Jul 22 2009

Girl With Half a Brain

The PNAS this month reports a case of a 10 year old girl who was born with half her cortex – the right hemisphere – missing. Before the 7th week of gestation her right hemisphere stopped developing. This fact was not discovered, however, until she was about three year old, because she is remarkably neurologically intact.

This story is being reported by the media as surprising, but it is strikingly not so. It is certainly interesting, and the details of the rewiring of her brain are of interest, but I am decidedly not surprised.

Similar cases have been reported previously and we have a pretty good understanding of how and why this happens. If one part, even half, of the brain is damaged or fails to develop at an early embryological stage the brain still retains the ability to wire itself around the missing or damaged cortex.

This is due to the fact that brain development does not result from following a genetic blueprint. The brain is not built by following a plan the way a house is built from a bluprint. Rather, the genes determine the rules of development like an algorithm which then unfolds dynamically to create the brain. This process is called mapping.

Mapping allows for the brain to encode more information than the genes that code for building the brain. As the brain develops it maps itself to the body and to use. This way the genes do not have to tell how to wire up each retinal cell to its corresponding neuron in the visual cortex – they just encode for the process by which the visual cortex maps to visual input.

The earlier in the brain development process a piece of the brain is damaged or lost, the greater the capacity for the development process to remap and recover the otherwise lost function. This is a property of the brain known as plasticity, which we retain through adulthood but is maximal during embryological development.

There are cases, for example, of children born with one hemisphere missing but they are not paralyzed on the opposite side of the body – the side controlled by the missing half of the brain. This is because the surviving hemisphere picks up control of that side of the body.

This is also why there are cases of surgical removal of one hemisphere. Some children have a damaged hemisphere that is not functional. Whatever function they have is already moved over to the other hemisphere. Sometimes all the damaged hemisphere is doing is serving as a focus for seizures, so removing it will reduce or cure the seizures without causing any loss of function.

This new case is interesting because the researchers were able to determine how the child’s visual pathways remapped themselves to the surviving hemisphere. She was also born without a right eye, so we are talking only about the mapping of the left eye. Normally the medial part of the retina – the part closer to the nose – which sees left part of the world will map to the right hemisphere, while the temporal retina which sees the right half of the world maps to the left hemisphere (always opposite). In this case both halves of the retina follow different paths to different parts of the visual cortex in the left hemisphere. Therefore she can see both halves of the world.

Some of the reporting suggests that her vision, and even her neurological function, is “normal” or “full”.  While it is remarkably preserved, this is overstating it. She would still lack binocular vision, having only one eye. Also, she is weak on the left side, although she still has function. Also it is likely that her overall intelligence will be below average. She is making do without half a cortex, after all.

Cases like this are dramatic examples of the consequences of brain mapping and plasticity. These principles are already well established, however, and so there is nothing shocking or suprising about them any more. I was certainly surprised when I first heard of such cases in medical school – they are dramatic – but that was back in the early 90’s.

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