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.” |