Recent studies demonstrate that babies 12-18 months old have similar activity in their brains in response to spoken words as do adults, a fact that tells us a lot about the development of language function.
In the typical adult brain language function is primarily carried out in highly specialized parts of the brain – Wernicke's area (in the dominant, usually left, superior temporal lobe) processes words into concepts and concepts into words, while Broca's area (in the dominant posterior-inferior frontal lobe) controls speech output. The two areas are connected by the arcuate fasciculus and are fed by both auditory and visual input. Taken as a whole this part of the brain functions as the language cortex. A stroke or other damage to this area in an adult results in loss of one ore more aspects of speech, depending on the extent of damage.
Damage to this part of the brain in babies, however, does not have the same effect. When such children grow up they are able to develop essentially normal language function. There are two prevailing theories to explain this. The first is that language function is more widely distributed in infants than in adults, perhaps also involving the same structures on the non-dominant side of the brain. As the brain matures language function becomes confined to the primary language cortex.
The second theory is that brain plasticity allows non-damaged parts of the brain to take over function for the language cortex. Such plasticity exists even in adult brains, but is vastly more significant in babies, whose brains are still developing and wiring themselves. There is still a lot of raw brain material that has not fully specialized yet that can be coopted for whatever functions are needed.
The new research has implications for this debate. If the former theory is correct, then babies who are just learning language would activate their brains more broadly than adults in response to language. If babies show a similar pattern of activation, that would support the plasticity theory.
This latest research firmly supports plasticity as the answer. Researchers at the University of California used functional MRI scans and magnetoencephalography (MEG) to look at the brain activity of 12-18 month old children in response to spoken words. They found that their primary language cortex lit up in a similar pattern to adults. They further tested to see if the children had any sense of the meaning of the words. They showed pictures of common objects with either a correct or incorrect spoken word. The children showed increased language area activity when the words were incongruous to the picture – and the researchers showed this is the same increase in activity as seen in adults.
What this research implies is that the genetic program for brain design comes into effect very early in brain development. The language cortex is destined to be language cortex right from the beginning, as long as nothing goes wrong with this process.
It should also be noted that this study looked only as the response to individual words. What it says about the 12-18 month old stage of development is that children of this age are already programming their language areas and storing up words and their meanings. This research did not look at other aspects of language, such as grammar – the ability to string multiple words together in a specific way in order to create meaning. It also did not look at the visual processing of written words.
Any parent of young children will likely remember with great detail the functional language development of their own children. At this age, and even younger than 12 months, children do seem to be sponges for language. Once they start learning words, they do so very quickly. Young children also seem to understand far more words than they can say. I don't think this is mere confirmation bias (although that would tend to exaggerate the appearance of word acquisition), and research bears out that children can understand many more words than they can say. The ability to speak comes a bit later than the ability to assign meaning to specific words.
I remember that I played games with my children when they were about one year old, and still in the babbling stage. They could reliably, for example, retrieve specific toys by name (being very careful to avoid the clever Hans effect). I remember, in fact, be very surprised at how well they performed – they seemed to understand many more words than I would have thought given the rudimentary nature of their babbling. In this case, careful research confirms subjective experience – children learn to understand words spoken to them before they gain the ability to say them.
This makes sense from the point of view that it is very neurologically difficult to articulate. We take it for granted, but it does require dedicated cortex to pull of this feat. Also, think about how easy it is to become dysarthric – we start slurring our words even when we are just a little sleep deprived, or with a moderate amount of alcohol. It does seem to be a function that goes early when brain function is even slightly compromised, which says something about how demanding it is.
One more tangential point – it also strikes me that we tend to naively judge what is going on in people's heads by what is coming out of their mouths. This is not unreasonable in most circumstances, but there are many reasons why people may be more mentally sharp than is evidenced by their articulation. Young children are just one example – they may be babbling with their mouths, but there is more linguistically going on in their brains.