Last week, four-year-old Cleo and I encountered three large mutts outside our neighborhood café. As I stood sipping my latte and chatting with their person, Cleo respectfully held out her hand to one dog for the sniffing and asked him if he would like to be petted. This formality dispensed with, she stroked his fur and then tilted her cheek invitingly for him to kiss. When the dog obliged, she let off a peal of such deeply gratifying giggles, everyone on the surrounding sidewalk turned and laughed with her.
As we walked on down the avenue, she said, "Mommy? I think I'd rather be a dog."
"Really?" I asked. "What do you think you'd like about that?"
"I'd like to know how it feels to chew up a bone."
"Oh, yeah," I said, nodding. "Anything else?"
"Yes -- then I could have my own water bowl."
What a thought: I think I'd rather be a dog. I'd like to learn lithography. I'd like to see Alaska. But be a dog? Cleo still sees birth into another species as a viable path in life. Her openness to every fold of existence, the way she gives each possibility equal weight, makes me giddy.
Neurobiologically, Cleo is still in the Playdoh stage of life. Her body produced tens of billions of brain cells while she was in utero, and ever since they came to be, these neurons have been learning to communicate with each other, reaching out long, skinny extensions called axons and forming points of contact with other neurons called synapses.
Even as Cleo thumped around inside my belly, giving me heartburn, a process of synaptic pruning began. Any recurring stimulus -- like the familiar rhythm of my step or the voices of her dad, Patrick, and then three-and-a-half-year-old sister, Zoë, who sang with their mouths pressed up against my tummy -- activated certain synapses in Cleo's brain repeatedly. The connections that were used often became stronger, but synapses that lay dormant atrophied and became weaker, just like muscles do when you don't use them.
This developmental sculpting process continues now; the selective strengthening of some synapses and pruning away of others gives shape to the vast, complex set of circuits that will allow Cleo's neurons to share, integrate, and store information as she grows and learns. Every experience she has changes the architecture of those connections in her brain. A tiny, seahorse-shaped structure called the hippocampus deep inside her head is just now beginning to store her conscious long-term memories. She's still in that developmental window wherein kids can learn any language from Japanese to Urdu with equal agility.
And even four years from now, when Cleo is Zoë's age and has passed out of that most flexible, trainable stage of language development, she'll still have a second wave of brain growth in store for her just before puberty, followed by another burst of pruning that will last well into her twenties.
I feel so static by comparison, so far from that time of possibility. The path keeps narrowing. I can just hear the doors quietly clicking closed as I begin down the gentle slope of my forties. I'll never be a surgeon or a track star; I probably won't ever bobsled or blow glass. My habits -- shlumpy clothes, too much coffee -- are so entrenched.
Scientists suggest that to lessen the chances of getting Alzheimer's disease, one should remain mentally active. I jumped off the career fast track eight years ago, when I had Zoë, and then off the slow track as well when Cleo came. Is my mind active enough now, or am I slouching toward the same demise as Ma, whose dementia reduced her from a lively, laughing, opinionated woman to a fretful, forgetful, unmotivated being before she died? I can't help but imagine a similar fate for me. I try not to get depressed about it, though -- ironically, it turns out that depression may cause a shortage of the very chemicals needed for neural plasticity and cell survival in the brain.
Even if Alzheimer's doesn't get me, and even if I manage to stay clinically un-depressed, am I simply stuck here, all grown up, unchanging?
It turns out that the answer is no. Once, scientists thought that you got a set of neurons at birth, they hooked themselves up properly according to hardwired genetic instructions, and then they just died off from that point on. Now we know that adult brains can grow, too.
It's true that we're each born with a set of neurons, and many of them die off during our lifetime. It's true too that there are neurobiological windows of opportunity -- most famously, the language stage that Cleo is in. (At the rate she's going, Cleo has a good chance of learning Dog.) But it seems our brains are much more malleable than we once thought. Not only do synapses form and continue to be tuned in response to experience throughout adulthood, but counter to last century's dogma, it turns out that brand new nerve cells are actually born into the brains of middle-aged grown ups.
The neurons born into our grown-up brains are few, and so far, they have only been observed in certain areas of the brain -- but I see this as a good thing. If brain cells propagated the way epithelial cells do, I guess we'd have to continually shed our memories and personalities the way we shed skin. Still, the birth of these fresh neurons and the continual reshaping of synapses raise the possibility of some renewal even later in life.
Here is how I'm beginning to envision it. Past a certain age, my brain began to develop more slowly, but that's okay. Now is the time to make the delicate changes, the incremental, perfecting changes inside my head. These can still be important and even dramatic in the way they affect my life, but they are changes in exquisite detail rather than in gross outline. I imagine, for example, that as I struggle in my relationships with Patrick and the girls, tiny changes in the shape and strength of certain synapses in my brain produce a minute mental shift, and I become a sliver more accepting of our differences. As I reach again and again for the right word, my mind slowly reshapes itself to accommodate the practice of writing. And if I walk into the garden each morning and whisper "Thank you," maybe I am, in effect, clearing a path through my wet thicket of brain cells to a place of gratitude.
Of course, as I get older, my mental processes, like my physical ones, will eventually slow down. But it has been shown that if you simply give healthy older people more time, they remember as well as younger people do. Apparently, many elders need a slower world, not a simpler one. When I'm not in the car behind the elders, I rather like this idea of clear-minded deceleration.
So let Cleo and Zoë ramp up into the frenzy of their young adult life. Let them experience the massive formation, pruning, and reshuffling of their billions of synapses. With me, change is a subtle thing, a fine-tuning, a smoothing and tending affair. The adage that you can't teach an old dog new tricks is cute, but it may not be true after all. Perhaps the new tricks of us old dogs are just a bit more refined.