Fast Plasticity

Among the many wonders of neuroscience -- and central to the discipline -- is the brain's plasticity, its ability to rework synapses and networks to respond to new challenges and experiences. In this dynamic lies the physical explanation of the fluid nature of experience, thought, and consciousness. This is why I find so fascinating the work of those who proposed and discovered the mechanisms underlying this plasticity, such as Ramon y Cajal, Donald Hebb, and Eric Kandel. These and other researchers showed the fundamentals of how changing synapses allow our brains to learn new lessons and behaviors or change old ones.

Some of the most fascinating current research is adding detail to the basic picture of plasticity painted by Kandel and others. The latest to catch my eye is excting indeed, as the research team at the Brain Mind Institute of the Ecole Polytechnique Federale de Lausanne, led by Pierre Magistrietti, has shown the brain can accomplish significant rewiring in just hours -- "suggesting," as the press release puts it, "that the brain is evolving considerably even during the course of a single day."

Markran and Le Bé came to these findings by studying neuron clusters from the neocortex of neonatal rats. As the EPFL release put it,

Markram and Le Bé found that instead of growing preferentially towards specific receivers, neurons actually have no particular affinity for any other neuron, but instead remain in a state of perpetual readiness to reconfigure circuits. They found that over the course of just a few hours, connections are formed and re-formed many times.

"The circuitry of the brain is like a social network where neurons are like people, directly linked to only a few other people," explains Markram. "This finding indicates that the brain is constantly switching alliances and linking with new circles of "friends" to better process information."

In their samples, the rewiring process was occurring continuously at a slow pace. By exciting the sample with glutamate, they found that the rate increased markedly. This suggests that with a strong new experience, the brain accelerates its reconfiguration process, allowing new connections to be made, tested, and strengthened, and weaker ones removed so that the brain is quickly better adapted to the new situation.

This continual rewiring of the microcircuitry of the brain is like a Darwinian evolutionary process," notes Markram, "where a new experience triggers a burst of new connections between neurons, and only the fittest connections survive."

Funny he should say that. As soon as I started to read of this demonstration of fast rewiring, I couldn't help but think of how Peter and Rosemary Grant, closely studying beak changes in Galapagos finches, showed vividly that the mechanisms of natural selection actually worked much more quickly than anyone had realized. This story is told wonderfully in Jonathan Weiner's Beak of the Finch, and Harvard geneticist Arhat Abzhanov, working with the Grants, is now mapping [Nature, subscription required] some of the genes responsible for the crucial variations in beak size.

Tracing selection's dynamics down to the genetic level seems a wonderful proof of Darwin's theory -- and fitting, coming on the finches Darwin (or at least Darwin's popularizers) made famous. Likewise, the work Markran, Le Bé and others are doing to particularize and refine the dynamics of neural rewiring confirm and solidify the larger concept of the brain's plasticity and the importance of shifting networks. This isn't necessarily earth-shaking science. But it falls so elegantly and satisfyingly into a line of inquiry that its confirmation and implication seem correspondingly enlarged.

(The EPFL release, by the way, says the paper is supposed to be published on the PNAS Early Edition site, but I couldn't find it; I'll take it kindly if someone can send me a working link to it so I can share it here.)