Over time, some mice became more adventurous and curious and others less so. When they euthanized the mice, those that were the “wanderers” at the end of the study were also those who experienced the greatest proliferation of adult-born neurons (neurogenesis).
At least some parts of our character (sense of self, or identity) are not accounted for by genetics.
A fascinating new neuroscience experiment probes an ancient philosophical question—and hints that you might want to get out more
May 28, 2013 | By Jason Castro
Imagine we rewound the tape of your life. Your diplomas are pulled off of walls, unframed, and returned. Your children grow smaller, and then vanish. Soon, you too become smaller. Your adult teeth retract, your baby teeth return, and your traits and foibles start to slip away. Once language goes, you are not so much you as potential you. We keep rewinding still, until we’re halving and halving a colony of cells, finally arriving at that amazing singularity: the cell that will become you.
The question, of course, is what happens when we press “play” again. Are your talents, traits, and insecurities so deeply embedded in your genes that they’re basically inevitable? Or could things go rather differently with just a few tiny nudges? In other words, how much of your fate do you allot to your genes, versus your surroundings, versus chance? This is navel gazing that matters.
In the absence of a time rewinder, the next best experiment is to do what Julia Freund and her colleagues did in a simple, yet remarkable recent study. These investigators placed genetically identical individuals (mice in this case) in a common environment, and asked whether systematic behavioral differences could still develop between them. An answer of “Yes” would mean that there are sources of behavioral variability – “individuality,” if you will – that aren’t accounted for by the combination of genes and common environment.
In their experiment, Freund and her colleagues housed 40 genetically identical mice in a so-called “enriched” environment, and monitored their behavior over a period of three months (about 10 to 15 percent of their lifespan) during their early life. The enriched environment was very generous as far as lab-mouse accommodations go, with an approximately 36 square foot footprint, and a multi-tiered arrangement of platforms, nesting boxes, and interconnecting tubes. In these conditions, mice can exhibit a more natural set of exploratory behaviors than in the more typical confining cage.
What made this study different from, say, a study of human twins is that the subjects’ movements could be tracked in extraordinary detail over a significant portion of their lifespan. Each mouse in the study was tagged with a radiofrequency ID (RFID) transponder, whose location was monitored by one of twenty antennas inconspicuously arranged among water bottles, tubes, and nesting boxes. Every movement, chase, and sedentary spell was recorded and logged.
To study potential differences in behavior among the mice, the experimenters used a measure called “roaming entropy.” Basically, this captures how often you get out, and with how much variety. If you’re someone who mostly just darts between work and home, your roaming entropy is low. If you’re the kind of person who could conceivably be just about anywhere at any given time, your roaming entropy is high.
Initially, the mice were fairly uniform in their roaming entropy. As the weeks progressed, however, the population started to diverge, with some mice being markedly more exploratory than others. If we take the tendency to explore as a kind of crude trait, then this is one trait that elaborates over time, in a way that isn’t strictly determined by genes or available resources.
The most interesting part of the study, however, came when the researchers examined the brain changes that paralleled the changes in exploratory behavior. Before ending the experiment, the mice were injected with a compound that’s selectively incorporated into dividing cells, and hence labels adult-born neurons. While most neurons are fashioned during early development, there are a handful of well-studied brain areas in which new neurons are continuously produced even in adulthood.
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.
ABOUT THE AUTHOR(S)
Jason Castro is an assistant professor of psychology and neuroscience at Bates College.