left-handedness and minor brain damage

[elfkin477]

I've read several times that doctors theorize that left-handedness might be caused by minor brain damage/injury at birth through O₂ deprivation (which certainly might be true in my own case, the nurses appearently thought that not inconviencing my mother's doctor sooner on a holiday was nearly worth my life) and even Cecil aludes to it in a column here http://www.straightdope.com/classics/a5_092.html . However,
It remains a theory, one which doctors admit to not being able to prove conclusively. Why?

My question really has two parts. The first part is could they do a CAT-scan or MRI on a left-handed person and find the theorized minor damage? If they can, why haven't they been able to conclusively prove that it? I would think if most of the lefties they scanned had the damage, they'd be able to say "this is what causes it" without doing all the hedging they've done so far.

[Qadgop the Mercotan]

My wild-ass educated guess: The 'damage' hypothesized to be present is probably so subtle that our modern methods of scanning wouldn't pick it up. Perhaps if PET scanners were more sensitive, it could be seen, but they're still trying to correlate the results of PET scans (which show brain function) with the results of MRI or CT (which show brain structure).

Perhaps Neurodoc will stop by and elaborate on what's really going on. I'm sure KarlGauss knows more about it than I do too.

QtM

[Alex B]

All this talk of left handedness being derived from some deficiency in the brain seems to my (albeit left handed and damaged) brain to start from an assumption that right handedness is the 'correct' way to be and any deviency from it must be due to a problem in the person concerned.

The study is searching for the 'cause' of left handedness, as if we all started right handed (i.e. normal), whereas surely the real question is why we have any bias either way. (Of course, it is entirely possible that the study is part of a wider investigation, but you make no reference to it.)

For the record I suffered no detectable lack of O2 during birth and I am left handed, while my brother did, almost suffering severe brain damage, and he is right handed.


Alex

[micilin]

The brain damage thing is - I think - during development of the foetus. AFAIK there's a stage where 'normal' people's brains become asymetrical by increasing the size of the left hemisphere relative to the right. If theres O2 deprivation at this stage the hemispheres remain undifferentiated thus giving you a higher chance of being left handed.

Brain damage might be a strong term for what happens, but it is 'normal' for people's brains to be larger on the left. So what? It's also 'normal' for people to have their hearts on the left, and nobody gets upset about that asymmetry.
Anybody wants to seriously call me brain damaged can go right ahead:
sticks and stones etc

What I want to know is what's up with the supposed increased instances of alcoholism, mental illness, and the 9 years shorter lifespan that lefties are supposed to experience? Surely its not all because we find scissors are awkward to use?

[Neurodoc]

About 90% of humans are left-hand dominant. There is some genetic or at least familial determination of handedness. The Kerr family surname is associated with sinistrality, for example. But identical twin concordance for handedness is far from 100%...in fact it is rather low. Many identical twins are opposite in terms of hand dominance, which raises the question of neurological "mirroring."

Absent demonstrable brain injury, there is no reason to believe that left-handedness is related to brain damage.

The most interesting fact about handedness is that about 70% of left-handers still show left hemisphere dominance for language function. About 96% of right-handers show left hemisphere language dominance.

[Neurodoc]

Error in above post...90% of humans show RIGHT hand dominance...sorry.

[choosybeggar]

Originally posted by Qadgop the Mercotan


Perhaps Neurodoc will stop by and elaborate on what's really going on.
QtM

And then

originally posted by Neurodoc
About 90% of humans are left-hand dominant. There is some genetic or at least familial determination of handedness. The Kerr family surname is associated with sinistrality, for example. But identical twin concordance for handedness is far from 100%...in fact it is rather low. Many identical twins are opposite in terms of hand dominance, which raises the question of neurological "mirroring."

Absent demonstrable brain injury, there is no reason to believe that left-handedness is related to brain damage.

The most interesting fact about handedness is that about 70% of left-handers still show left hemisphere dominance for language function. About 96% of right-handers show left hemisphere language dominance.

Why shoot, I coulda' told ya' that.

What I really want to know is why does the left cerebral hemisphere exert sensorimotor control over the right hemibody of bilaterally symmetric vertebrates, and vice versa?

[DVous Means]

In my opinion, as we find out more and more about the human brain and how it works, the more we realise how much we have yet to learn.

[bashere]

What I want to know is what's up with the supposed increased instances of alcoholism, mental illness, and the 9 years shorter lifespan that lefties are supposed to experience? Surely its not all because we find scissors are awkward to use?

The 9 year shorter lifespan is incorrect. The study that discovered that fact was badly flawed.

This site: http://www.stats.org/statswork/left.htm

briefly discusses the problems with the '91 study.

[Neurodoc]

Originally posted by choosybeggar
Why shoot, I coulda' told ya' that.

What I really want to know is why does the left cerebral hemisphere exert sensorimotor control over the right hemibody of bilaterally symmetric vertebrates, and vice versa? [/b]

If you can get to a university library that has the journal Medical Hypotheses in its collection, you can read two articles I published ond the subject: 1) Why are vertebrate nervous systems crossed? (1995) 45, 471-475; and 2) Why do we speak with the left hemisphere? (1999)52(6), 497-503.

Both of those articles will, I think, answer your questions about the subject.

[Lao Tzu]

I tend to agree with a radical and rarely-mentioned explanation for "handedness". It basically goes like this: Due to the "violent" nature of most births (bright, noisy hospital rooms; held upside down and spanked; etc), babies' raw senses are shocked. This crazy new world they are pulled into overstimulates the left side of their brain (which deals with language, reasoning), leading to right body dominance.

Makes sense, considering that people who have suffered severe trauma - like abused children and veterans - often show abnormal brain waves in the left hemisphere. Frederick Leboyer's underwater childbirthing method supposedly yielded children who are predominately left handed. Can't remember the exact figure, but it was something like 70% lefties. I wouldn't be surprised to learn that most people are inately left handed.

[choosybeggar]

Originally posted by Neurodoc

If you can get to a university library that has the journal Medical Hypotheses in its collection, you can read two articles I published ond the subject: 1) Why are vertebrate nervous systems crossed? (1995) 45, 471-475; and 2) Why do we speak with the left hemisphere? (1999)52(6), 497-503.

Both of those articles will, I think, answer your questions about the subject. [/b]

Thanks for the cites, ND, but we don't have the journal and I can't get either article online without coughing up $35. Do you have any reprints? Email me if this is a possibility. Again, thanks.

[Turlough]

What I want to know is if left-handedness results from minor brain damage, what does it mean for all us ambidextrous people out there? (and I know its uncommon)

[gump]

I work with MRIs, and one place that I frequent does a lot of work with terminal brain cancer patients. Some of the people come in for their first MRI very somber, like I guess most of us think we would if there was suspected cancer.

Other people come in light as a feather, remarking things like, "Oh, I know that I'm fine, and the doctor just wanted me to get this silly test anyway."

The MRI technologist says that after a while she noticed a difference in the locations of their tumors, and she could predict if it was a left brain or right brain cancer.

MRIs and CTs are just beginning to explore brain function. PET scanners and Magnetic Source Imagers also are potentially powerful tools, and the next decade will see some cool stuff in brain discovery

[slortar]

Sounds kinda iffy. I know exactly why I'm left-handed--when I was starting to learn how to write in kindergarden, I'd have problems forgetting which hand I was going to use. At one point I asked my Mom about it and she responded "You're left-handed, of course." I've been left handed ever since.

[Crafter_Man]

I’m left-handed, and my wife says I’m brain damaged. So maybe there’s something to it…

[Lao Tzu]

Interesting, Gump. Are you implying that the technician could tell which side the tumor was on by their personality (somber vs. light as a feather)? If so, which side does she think corresponds with which personality?
_________________
"It is an ironic habit of human beings to run faster when we have lost our way." -- Rollo May

[Neurodoc]

Originally posted by Lao Tzu
Interesting, Gump. Are you implying that the technician could tell which side the tumor was on by their personality (somber vs. light as a feather)? If so, which side does she think corresponds with which personality?
_________________
"It is an ironic habit of human beings to run faster when we have lost our way." -- Rollo May

I would guess that the right-hemisphere patients would be the most unconcerned. There is a peculiar syndrome in patients with non-dominant hemisphere lesions (usually right), called agnosoagnosia, meaning roughly "I don't know nothing about it and there ain't nothing wrong with me." I've had patients with non-dominant hemisphere strokes who, when I held their paralyzed hand in front of their noses and asked, "Whose hand is this?" have said "Yours."

[Acco40]

Originally posted by Turlough:

What I want to know is if left-handedness results from minor brain damage, what does it mean for all us ambidextrous people out there? (and I know its uncommon)

How screwed up am I? For any large-muscle activity, I am predominantly right-handed (throwing, frisbee, golf). For small, intricate activity, I'm a lefty (writing, using a utensil). I can throw relatively well left handed though, too. Does this qualify am being ambidextrous?

So what's my deal, doc?

[Crafter_Man]

Originally posted by Acco40
Does this qualify am being ambidextrous?

“I’d give my left arm to be ambidextrous.” - Unknown