What makes me, me? The cellular correlates of individuality

Somewhere in Germany, a group of 40 genetically identical females are being constantly watched. Implanted with radio-frequency identification transponders (RFID) since 4 weeks old, they are allowed to roam free in a rich, 5-storey mansion, with 20 antennas monitoring and recording their whereabouts. 3 months later their brains will be examined for traces of emerging personalities.

While sounding sinister, that was the set up of an experiment investigating the neurobiological reflections of individuality. Why is it that maternal twins, growing up in the same environment, nevertheless develop different personalities? What processes in the brain are responsible for this effect? And the age-old question: what makes me, me?

Why do genetically identical mice have different "mouse personalities"? Source: http://vetmed.duhs.duke.edu/

Freund J et al (2013) Emergence of Individuality in Genetically Identical Mice. Science, 340 (756).

Scientists set out with the idea that the way individuals interact with the environment will cause the brain to physically change. Personalities, then, emerge as a result of these changes. This simple hypothesis is –unfortunately- very hard to prove experimentally, since brain rewiring happens at both synaptic and network levels. This makes changes very hard to track.

Another way to approach this is to look at adult neurogenesis, or the birth of new neurons. We know that interesting and new environments trigger the brain to generate new neurons, which integrate into existing networks and causing the brain to change. Could it be then, that twins develop different personalities because of differences in neurogenesis?

Creepy, but still a paradise.

Researchers tagged 40 inbred young female mice with RFIDs and followed them for 3 months as they explored their 5-storied mouse paradise. During this time, the “blank slates” increasingly shed their “sameness”: some roamed every corner of the mansion, spending most of their time feeding their wanderlust; others stayed near their home area, only occasionally venturing out of their comfort zones. Many didn’t seem to care where they stayed.

3 months later, all of the mice grew bigger brains with many new neurons and astrocytes (“supporting” cells). This isn’t surprising, since physical activity by itself stimulate neurogenesis. However, mice that regularly explored large areas had significantly more new neurons than those that stayed put. In other words, in each animal, the degree of exploration positively correlated with the amount and speed of new neurons born. Looking at all the rats as a group, the increasing difference in exploratory behavior could also be partially explained by different levels of adult neurogenesis.

What to make of all this? We know for sure that adult neurogenesis has something to do with the divergence of mouse personalities. This could mean many things. Different exploratory behaviors may be due to differences in the mice’s ability to generate new neurons to begin with. Although genetically identical, minute changes may still happen in the genome during gestation, which could account for differences in neurogenesis. Conversely, mice may have equal ability to generate new neurons, but those that explore more are cognitively challenged more, hence give birth to more new neurons as an adaptive response. Or perhaps, the mice differed in their capability for neurogenesis (amount, speed) at the beginning, which was further amplified by their different interactions with the environment. Or maybe neurogenesis isn’t even an important factor – perhaps small changes in the genome or how genes are expressed change the animal’s perchance for exploration at birth, and it’s subsequent interaction with the environment is reflected in it’s level of neurogenesis.

How to interpret the data? Just keep in mind: correlation does not imply causation. Source: xkcd.com He's awesome.

As of now, we don’t know the cause-and-effect of neurogenesis and personality. What we do know is that the brain is extremely malleable, constantly re-sculpted by our interactions with the environment, which could underlie something as essential as our personalities. Perhaps the animal model in this study can –in future studies- help us provide a scientific and neurobiological answer to the big question: what makes me, me?

Freund, J., Brandmaier, A., Lewejohann, L., Kirste, I., Kritzler, M., Kruger, A., Sachser, N., Lindenberger, U., & Kempermann, G. (2013). Emergence of Individuality in Genetically Identical Mice Science, 340 (6133), 756-759 DOI: 10.1126/science.1235294