Infant Brains Wire to the World Around Them

Human babies are much more dependent on their caregivers than many other newborns in nature – horses can walk shortly after birth, and an infant chimp can cling to its mother’s hair. It takes months before a human infant can move their limbs with intent, to gesture, or hold an object. Other developmental milestones aren’t seen for years following birth. As a species, we are born still under construction; still developing, a process that takes about 25 years in total. Many scientists have speculated as to why this is, though nobody knows for sure. We can’t distinguish nature vs. nurture in this context, as they are so intimately intertwined. We can, however, contemplate the advantages that this key period in human development may bring to our species.

To a substantial extent, a baby’s genes are guided by the environment – a phenomenon that scientists call ‘epigenetics’. Take vison as an example, whereby the brain areas most centrally involved in visual perception will develop normally after birth only if a baby’s retinas are regularly exposed to light. Additionally, when you cradle a newborn baby in your arms, you present your face at just the right distance to teach their brain to process and recognise faces.

Tuning and Pruning

As information travels from the world into the newborn brain, some neurons (cells of the nervous system) fire together more frequently than others, causing gradual brain changes that we’ve called ‘plasticity’. These changes nudge the infant’s brain towards a higher complexity via two processes called ‘tuning’ and ‘pruning’.

‘Tuning’ refers to a strengthening of the connections between neurons, particularly those which are used frequently or are important for budgeting of the resources of your body (water, salt, glucose, etc). Well-tuned neurons are more efficient at carrying and processing information than poorly-tuned neurons, and are more likely to be reused in the future. Neuroscientists like to refer to this by saying “neurons that fire together, wire together”.

Meanwhile, less-used connections weaken and sometimes die off completely. This is the process of “pruning”, the neural equivalent of “if you don’t use it, you lose it”. This is a critical process in the developing brain, where humans are born with far more connections than they will ultimately use in their lifetime (twice as many, to be more precise!). This is helpful in the outset, allowing infants to tailor themselves to diverse environments, but is a huge burden on the metabolic system. In the long-term, it’s a waste of energy for the body to maintain unnecessary neural connections, when pruning could make room for more learning; for more useful connections to be tuned and greater efficiency.

These two processes happen continuously and simultaneously, driven by the physical and social world outside the infant’s body and by the growth and activity inside it. Both processes continue throughout our lifetime.

The Attention ‘Spotlight’

An example of tuning and pruning can be seen in how we develop our attention skills. Have you ever been in a crowd, not really listening or attending to the conversations happening around you, and then someone speaks your name and you immediately orient to it? (scientists refer to this as the ‘cocktail party effect’). Our adult brains can effortlessly focus on one thing and ignore others, similar to a spotlight in the darkness, because it’s able to ascertain what is important and disregard other details as irrelevant. Our brains focus its spotlight of attention continually and automatically, and often we’re unaware that it’s happening.

Interestingly, the newborn brain doesn’t have a spotlight – It has more of a lantern, which illuminates a wide area of its physical environment. Newborn brains are not able to distinguish what is important from what’s not, so they can’t focus as adults do. They still lack the wiring that narrows their lantern into a spotlight. Caregivers are the missing ingredient in their social world. Their role is to guide the baby’s attention to things of interest, such as a toy and look at it. For example, the mother will look at a toy (In this instance, a cuddly toy dog), then look back at the baby, guiding her infant’s gaze. She will turn to her baby and say with intent ‘what a cute little doggie’ in a singsong tone. The mother’s speech, combined with the back-and-forth gaze switching (which scientists refer to as ‘sharing attention’) alert the baby that the toy dog is significant – that is, they should care, and learn about the toy. Little by little, this teaches an infant which parts of the environment matter and which parts are irrelevant. They are then able to construct their own environment and disregard what they believe to be unimportant.

Distinguishing Sounds from Birth

Another example regards the development of the senses, specifically language. In the first few months of life, babies are surrounded by all kinds of sounds, including the sound of other people speaking. Newborns, with their lantern of attention, take in all the sounds around them. When tested in a lab, newborns are able to take in and distinguish a wide range of language sounds, including those that are not from their native language or that they do not hear often. Over time, however, tuning and pruning will wire the baby’s brain based on the vocal sounds that they hear most often. Sounds that are frequent cause certain neural connections to be tuned, whilst those that are not heard regularly are pruned (they are treated as noise to be ignored, and will fall out of use). Scientists believe that this is one reasons why children may find it easier to learn languages than adults do. Different spoken languages use different sets of sounds, for example Danish has twenty or more vowel sounds. If people interacted with you in multiple language when you were a baby, then your brain was likely tuned and pruned to hear and distinguish the sounds in those language. If you heard only one language as a baby, then you’d need to relearn the ability to hear and distinguish those sounds outside your own language as an adult – a much harder task.

We have the kind of nature that need nurturing. Our genes require a physical and social environment to produce a finished brain. This means more than we recognised even a few decades ago; it’s up to other humans to create the world that our infants are wiring themselves to, and to grow little brains that thrive.

References:

Feldman Barrett, L. (2020). Lesson no 3. little brains wire themselves to their world. In L. Feldman Barrett, Seven and a half lessons about the brain. (pp. 47-63). London, UK. Picador.