Sunday, May 6, 2007

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.

4 comments:

rbh said...

You seem to say that being left handed is okay. That to force a lefty to be a righty is bad, but then why do Waldorf schools persist in making lefties use their right hands?

Emily G. W. Lilly said...

RBH--

While there are probably some teachers in some schools that still tow the old Steiner line (of forcing all children into righties), you will find that most do not.

My one daughter has 3 lefties in her class, and while the teacher started the year emphasizing how to pick up implements with the right hand, any child that wants to can switch to the left.

Part of the First Grade Assessment (I think it is universal; they certainly did it at both schools we have been to!) is to check for dominance in hands, feet, eyes and ears. One ambidextrous child that I know had to go through the assessment 3 times because he had no dominance at all! His parents, fully convinced that he could really learn to do anything with either hand, went on to ask the teacher to coax their son into using his right hand to write. The mother had gone to a Waldorf school as a child, and one of her friends had been forced into righthandedness, with awful results. She was prepared to defend her son's right to be a lefty if it was necessary. But since she had seen his mixed dominance herself, and then the assessment came through the way it did, she is confident that her son will not suffer the same brain-damaging fate as her friend.

Ambidextrous people are better off learning certain skills (like writing and cutting) with their right hands, as the world is better suited to being right-handed. (Please note: I am ambidextrous, but because I showed a slight preference for the left at the age of 6, I grew into using my left hand for writing. There are definitely days when I wish I had learned to use my right hand for more things as a child!)

If you (or anybody else out there!) knows of Waldorf teachers and schools with an anti-lefty policy, I'd like to know--I can search the handedness studies to get out the hard facts about the differences in brain organization between lefties and righties.

KateGladstone said...

Re:

> If you (or anybody else out
> there!) knows of Waldorf
> teachers and schools with an
> anti-lefty policy, I'd like to
> know--

Note this from a mom about the Waldorf school her children attended, at the bottom of a discussion page about the significance of left and right in Waldorf/Anthroposophy:

http://uncletaz.com/diana2.html

" ... At our ex-school, one of the
teachers sent a flyer home to her
her class parents discussing the
need to change left handedness. I
know of several parents at our old
school being told by the teacher
that their child must change
hands. And [daughter] Sandra also
said she was required to change
hands."

Fiona said...

I know for a fact that the Waldorf school in North Vancouver, BC, Canada is enforcing right handedness. I'm looking into the Nelson BC school, where I'm considering sending my daughter, to find out what their policy is. If they enforce right handedness I will go with public school.