Scientific Method / Science & Exploration

Premature babies’ brains process speech before they’re fully formed

Study shows distinct activity even as neurons are migrating into place.

The human brain has a remarkable capacity for interpreting speech, with large areas of the brain given over to tracking the sound and interpreting it as language. The neurons that manage this capacity are put in place during our embryonic development, and these are able to respond to sounds shortly after birth. But now, a new study looked at brain activity in premature infants, and it showed the networks that respond to syllables are already active well before most infants are normally born.

What makes this all the more remarkable is the networks are still under construction at that time.

The work relied on infants born at about 30 weeks of gestation (normally, birth comes at about 38 weeks). These babies are typically so young they spend most of their time in an intensive care unit, so it's not possible to take them to an MRI tube to track their brain's function. Instead, the researchers took advantage of the human body's relative transparency to certain infrared wavelengths.

It turns out some of these wavelengths are absorbed by different forms of hemoglobin, the oxygen carrier of red blood cells. Using these wavelengths, the authors could distinguish between oxygenated hemoglobin and molecules that had already delivered oxygen to nerve cells. By fitting a light source and series of sensors to the infants' heads, they were able to roughly track where in the brain more of the oxygen was being used, and thus where the brain was most active. The resolution isn't as good as in MRI, but the speech centers of the brain are pretty well mapped, so this wasn't as much of a drawback as it could have been.

Clearly, even full-term infants don't understand speech. But their brains are sensitive to speech, showing increased activity when the syllables change or a new speaker's voice is heard. So, the authors tested both, playing a repeated set of "ba" sounds, but sometimes changing the speaker, or other times slipping in a "ga" instead. And, just as in infants and adults, the activity appeared in specific areas of the brain and showed distinctive left-right differences.

So, the data clearly suggests the ability to process sound associated with speech is already present at 30 weeks after conception. Again, the striking thing about that is the areas of the brain that do this processing aren't fully formed by this point in time.

The areas studied by the author have a distinctive layered structure, formed by the progressive birth and migration of different populations of neurons—those born first for a layer that ever successive population has to migrate through, with the next population forming a second layer that everything after them also has to migrate through. Once the layers are completed, connections among neurons in different layers are formed. At the stage the authors are looking at, the cells are still migrating. Only some initial connections are in place.

It's often difficult to distinguish between the things our brains are structured specifically to do and things our brains are structured to have the capacity to learn. The fact these areas of the brain can pick out speech differences even before the final structure is in place, however, provides some support to the idea that some capacity to speech is inherent to the brain.

PNAS, 2013. DOI: 10.1073/pnas.1212220110  (About DOIs).

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