Thursday, April 15, 2010

(A little note about my extended absence: After a promising start, I let this blog go by the wayside just about a year ago, shortly before I had my first baby. I find being a parent is absolutely fantastic and also supremely absorbing and time-intensive. My daughter is now ten months old and I'm just starting to recover my interest in the world around me. It seems like a good time to revive the blog.)

Now that I spend three days a week at home with my daughter, I'm listening to a lot of talk radio to get my grownup time.

Yesterday morning while supervising the gleeful mashing of banana into hair, I heard a fascinating NPR interview with Barbara Strauch.

She's the science editor for the New York Times who recently wrote a book called "The Secret Life Of The Grownup Brain."

I've yet to read it (though it's now on my list!), but based on the interview Ms Strauch's book is an exploration, not of the decline and fall of the aging brain, but the changes - some for ill but some for good - that come to cognition with middle age. The young brain may be better at rapid calculation and short-term memory, but the aging one seems to be better at recognizing where information fits into existing patterns.

"We are better at getting the gist of arguments," she says. "We are better at recognizing categories. And we're much better at sizing up situations. We're better at things like making financial decisions, which reaches a peak in our 60s. Social expertise -- in other words, judging whether someone's a crook or not a crook, improves and peaks in middle age."

Now this doesn't quite fit with the common perception that the brain just gets mushier and mushier with age. Interviewer Terry Gross was posing questions from the point of view that the brain goes into a smartness free-fall after somewhere around age 25. As evidence, she pointed out that synapses - the electrical connections between nerve cells - decrease in number over time. She assumed this meant a parallel decline in cognitive capacity, as if synapse number were equivalent to 'smartness dose.' More connections mean more braininess, right?

Well, it may be a little more complicated than this.

One might think that you'd gain synapses as you learn things; but actually, the brain has the most synapses at a time when it knows the least. Synapses are overproduced in childhood and then reduced as the brain matures. As you grow and learn, some existing connections are strengthened and new synapses are formed - but more are weakened and pruned away.

J Comp Neurol. 1997 Oct 20;387(2):167-78.
Regional differences in synaptogenesis in human cerebral cortex.
Huttenlocher PR, Dabholkar AS.

This pruning isn't simply the death of brain tissue and reduction in cognitive capacities. It is more like specialization: as a sculptor carves away excess marble to reveal his intended form, so the brain prunes away excess connections to reveal a latticework of meaning. We are still pretty far from understanding exactly how this works, but here is a really nice paper from a few years ago that proposes a model of how changes in the strengths of nerve cell connections might underlie learning:

Fusi, S., Drew, P. and Abbott, L.F. (2005) Cascade Models of Synaptically Stored Memories. Neuron 45:599-611.

So Strauch's thesis, that the middle-aged brain is better at putting things into context, parallels the common understanding that as we gain life experience, we are more able to assimilate new information in terms of the many things we already know. It's called wisdom, and older people in traditional cultures around the world are revered for it. Funny that we should require all the power of modern neuroscience to remind us of this!