During the first 6 months of life, human babies develop at a rapid pace, hopping from one developmental milestone to the next. While newborns have only blurry vision and can’t move in a very coordinated fashion, 6-month-old infants already interact well with their environment. 6-months-old infants can for instance recognize their parents and distinguish them from strangers, they can turn around, and can even begin to learn sign language. In these first 6 months of life not only the abilities of the babies develop rapidly, but their brains change rapidly as well.

The adult brain is a strongly interconnected, complex organ. Different regions in the adult brain communicate with each other via large nerve fiber pathways, referred to as fiber bundles. In the adult brain, these bundles are wrapped in a fatty substance called myelin. Myelin wraps insulate the bundles – similar to a well-insulated cable – thus enabling rapid transmission of neural signals across brain regions that are far apart. But how do these bundles look like in infants? Are they present from birth? When do myelin wraps form around the bundles? This is what our international team from Stanford University in the USA and the Philipps-University Marburg in Germany set out to explore.

To understand how the brain connections of infants change early in life, we leveraged magnetic resonance imaging (MRI) and acquired data from the same infants at birth, at 3 months of age, and at 6 months of age. This was not easy, as it is important that participants lie still while the data is being collected so that the MR images are not blurry. Of course, we could not explain to these young infants that we would like them to stay still. We bypassed this issue by collecting data only at night when the infants were naturally sleeping. During many late-night sessions, we acquired three different types of MRI data at each time-point: an anatomical image of the brain, diffusion-weighted data, and quantitative MRI of R1 relaxation rate (measured in 1/seconds). While the anatomical image gave us a first impression of what the infant’s brains looked like, the diffusion-weighted data allowed us to trace the fiber bundles in each infant’s brain, and the quantitative R1 measurements provided information related to the degree of myelination, i.e., the amount of myelin wrapped around each bundle. By combining these different types of MRI data, we could hence assess how the connections in infants’ brains change over the course of their first six months of life.

We began by tracing the bundles in each infant. For adults, there are well established software tools that use diffusion-weighted MRI data and from this data trace the bundles and assess their properties in each person’s brain. However, we quickly realized that these tools are not ideal for infants as the infant brain is significantly smaller and shaped a bit differently than the adult brain. Thus, first, we developed a new software tool that is specifically designed for infant data (e.g., by using a baby brain template) and overcomes these challenges. The developed software, named baby automated fiber quantification (babyAFQ), identifies 24 bundles in individual infant’s brains. You can see these bundles in different colors in the above image of an example infant’s connections. We also made babyAFQ openly available so that other researchers can use it. It is interesting that with babyAFQ we can identify already in newborns all the bundles that we typically assess in adults, suggesting that these bundles are likely established by the time we are born.

Next, we compared the degree of myelination of different bundles in newborns, 3 and 6 months-olds using quantitative R1 measures. Strikingly, we found that the myelin content of infants’ bundles increases rapidly during the first 6 months of life. We also found that myelin does not grow at an equal pace across the brain – not only are some bundles myelinated faster than others, but the rate of myelin growth even changes along the length of each bundle. We discovered two factors that govern how quickly a specific location in the infant’s brain gets myelinated: First, locations that have the least myelin at birth, get myelinated faster. This may enable all the connections in the brain to quickly get wrapped with some minimal amount of myelin, which may be essential for global communication across the brain. Second, we found that locations towards the back and top of the brain get myelinated more quickly than other locations. The back and top of the brain are responsible for our vision and motion, respectively. This may enable the most critical brain functions to communicate effectively already early on in life. Overall, we found that myelin is build more quickly during the first six months than any other period of our life.

By describing how the brains of healthy newborns are wired up during the first 6 months of life, our study provides an important reference point for future developmental and clinical work. For example, it would be interesting to probe if myelination follows the same developmental trajectory in infants born too early as in the full-term infants we investigated here or not. Further, we found that there is still less myelin wrapped around the bundles of six months-old infants than there is in adults. As such, it would be particularly exciting to follow the development of myelin over an even longer period of childhood in future studies. Finally, while we find that myelin grows rapidly during early infancy, the fiber bundles themselves appear to be established at birth. Future studies examining babies’ development while still in the womb could probe how and when these bundles are built.