Feb 19 2018

Brain Plasticity in Infants

A new study looks at the brains of young adults who suffered a stroke in the language center of their brains as infants. They found that the subjects developed normal language, which just relocated to the mirror-image other side of the brain. This is not surprising, and reflects our evolving understanding of how the brain develops and functions.

For most people language localizes to the left frontal and temporal lobes of the brain. Broca’s area in the frontal lobe is involved in speaking, in the subtle motor output necessary to precisely articulate words. Wernicke’s area is in the temporal lobe and is involved in translating words into ideas and ideas into words. The two areas are connected by the arcuate fasciculous. These are the central language areas. There is also surrounding cortex which is necessary for communication between the language structures and other parts of the brain.

For most people the language area is on the left side of the brain. Meanwhile, the mirror right side of the brain is involved with understanding and producing speech intonation – knowing when someone is asking a question or being sarcastic. The right side is also involved with music and singing.

We also know that brains are plastic, meaning they can change the structure of their connections as necessary. People often use a computer analogy when talking about the brain, but the analogy to digital computers is flawed. Computers are hardware that run software, but brains are neither hardware or software – they are wetware, which is both at the same time. The connection of neurons in the brain is where information is stored and processed, and those connections change as a result of the processing, which alters the memory.

In addition, not just the connections but the anatomy can change in response to use and need, but in a constrained way. The brains of people who play the violin from a young age are different – the part of their cortex that controls their non-dominant hand, the one that works the strings, is hypertrophied.

This new study fits into this model and extends it. The researchers found that the subjects with a dominant hemisphere stroke as an infant developed normal language, but still has a bit of a limp or decreased motor function on their dominant side. Imaging showed that their language function had relocated to the opposite hemisphere, but in the mirror locations.

These findings reflect several concepts important to our understanding of brain function.

First is the basic concept of plasticity itself, the ability of the brain to alters its structure with use and demand. Plasticity is maximal as a fetus or neonate, and then decreases as we age, although never going away completely. Further, there appears to be developmental windows with maximal plasticity for certain functions (including language). For example, you need to develop the wiring for binocular vision by a certain age.

Plasticity is also limited by the parts of the brain involved. It seems that only the mirror cortex can take over language function, not other parts of the brain. In some people, after a language area injury, other parts of the cortex will become more active during speech, but they will not function as well. They are trying to compensate, to take over some of the lost function, but they are just not organized for speech.

Putting all of this together to give us a view of how the brain probably works – it seems that the brain is not a uniform mass of neurons, but is anatomically divided into discrete areas and networks with different kinds of neurons connected in specific ways. Some scientists refer to these basic structures as brain modules, which are connected in brain networks.

But these modules and networks are only semi-specific. They are optimized for a certain kind of processing, but that processing can be used for many different specific functions, and can participate in many different networks.

This explains why language relocates to the opposite hemisphere. It now seems that when language develops, both hemispheres have language capable cortex in the frontal and temporal lobes. The two hemispheres then divide up the work, with words and articulation going to the dominant hemisphere, and emotional content and music going to the other. They subspecialize, but both have the basic architecture capable of language. So when the primary language cortex is damaged, the other hemisphere has the function necessary to take over. The visual cortex, however, has a very different structure and could not function for language.

We might see the cortex as having at least three layers of specialization when it comes to the structure of the neurons and supporting cells that determine function. Cortical neurons have a basic structure that determines the kinds of processing they can do. With development, appropriate cortex is adapted to whatever functions are necessary, essentially determined by use. That function is then coded with the specific details of your environment, including culture and family.

So, for example, the language area has the capability of developing language function, but will only do so if a child is exposed to language. Further, the language area will reflect the specific language or languages learned. It will learn the phonemes and grammar of the primary language to which a person is exposed.

All of this reflects the fact that brains are an adaptation specifically to quickly adapt an organism to the environment. Brains learn, they change, they remember – all of which gives the ability to adapt dynamically to changing behavioral needs.

This is why it is misleading to talk about a brain being “hard-wired” for some behavior. That is not how the brain works. But neither is behavior entirely learned. The old nature-nurture debate has been resolved – the brain is both. That’s kind f the point.

Evolution, genetics, and development determine the basic structure of the brain and the kinds of processing the brain can perform well. This translates in a person to their propensities, their talents, their strengths and weaknesses. But all of this interacts heavily with the environment – what they do and what they learn. People can change, learn, and adapt at any age, however it does become more difficult as we get older. Also, deeply learned patterns at a young age may be very difficult to change.

Also, some aspects of personality seem to be more genetically and developmentally determined than others. We are born with a personality, but again we can look at it like a general propensity that can manifest in many different specific ways determined by environment. So we are not blank slates, but neither are we the slaves to neural destiny.

This kind of research, in addition to giving us insights into healthy neurological function, may also help us treat patients with various kinds of brain injury. How can we optimize plasticity, and improve recovery? Is it possible to de-specialize a part of the brain so it can take over a different function? Perhaps not, but it’s an interesting idea.

Also, this kind of knowledge will be key to developing brain-machine interfaces. We need to know what kind of processing the brain is doing. We may be able to make brain prosthetics that take over for lost or damaged brain functions. As we develop neural-net computer chips, understanding how the different parts of the brain are fundamentally organized and how that relates to their function will be important.

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