Jun 17 2010
Half a Brain
This morning I found the following e-mail in my inbox:
I would be very interested in your opinion about the following case. I have a friend who recently found out that her 25 year old son does not have a right hemisphere. The two neurologists that have studied his case have different explanations for the missing part of his cortex. The young man has slightly lower than average intelligence although he has difficulty in some areas as would be expected. However, he was able to graduate from high school and then later attended the California Art Institute where he studied graphic design. What I cannot believe is that he lived with this for so many years undetected. His parents are both professionals and he has a brother who is of above average or gifted intelligence. How has he been able to function this way? I have studied the idea of plasticity but I have never heard anything like this. Is this unusual? My friend is researching to find out more about this and she is considering contacting someone at UCLA since it is about four hours from where we live. Your thoughts and suggestions would be highly appreciated.
This is a great question. When I first heard of cases of people found to be missing an entire hemisphere of their brain I too was stunned, but it does make sense. First a little background neuroanatomy.
The basic structure of the brain includes the brainstem, which is the most primitive part of the brain. It is sometimes called the lizard brain, since reptiles have little more than a brainstem and a tiny ribbon of cortex. The brainstem is where basic functions, like breathing, pupillary function, and sleep-wake cycle are controlled. You need a functioning brainstem to be awake, and therefore severe brainstem damage results in a deep coma.
At the base of the brain there also sits the cerebellum, which connects through three peduncles at the level of the brainstem. The cerebellum functions almost as its own self-contained brain and is involved in motor coordination.
The diencephalon includes the thalamus, hypothalamus and related structures. This is the “interbrain” and contains the relay-station between the cortex and the sensory input and motor output. It also contains in the hypothalamus the neuroendocrine systems – the connection between neurological function and hormonal responses. This also controls basic drives like hunger.
Finally there is the telencephalon – the ventral telencephalon includes the basal ganglia which is involved in setting the gain for voluntary motor control. It is the basal ganglia that is malfunctioning in diseases like Parkinson’s disease.
The dorsal telencephalon is the cerebrum, which is comprised of two cerebral hemispheres. This is the most evolved part of the vertebrate brain, and is the hierarchically highest level of control. This is where you have your memories, experience sensation, have emotions, and do your thinking. Each hemisphere can function as an independent brain but they are massively connected in the middle through one big cable – the corpus callosum – and several smaller ones. The result is one functioning brain and mind.
Specific functions are localized in the cerebral hemispheres, and these are divided into those that are lateralized, those that are bilaterally lateralized, and those that are bilaterally redundant. A lateralized function exists only on one side of the brain. Language, by definition, is in the dominant hemisphere only (the left hemisphere for most people), mathematics is dominant only, while visuospacial processing is in non-dominant only.
Bilaterally lateralized functions exist on both sides of the brain, but each has an independent function – they are not redundant. This includes motor and sensory function. The cerebrum is organized so that it maps to the opposite side of the body and world, so the right hemisphere controls the left side of the body, receives sensation from the left, and sees the left side of the world.
Bilaterally redundant function exist in both hemispheres and the function of one side can be lost without noticeable loss of overall function. Much of the higher functions of our frontal lobes are bilaterally redundant, such as executive function and inhibition of behaviors. Memory structures are also redundant, but there may be a dominant side that provides a disproportionate amount of function. Hearing is also bilaterally redundant.
With all that in mind, what happens if you are missing one cerebral hemisphere? Anatomically, all basic functions would be intact. What we would be missing are all the lateralized cortical functions – so we might expect someone to be paralyzed and numb on the opposite side, to not be able to see that side of the world, and to be missing language if the dominant hemisphere is missing, or visuospacial function if the non-dominant hemisphere is missing. This is exactly what we see if someone has a large stroke taking out most of one hemisphere.
But there is a property of the brain known as plasticity – the ability to repair itself, to form new pathways, and to change the pattern of hardwiring in response to use and need. Plasticity is able to repair brain damage to some degree (never perfectly). Even older adults have some degree of plasticity – but plasticity does vary greatly with age. Young children have a remarkable ability to heal around brain damage, while the elderly has a very limited capacity to do so.
What about fetuses? What if someone had a stroke in one half of their cortex while they were just a few months old in the womb, due to a developmental anomaly, for example? While in the fetal stage of development brain plasticity is extreme. If a fetus loses one hemisphere, the other hemisphere will completely take over its function. The brain is still organizing itself and mapping itself to the world and to its various functions, and it uses what raw material it has. If there is only one hemisphere, is maps that hemisphere to all needed functions. There are numerous published cases that demonstrate this.
Other possible causes include the failure of one hemisphere to develop at all. An infection may also damage the brain in utero, but this is less likely to be in just one hemisphere.
As the e-mailer indicates, however, while all function is present, overall intelligence is decreased. Despite the popular myth (although now widely debunked on the internet) we use more than 10% of our brains. In fact, we use all of it. If someone is missing a huge chunk of cortex, that is less raw material for brain function, and overall function will be decreased. It really is as simple as that – driving with a 2-cylinder engine is less peppy than a 4-cylinder engine. In this case the subject of the e-mail comes from an above-average intelligence family and has below-average intelligence, almost certainly due to his missing half his cortex.
To be clear, volume or weight of brain is not the only determining factor in intelligence. The structure of the brain – even details like number and pattern of connections – is very important. So much so that you cannot measure how intelligent someone is by weighing their brain. But that does not mean that for any individual, losing part of their brain does not also translate into losing brain power. Size does matter – it’s just not the only factor.
These cases are fascinating, but are not mysterious. They are in line with our understanding of brain development and function, specifically the plasticity of the brain and the manner in which the brain develops through mapping of function to use and input. They are dramatic cases that demonstrate the extremes of these principles.
21 Responses to “Half a Brain”
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Superb write-up on a fascinating topic.
Steve, how about a post on plasticity? Some people use plasticity as if it’s somehow a ‘magic sauce’ or pretty damn close to what was once called “the soul”. (Vaughn Bell of Mind Hacks points out there is also very little consensus about what it actually means). Do we know much about the mechanisms of plasticity? How it evolved? (Or is it an inherent by-product of having a brain at all? Maybe it was just fine-tuned?). Its occurrence in non-human animals? etc.
Very interesting topic, a question on a somewhat related topic: I am only really familiar with the concept of a “Lobotomy” from movies and pop-culture. As I understand it (from those limited sources), the process consisted of surgically severing the single link between the two half’s of the brain and this causes the patient to become like a zombie that just sits in a wheel chair in the back of the mental facility and drools on him/herself. In the movies, this method is used to control a patient that cannot be otherwise controlled and could be a threat to themselves and others. Now that I’m talking about it, this was actually recently portrayed in the movie “Shutter Island”, only in that movie they would go in under the eye lid to severe the connection. It seems like most of these types of things portrayed in movies are somewhat based on fact, but mostly exaggerated. Steve, can you or anyone else here that may know more on the topic elaborate some on what a Lobotomy is or isn’t? ( I suppose was or wasn’t would be more appropriate) Thank you very much for your time and dedication to truth and reason. Keep fighting the good fight.
I wrote about plasticity here: http://www.theness.com/neurologicablog/?p=1777
and here: http://www.theness.com/neurologicablog/?p=142
And wrote previously about the half-brain phenom here: http://www.theness.com/neurologicablog/?p=582
Most informative post. Thanks.
This seems to seriously downgrades the effectiveness of the insult “if you had half a brain you’d be dangerous”.
Another slightly off-topic question:
The wiki entry on peduncles describes the cerebellum-cortex negative feedback loops involved in damping motor commands to smooth movements. This sounds rather time consuming (relatively speaking, of course). In executing time-sensitive but arguably automatic movements involved in activities like sports, playing musical instruments, reacting to flying objects, etc, is any of that feedback processing by-passed in order to speed up reaction times?
Thanks.
“Despite the popular myth (although now widely debunked on the internet) we use more than 10% of our brains. In fact, we use all of it. If someone is missing a huge chunk of cortex, that is less raw material for brain function, and overall function will be decreased. It really is as simple as that – driving with a 2-cylinder engine is less peppy than a 4-cylinder engine.”
I feel the need to clarify this analogy. We use “all of it” because we don’t keep “all of it” during development. We start out with excess, and like you described, the neurons grow or die and connections strengthen or weaken based upon activity/use. Thus in your analogy if a person who starts off with “half a brain” he/she is like a designed 2 cylinder engine (with some limitations) versus 4 cylinder engine (undamaged brain), but if a person loses (damages) brain much later in life it is like a 4 cylinder engine that is damaged. So damage to half of a 4 cylinder engine will likely cause that engine to fail, versus starting off with a 2 cylinder engine which may not be quite as peppy but can function fairly well.
This is from Ganzzaniga et al. 2009.
“Most of what we know about plasticity is from animal studies. The resuts suggest a cascade of effects operating across different timescales. In both Humans and other animals, changes in cortical mapping can be detected essentially immediately after the change in sensory input or motor activity. But additional components of plasticity take longer to appear.
…
Rapid changes probably reflect both the unveiling of weak connections that already exist in the cortex, through both release of from inhibition and changes in the efficacy of synapses. Longer-term plasticity may result from the growth of new synapses and/or axons.
Well, now I have a better understanding of my acquaintances with half a brain. Thank you.
Now, if you would, I would like a better understanding of those who are half baked.
Much appreciated.
@ Charles W:
I heard that the visual cortex does not simply react to visual stimuli but proactively predicts what it is likely to see in any given context, allowing for quicker reaction times and saving energy. And that’s why optical illusions work on us. And I remembered that cuz I thought it was cool, but I don’t remember where I heard it from. George Hrab, maybe.
“Now, if you would, I would like a better understanding of those who are half baked.”
Do you mean: is a half-baked half brain equal greater than a fully baked whole brain? I would think so.
Well thanks a lot Steve! Now I’ve got the “Escape (The Piña Colada Song)” earworm !
And now my friends….so do you.
@ Chaos4zap
Your post is actually referencing two different procedures.
A lobotomy is a process in which a portion of the dorsal telencephalon (cerebrum) is damaged and/or disconnected from the rest of the brain. Historically, this was used on patients who “needed” to be…calmed down. Methods included an icepick slipped into the skull through the nasal or orbital cavities. It’s a terrible, barbaric procedure that was used excessively when it was first discovered. I won’t go into all the horrifying details, but I’m sure wikipedia will if you’re interested.
The surgical procedure that cuts the corpus callosum is a “procedure of last resort” that is still used today when someone is suffering from severe epilepsy. Unsurprisingly, it’s called a corpus callostomy. Epileptic seizures are (in my limited understanding) kind of like contagious neural storms. Parts of your brain can be prone to them, and then one occurs it can spread rapidly to the rest of the brain. As you can imagine, this kind of sucks.
The idea between cutting the connection between the two hemispheres is to prevent a seizure from crossing over from one and screwing up the other. People who undergo this procedure have their personalities and intelligence intact. They do see some very strange, but altogether less harmful, side effects from the procedure. For instance, if they are shown a math problem in the left half of their visual field, they may not be able to solve it until they say it out loud (thus informing the other side of their brain about the problem).
“They do see some very strange, but altogether less harmful, side effects from the procedure.”
For those unfamiliar:
http://www.youtube.com/watch?v=lfGwsAdS9Dc
Once neuroscientists come to a better understanding of brain plasticsity do you think it should be possible to engineer drugs to improve the mechinisms behind it?. Which could prehaps help stroke patient recover.
“lobotomy”
Technically called a “prefrontal leucotomy”
“Unsurprisingly, it’s called a corpus callostomy.”
It may surprise you then to know that it’s actually called a “corpus callosotomy”
What does half a brain look like? Does it look like a normal brain sliced in half, with empty space filling half the skull? Or is it just one rounded lump of brain, without the two halves you would normally see? Does a half-brained person have a smaller head than other people?
Diane,
“What does half a brain look like?”
Google is your friend.
http://www.dailymail.co.uk/health/article-1200958/Girl-born-half-brain-person-world-fields-vision-eye.html
In this 10 year old girl, the right hemisphere failed to develop in utero. The MRI shows absence of the right hemisphere. It also shows that the right optic nerve has re-routed to the left side of the brain so that she has a complete visual field.
Very interesting. One question, you say ‘This is the most evolved part of the vertebrate brain’. This doesn’t make sense to me, unless you’re saying that evolution involves a direction or end. Surely reptile brains have changed over eons and so are just as highly evolved, but lack some structures that we have. Can you clarify what you meant? Thanks.
Brian, don’t get cute.
I think you know exactly what he meant.
Recently I had a discussion with a guy somewhere on the Internet, he presented me with two following cases:
http://www.newscientist.com/article/dn12301-man-with-tiny-brain-shocks-doctors.html
http://www.metacafe.com/watch/205285/amazing_kid_with_only_half_a_brain/
And there are even more extereme examples to be found on the Internet.
Of course you can try to explain how these people’s brains retain their normal functionality – in the first case, the brain developed all the minimally sufficient structures, and in the other – the guy had undergone a long rehabilitation (though he lost half of his brain at the age of 14!)… But still, there is something really strange about those cases, giving us a hint that perhaps there is something we overlook at the very basic level of human brain’s functioning?
Hi my name is Narelle I am 39 and have cerebral palsy I had a reaction to a needle when I was 3 months old and left me with half a brain the doctors said I would never walk or talk I can not do money or maths I use to have epilepsy I had the operation to cure that in 1997 and have been seizure free since and now live on my own and drive a modified car I live in Perth Australia
From Narelle