Brain differences in TS
Better?The human brain has two halves: left and right. You've probably heard the expression "She's right-brained" or "He's left hemisphere dominant". This is because the left half of the brain (when looking at the brain from behind) is usually associated with analytical ability [and people who are right handed], while the right half is more artistic [and left handed]. It has even been found, thanks to serious brain injury, that people who have one half of the brain damaged become dominant in the other side and its associated traits. I remember very vividly studying a woman who spent most of her life as a right-handed engineer until she had a stroke on the left side of her brain. She then became left-handed, and quit her job in engineering to become an artist in stained glass.
These sort of studies have also discovered something weird(er). In cases of severe epilepsy the practice in the late 1980's/early 1990's was to sever the corpus callosum. The corpus callosum is a "tube" that connects the left half of the brain to the right. The reason this is necessary is obvious in the patients who had this operation.
You've heard of the expression that "the left hand didn't know what the right hand was doing"? Well, this is literally what happened. Patients would find themselves in strange situations. One man was getting dressed one morning with his dominant hand (I forget whether it was right or left-sorry.) while his other hand was undressing him. People would be trying to put on makeup with one hand, while the other would smear it around the room.
Having done tests on these patients really helped our understanding of the human brain. For example they would take a person and show them something like this:
| GOLF |
They would cover one eye, then get the patient to write down what they saw with each eye. When the left eye was covered; the patient reported seeing:
When the right eye was covered, they saw:
| GOLF |
Now, what does all this have to do with Tourette syndrome??
In a study by Plessen KJ, et al. (Nov 2004) it was discovered that there was a difference in corpus callosum (CC) size in TSers than in the control subjects. Interestingly it was found that children and teenagers with TS has smaller CCs than the general population while adults with TS had larger CCs than the general population.
This immediately suggests that there is a difference in Childhood Tourette syndrome and Adult Tourette syndrome; perhaps even pointing to a reason why most children outgrow their Tourette's in their late teens.
The authors of the article suggest that the smaller CC size in children and teens means that "inhibitory feedback" between the hemispheres is reduced. I've mentioned inhibitory feedback before, but may not have used those exact words. So here is an explanation:
Say you have a human brain with 2 glands; A and B.
Gland A produces a chemical (a) which floods the brain and
causes some action (say heats up the brain):
Now Gland B realises that it is too hot in the brain; so it releases chemical
b into the brain. Chemical b goes to Gland A and tells it to stop
producing chemical a. This is inhibitory feedback.
Of course, when chemical a is not being produced, the temperature falls.
So by using inhibitory feedback, the temperature of the brain is kept
constant.
This is roughly the same mechanism used in the TS
brain, but instead of chemicals the brain is communicating via. neurons (brain
cells that pass electric pulses back and forth). So the theory is that
in a "normal" brain the message that says to the body 'MOVE!' will be
regulated by the other side of the brain which will say 'DON'T MOVE, THIS IS
NOT IMPORTANT!' when a tic comes through. In a TS child's brain,
however, there isn't enough room for the 'DON'T MOVE, THIS IS NOT IMPORTANT!'
message to squeeze across the tube connecting the two sides of the brain
because it's too narrow; so the tic is not stopped.
This sounds a bit funny for those who know that the brain doesn't work that way,
with big electrical impulses trying to squeeze through a tiny tube, but in
reality that's pretty close to the truth.
This abnormality in juvenile TSers brains can also partially
explain why we often have difficulty in processing two or more stimuli at once.
(eg. holding a conversation while watching TV)
The only suggestion I have as to why Adult TSers have a
larger CC than normal is my favourite "equilibrium" argument.
Which, if you're not sick of me repeating it by now, is:
The human body has, in its genetic makeup, an idea of what is a 'normal' function. Now, a stunted growth in a child's CC would possibly cause the brain to yell out "Hey, our messages aren't getting through! Let's send growth hormones to the CC and make it bigger!" But squirting out random hormones is not an exact thing. So the CC may grow too big! Once this has happened, the brain says "Now I'm getting too many signals...ignore a certain percentage of all messages before I burn out!" The net effect of this is you have the same levels of information transfer being processed by the brain as in a smaller CC size.So perhaps the reason some people don't grow out of TS, and in fact get worse as they age is that their CC has overshot its ideal size and is now trying to compensate!