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.

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