O.M.G.!! The toes and fingers are so tiny! The face is so round and appealing. The eyes are so, umm..., unfocused. And that little newborn brain. If only we knew what this tiny new creature was thinking. Hmmm. We don't know what a newborn thinks, but we do know that its brain is pretty undeveloped.
And by undeveloped I mean that it has just begun to set up shop. The baby arrives with some basic shelving-the parts of the brain (cerebrum, cerebellum, etc) are fleshed out and there are many nerve cells (neurons). But after birth and for the next five years the human brain more than triples in size, mostly because the cells already present at birth become more complex and interlinked.

In the beginning, neurons are not very networked. By analogy, think about when you first started a Facebook page. You asked a few friends and family members to friend you. Then Facebook started offering up people you might know and you chose to friend some of them. Others started to find you-former school chums or work colleagues, etc. Before long you were networked in an ever-growing web of connections.
That's sort of what happens to the brain after birth. Neurons use specialized structures called synapses to communicate with one another. The more synapses in a given space, the more information can pass back and forth. Neonates start life with a low synapse density. By 3 months this density doubles, and by two years it has more than tripled! At the same time, each neuron transforms from a cell with a simple branching pattern to a multi-pronged bush, the fine branches from one bushy cell connecting to other cells via those aforementioned synapses. By adulthood there is an exuberance of branches-as much as 800 times more than at birth, permitting ever more complex behaviors.
Now here's the kicker. All this branching and synapsing is not random. The infant pops out of the birth canal into a new world of sensory assault. He or she experiences light, sound, touch and smell at levels far more intense than in the sheltered womb. The poorly interconnected neurons residing in that tiny brain respond to this heightened environmental stimulation. How this heightened stimulation shapes the baby's brain is the topic of Part 2 of my Nature VERSUS Nurture series (see Part 1).
And speaking of heightened environmental stimulation, I promised the reader that I would talk about sex this time. And so I will-- starting with rats, and the results of more than 20 years of research done by University of Massachusetts psychologist Celia Moore. Her work has been surprisingly under cited and definitely under-reported by the popular press. Moore first reported that rat mothers lick the private parts of their newborn male pups more than those of their female pups. (The licking behavior is called ano-genital licking or AGL for short). Years of experimentation revealed that the odor caused by higher levels of testosterone in the urine of baby males induces more licking. But most interestingly for this entry's "brain building" theme is that the sex differences in AGL produced sex differences in brain structure.
Scientists have noted for some time that groups of nerve cells (called nuclei) in the adult rat's brain stem that send connecting fibers to the penis is larger in males (hardly a surprise). But what WAS surprising was that when Moore used a paintbrush to raise the AG stimulation levels in female pups the size of their adult nucleus also increased. In other experiments, Moore lowered levels of AGL by blocking the mother's ability to smell. In this case, males who had been licked less as newborns, took longer to ejaculate as adults. Why? Because, compared to normally-licked males, they had fewer motor neurons in the part of the spinal cord associated with ejaculation. The general lesson: greater stimulation led to greater nerve development in the brain and spinal cord; diminished stimulation produced diminished connectivity.
The story Moore tells is more intricate than the nugget I have extracted for this blog entry. And I will return to that intricacy in future entries when I am ready to put a name to the alternative to Nature vs Nurture. But not yet. First (in the next blog entry, that is), I want to say a little more about human sensory experience and brain development, and hopefully tell a stimulating story about ducks.

 

Further reading:

Fausto-Sterling, Anne (2000) Sexing the Body: Gender Politics and the Construction of Sexuality (Basic Books)

Fine, Cordelia (2010) Delusions of Gender: How Our Minds, Society, and Neurosexism Create Difference (Norton Publishers)