New research suggests that the
subiculum, a part of the hippocampus that is rarely studied, is central to the
memory retrieval process. This work also indicates that memory formation and
memory recall follow different neural circuits. This work represents the first
identification of this recall circuit in vertebrates, although scientists did
identify a similar mechanism in Caenorhabditis elegans, a roundworm. Scientists
used laboratory mice that they had genetically engineered and conditioned to
react to changes in light so that a reaction to green light would be tied to
the subiculum neurons, which could then be inhibited. They then manipulated
memory formation and recall using fear-conditioning events so the mice would
make associations that were more or less pleasant in specific situations. Existing
research shows that this kind of memory-encoding process stimulates CA1, a
region of the hippocampus. CA1 interacts with the communication center common
to the hippocampus and the entorhinal cortex.
Although scientists previously
believed that the same circuits which formed engrams recalled them, this team
found that this isn’t the case. Instead, recall uses a pathway that deviates
away from CA1, cutting through the subiculum to reach the entorhinal cortex
instead. The team also found that the fear conditioning process which affected
the subiculum inhibited only the ability to recall, but not the ability to form
memories in the first place. The scientists hypothesize that it might be
important to have these pathways distinct in order to enable us to update
memories with fresher information and revise what we remember. They also
believe that the presence of stress hormones, which are released during the
recall of upsetting memories, might make two different pathways advantageous.
Either way, the researchers point out the importance of the work in the
understanding of the mechanisms of memory and speculate that, in the future,
these results might be relevant to the study of Alzheimer’s disease.
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