Definition: Lateralization

The brain is divided into two hemispheres; for many tasks, either one side or the other is in charge. So when neuroscientists say things like, "Linguistic function is lateralized in the left hemisphere," they are basically saying, "The talent of language happens on the left side."

Thus far, it is pretty straightforward. However, there are some nuances that are worth mentioning here.

The Motor Cortex
This section of the brain is special: first of all, part of it resides on each side of the brain (i.e. there's a bit of it in each of the two hemispheres), which does not happen with most other functions. It is what controls our movements. The fascinating bit is that the left motor cortex controls the muscles in the right side of the body, and visa versa.

How does that happen? It comes down to brain development before birth. In the early fetus, there is a brain attached to what will be the spine in a nice, straight line. Then, as the brain folds on itself (making all of those curvy, wiggly lumps), it flips over. It does a somersault. Pretty wild, eh? The result? The left side of the brain is attached to the muscles on the right side of the body (and the right to the left muscles). So, when you wiggle your left thumb, you are using a particular set of neurons in the right motor cortex.

The senses of the skin (such as touch, temperature, pain, etc) do this as well.

Right-Handedness Versus Left-Handedness
All claims of lateralization are always made based upon right-handed subjects, because they are all wired the same way. So, the statement above about language being on the left only applies to righties. Another typical left-brain item is logical reasoning. Many studies have shown that emotional reasoning and music are both housed in the right brain. Again, these things only apply to righties.

And what about the lefties? I'll have to do this more thoroughly later (I need to find an article to review that covers all of the details), but here is the basic gist: a simple majority of lefties (about 60%) are exactly the reverse of righties (so language and logic are in the right brain while music and emotions are off in the left); about 10% are built like righties; and the remaining lefties do not show lateralization at all! They have language and logic in the left and the right; they have emotions and music in the left and the right.

This is why brain scientists are not interested in left-handed subjects when doing studies to determine where in the brain certain functions are located. There are some researchers who are trying to figure out what exactly is going on in lefties' heads, but they could not get started until much of the brain had been mapped out in righties: this makes sense, since they could not possibly be able to understand anything without a base-line map.

Lefties' motor cortices work exactly like those of righties: the right motor cortex makes a person move their left thumb.

It is not too hard to understand why it is that forcing a lefty into being a righty can cause permanent brain damage!

In sum
Lateralization at its most basic is about which bits of thinking are located in which places of the brain. However, the generalizations only apply to right-handed people. Also, certain functions happen on both sides, but each side of the brain controls the opposite side of the physical body.

Definition: Neural Plasticity

I have been trying for two weeks to come up with a simple, straightforward definition of "neural plasticity" with a nice example, and I have been completely stumped! Hopefully, I will be clear enough!

To understand "neural plasticity", you have to go back to the original definition of "plastic". I always think of some hard substance; but really, to make a hard, plastic item, they pour into a mold a liquid that is able to harden into any shape. This is the key piece: to be plastic is to be infinitely shape-able.

What does it mean for your brain to be plastic? Basically, there is a bit of wiggle room within brain function. For instance, an area that is not being used for anything can be taken over by neighboring areas. Also, there may be one ideal part of the brain for learning a particular type of knowledge, but other areas can step up to bat in a pinch: I will definitely be discussing this when we get to teaching reading.

Usually, each section of brain is prepared to engage in a particular type of activity. The ability to switch from one function to another is remarkable and is quite handy for those who suffer from brain injuries. However, there is a limit to this type of self-repair.

Exactly how plastic the brain is is an active area of research; the concept figures prominently in many research studies.

Definition: Critical Window of Development

The quick explanation: A critical window is the period of time during development that the brain is open to a particular type of experience to result in a particular talent, and after the window closes, this talent can no longer be learned.

Boy, that's a mouthful!

Here's a pretty straightforward example from vision: the condition called strabismus (lazy eye). With this condition, the muscles that move the eyes around do not act in a coordinated way; one eye lolls about while the other is looking at the object that the person is interested in. There are ways of fixing this problem. However, if it is not fixed by the age of 4, then there is an aftereffect: the person is never able to see in 3-D. They can take guesses based on relative size and looking at shadows, but they cannot take the information from each eye and integrate it into a single picture with depth, the way most people can. I have a friend whose strabismus wasn't fixed until too late, and she cannot drive at dusk and dawn because the hazy light removes all of her depth cues. She can't tell if things on the road are 10 feet versus 50 feet away, which can be a major safety issue.

Once the critical window for depth perception has shut, there is no way that a person can learn this vital skill. There are no surgeries or therapies that can make the brain use the pictures from both eyes at the same time to figure out how far various objects are. Try spending 10 minutes walking around with one eye closed. You can estimate some distances (like my friend can, using shadows and sizes) but not all of them. Try it just after sunset to really disorient yourself!

Most people have 3-D vision, because most people have the experience of having their eyeballs pointing in the same direction before their 4th birthday. The only way that we know that there is a window at all is that there are some people who do not follow the regular pattern and have lifelong debilities as a result.

It can be tricky figuring out when developmental windows open and close. After years of research, however, it is a given that they do exist. What makes a brain become receptive to the input (the window opens)? What makes it no longer receptive (the window closes)? That is still an open debate; for numerous windows, however, it looks like the rush of hormones at puberty are the trigger. Many windows close between 12 and 18 years of age. Of course, several others open up at this time, too, so I'm not saying puberty is bad per se, but that the hormones are a needed part of proper brain development, acting almost like a timer.

You can try to teach someone something until you are blue in the face, but if it is before their window has opened up, or after it has closed, then they will not learn it. Or they might learn it a little bit, using the wrong part of the brain, but will never be good at it.

Not everything has a critical window. Bicycle-riding, like all talents that rely on man-made tools, does not have a window: once your sense of balance is pretty good, you could learn. Or you could wait 20 years, and you could still learn! I suppose that an elderly person who has lost their sense of balance and has never before tried to learn how to ride a bike would have trouble, but that is not because a specific "critical window for bicycle riding" has closed.

So, a quick recap: The potential for learning something becomes available inside the brain, the person has some time (usually measured in years) in which to learn it, and then a change happens inside the brain that prevents a person who has not yet had the right experiences from learning it. All critical windows open at a specific time and shut at a specific time during the course of development.