheimer's disease
often attacks the hippocampus and perirhinal cortex before other brain areas.
Co-authors on the
study are Andy Yonelinas, professor of psychology and at the Center for Mind
and Brain; Charan Ranganath, professor at the Center for Neuroscience; former
UC Davis graduate student Michele Lazzara, now project coordinator at the
University of Illinois at Chicago; and Robert Knight, professor of psychology
at UC Berkeley.
The work was funded
by the National Institutes of Health.
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