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).