A fascinating new neuroscience experiment probes an ancient philosophical question—and hints that you might want to get out more
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Choice, choices, choices… make us who we are.
Image: iStock/Crisma
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.
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.
ABOUT THE AUTHOR
Jason Castro is an assistant professor of psychology and neuroscience at Bates College.
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