New Neurons
The last form of plasticity we’ll
look at has been controversial because a long-held doctrine in neuroscience was
that, at birth, the brain has all the neurons it will ever have. As a result,
plasticity during the organism’s lifetime must be due to changes in these
neurons. However, neuroscientists have been expressing reservations about this
doctrine for years (e.g., Ramón y Cajal, 1913), and it turns out that those
reservations were justified. There is now clear evidence that new neurons
continue to develop throughout an organism’s lifetime and that this growth is
promoted by learning and enriched experience (Eriksson et al., 1998).
The evidence suggests, however,
that neurogenesis—the birth of new
neurons—is very slow in the adult human brain; and it seems that most of these
new neurons don’t survive for long (Scharfman & Hen, 2007; Shors, 2009).
It’s also unclear whether neu-rogenesis occurs in all parts of the adult
brain—and, in particular, whether it occurs in the cerebral cortex (Bhardwaj et
al. 2006). If it doesn’t, this may be a regard in which humans are different
from many other species.
In some ways, these results seem
backwards. One would think that the creation of new neurons would allow
flexibility for the organism and so would contribute to learning— and therefore
would be most prominent in species (including humans!) that are capable of
especially sophisticated learning. Yet it seems that we may be the species for
which cor-tical neurogenesis is least
likely. What explains this pattern? One hypothesis is that human intellectual
capacities depend on our being able to accumulate
knowledge, building on things we have already learned. This in turn may require
some degree of biological stabil-ity in the brain, so that we do not lose the
skills and knowledge we’ve already acquired. For this purpose, we may need a
permanent population of cortical neurons—and this means not introducing new neurons.
From this perspective, the absence of neuronal growthmight limit our
flexibility; but it might nonetheless be a good thing, helping to sustain
long-term retention of complex knowledge (Rakic, 2006; Scharfman & Hen,
2007).
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