How the Brain Grows Into the Body

Baby and Godmother Credit: kton25

Harvard Psychologist Stephen M. Kosslyn presents a fascinating conundrum concerning the development of a human embryo: In order for the brain to process the two images our eyes transmit to it in 3-D stereovision, complete with the ability to estimate distances accurately, it must know the distance between the eyes; however, at the moment of conception, there’s no way for the genes to know this distance, which depends on bone growth, which depends on the mother’s and infant’s diet.

So how do the genes do it?

What the genes did is really clever: Young children (peaking at about age 18 months) have more connections among neurons than do adults; in fact, until about eight years old, children have about twice as many neural connections as they do as adults. But only some of these connections provide useful information. For example, when the infant reaches, only the connections from some neurons will correctly guide reaching. The brain uses a process called pruning to get rid of the useless connections…

…the genes overpopulate the brain, giving us options for different environments (where the distance between eyes and length of the arms are part of the brain’s “environment,” in this sense), and then the environment selects which connections are appropriate. In other words, the genes take advantage of the environment to configure the brain.

So one metaphor for the developing brain is natural selection, producing an overabundance of neurons, and then killing off the ones that aren’t performing. This is just one of many reasons the whole Nature versus Nurture debate is considered silly. Is it genes or environment? Genes and environment are not dichotomous, but rather a feedback loop.

The brain’s need to properly interface with the body is why babies kick in the womb according to researchers:

Rat pups in their litter display frequent muscle twitches and non-directed limb and whole body jerks, which are similar to human fetal movements. By studying the relationship between these movements and neuronal activity in the sensory part of the cerebral cortex, the researchers determined that the information provided to the developing brain by these random movements are critical for creating the proper representation of the body in the sensory cortex. By analogy, spontaneous kicks babies perform during the late stages of pregnancy should perform the same service for the human sensor.

The developing baby kicks, not only to work out the joints and muscles, but also so the brain can wire properly into the muscles. So another metaphor is that the brain is doing science, positing a plethora of hypotheses in the form of neurons, experimenting, testing out the environment of the body, keeping those hypotheses that work, and tossing those that don’t. Not only are scientists, learning to live in the environments we are born into, but our brains, the organ that houses our consciousness, acts as a scientist as well.

Science is in our scaffolding.