Strikingly, the mice which were the “wanderers” at the end of the study were also those who experienced the greatest proliferation of adult-born neurons. While the usual caution of correlation not implying causation applies here, the result is still intriguing. Even after the genetic die are cast at conception, and after the bulk of the neural scaffolding is laid down in early life, the brain maintains a trickle of raw potential through its ability to grow a limited number of new neurons. The authors conjecture that these neurons are involved in tailoring and tuning our behaviors, applying context-specific corrections and adjustments to the more hard-coded aspects of our behavior. In their words, the ways in which we live our lives may make us who we are.
How, exactly does this happen? The authors concede that we don’t really know. This is not to discredit them, but simply to acknowledge that any experiment addressing something as profound, contested, and metaphysically tangled as the nature-nurture question is going to generate more questions than answers.
It could be the case, for example, that epigenetic changes, in which experience modifies patterns of gene expression, give rise to different life trajectories. Or perhaps the result is really hard-line determinism in disguise. Though nominally genetically identical, there are still minute genomic differences between inbred mice. Perhaps these are sufficient to give rise to trait differences that elaborate over time. Another question, of course, is how surprised should we be by the differences in roaming entropy that were observed? Are they comparable to what would be seen among less genetically related individuals of the same species? In other words, are we talking about the difference between type A and type B personalities, or just subtle shades of A?
Regardless of these specifics, this experiment is a potent reminder that our lives are a work in progress. If we’re indeed living out a kind of tape, then it seems to be one in which the tracks can be tweaked as they’re read, even if they’re rather deep. As your brain is shaped by the choices you make, there is room for chance and noise – room for you to be unique.
Are you a scientist who specializes in neuroscience, cognitive science, or psychology? And have you read a recent peer-reviewed paper that you would like to write about? Please send suggestions to Mind Matters editor Gareth Cook, a Pulitzer prize-winning journalist and regular contributor to NewYorker.com. He can be reached at garethideas AT gmail.com or Twitter @garethideas.