Very young brains process memories of fear differently
than more mature ones, new research indicates. The work
significantly advances scientific understanding of when
and how fear is stored and unlearned, and introduces new
thinking on the implications of fear experience early
in life.
"This important paper raises questions that are the
'tip of the iceberg' related to the very complex series
of events that occur as we learn to fear something. In
the real world, we become fearful, extinguish that fear,
reacquire it at another time, and then conquer it yet
again," says John Krystal, MD, of Yale University
and director of the clinical neuroscience division of
the VA National Center for Post-Traumatic Stress Disorder.
"Typically, we think about long-term, negative impact
of fear learning, such as lifelong problems with anxiety.
But this work highlights an avenue for adapting to early
stresses that apparently can occur only early in life:
to erase a learned fear from memory." Krystal was
not affiliated with the research.
Study co-authors Jee Hyun Kim and Rick Richardson, PhD,
of the University of New South Wales in Sydney, homed
in on the amygdala, using anesthesia to temporarily inactivate
it and therefore isolate its role. The amygdala is critical
for emotional learning and plays a central role in dulling
the memory of a fear.
Kim and Richardson trained rats that were 16 and 23 days
old--the human equivalent of children and budding adolescents--to
associate a specific sound with a mild shock to the foot.
After subsequent training, when the sound was not followed
by a shock, the animals' fearful reaction to hearing the
sound faded. Technically, this is known as "extinction,"
and depended on the function of the amygdala.
In a second round of training, the researchers reintroduced
the fear and tried to re-extinguish it. This time around,
they found, only the older rats were able to do so without
the amygdala.
The researchers concluded that the age at which the initial
extinction training occurred was critical to whether or
not the rats' fear faded the second time independently
of the amygdala. The authors suggest that in the very
young, it is primarily the amygdala that extinguishes
fearful memories, but that mechanisms independent of the
amygdala develop later.
This raises the possibility that fears unlearned at an
early enough age are, in fact, erased. As brains develop,
however, and related structures near the amygdala mature,
these structures take on a greater role. Thus, fear in
adolescence and later in life may not be erased, but instead
be, for example, inhibited by a process of overlaying
neutral memories on top of the initial fear reaction.
The initial memory could still exist and be called on
again.
"Extinction in the young brain might forever erase
early traumatic learning--but accepting this hypothesis
will have to wait for more research," says Mark Bouton,
PhD, of the University of Vermont, who did not participate
in the esearch. "What might change as the brain develops
is where and how fear learning and extinction are stored
and how they can be retrieved."
The findings appear in the Feb. 6 issue of The Journal
of Neuroscience. The work was supported by grants from
the Australian Research Council.
