Sharon Begley
|
Jan 28, 2009 11:01 PM
Ever since neuroscientists discovered a decade ago
that middle-aged and even old brains keep producing new neurons, they
have puzzled over a fundamental question: are these new recruits good
for anything, and if so, what?“Intuitively we feel that those new brain cells have to be good for something, but nobody really knows what it is,” said James (Brad) Aimone, a graduate student at the University of California, San Diego.
He and Fred
Gage, who led the paradigm-changing discovery of adult neurogenesis
(the paradigm being that we’re born with all the neurons we’ll ever
have and it’s all downhill from there), have an intriguing suggestion.
As they and Janet Wiles of the University of Queensland describe in a new paper in the journal Neuron,
neurogenesis at the entryway to the hippocampus, a region of the brain
that encodes memories, might help memory in several ways. New neurons
of the same age might somehow tag incoming
experiences—memories-to-be—that arrive at the same time in such a way
that the memories remain forever linked, a process they call pattern
integration. The newborn neurons might also tag new hippocampal
memories with something like a date stamp, so you know what happened
before and after other memories.
“By labeling contemporary events as similar, new neurons allow us to recall events from a certain period,” speculates Gage.
The reason this is speculative is that the suggestions arise from a computational
model that Gage, a neurobiologist at the Salk Institute for Biological
Studies, and his colleagues used to simulate circuitry in the
hippocampus and the region that serves as its entryway, called the
dentate gyrus. The scientists still need to study living brains.
But the
possibilities are intriguing. Scientists know that newborn neurons make
connections to mature brain cells and insinuate themselves into brain
circuitry. Then a new crop of neurons is born, eventually joining the
existing circuits, too. That sequence means that information reaching
the dentate gyrus passes through new neurons of a particular
generation, or class. As a result, information about events that
occurred around the same time can be tied together by the common
experience of passing through that generation of newborn neurons. So if
you think back to a vacation, thinking of the hotel you stayed at will
retrieve memories of the restaurants you visited and the sightseeing
you did.
“Current
thinking holds that when we bring up a certain memory, it passes back
to the dentate gyrus, which pulls all related bits of information from
their offsite storage,” says Gage. “Our hypothesis suggests that cells
that were easily excitable bystanders when the memory was formed are
engaged as well, providing a hyperlink between all events that happened
during their hyperactive youth.”
If
neurogenesis is indeed good for memory, the good news is that it's easy
to promote. Simple aerobic exercise (mall walking), learning, and
environmental enrichment increase the production and survival of new
neurons. Chronic stress impairs it.
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