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.

11 responses so far

11 Responses to “Girl With Half a Brain”

  1. artfulDon 22 Jul 2009 at 1:51 pm

    The full report on this case is available here:

    Note mention of the role played by epigenetic processes as well as genetic.

  2. Steven Novellaon 22 Jul 2009 at 2:38 pm

    artfulD – that’s the same link as the one I provided above. It is still preliminary online publication in advance of the final version.

    The mapping process I spoke of is epigenetic in the way that all developmental factors are.

  3. artfulDon 22 Jul 2009 at 2:50 pm

    Not to quibble but your link goes to the abstract, which then will have a link to the 13 page and remarkably detailed pdf file.

  4. artfulDon 22 Jul 2009 at 3:07 pm

    Also you stated: “Rather, the genes determine the rules of development like an algorithm which then unfolds dynamically to create the brain. This process is called mapping.”
    Which seemed to leave the role that epigenetics plays somewhat out of the picture – and in my view (and apparently yours as well) it was epigenetics that facilitated the algorithmic functions here.

  5. Steven Novellaon 22 Jul 2009 at 4:14 pm

    To clarify, the mapping algorithm is interactive with the environment, other parts of the body, and the brain itself – it does not unfold blindly. Therefore there are non-genetic factors that play a role.

  6. artfulDon 22 Jul 2009 at 4:26 pm

    In my view, it’s a strategic algorithm (or set of algorithms) that allows the process to adapt itself to the structure that will be available as the framework which will support the installation and activation of the strategic system. A necessary process in that individual primates especially will have diverse physical components that the brain will have to adapt to.

  7. Woodyon 22 Jul 2009 at 8:23 pm

    Doesn’t it seem odd that this girl didn’t have any brain imaging before age 3, since she was known to have microphthalmos and asymmetric motor function?

    This is interesting, but as Dr. Novella points out, not unique. I would be more interested in knowing how her other neurological functions are doing that normally map to the right hemisphere.

  8. superdaveon 22 Jul 2009 at 9:25 pm

    I remember learning about this process in a neuroscience class. It’s a brilliant example of how self sorting systems DON”T need a designer to achieve complexity.

  9. artfulDon 22 Jul 2009 at 10:14 pm

    I see it as a brilliant example of how our strategic systems are able over time to “design” themselves.

  10. HHCon 22 Jul 2009 at 11:27 pm

    Its a pleasure to read about this case, photos are impressive. For your information, The Serper method for cognitive retraining has a story and questions about a case involving a female patient who required surgical removal of half the brain due to a severe seizure disorder. Its content is for rehabilitation patients.

  11. sonicon 23 Jul 2009 at 5:11 am

    The reason that it is being called surprising is that the researchers found unexpected (and therefore surprising) responses–

    From the article (page 4 of 6)

    “Perhaps the most unexpected response
    is the rerouting of optic nerve fibers in the chiasm because the
    decision to cross or to stay on the ipsilateral side is thought to
    depend on molecular cues (Eph-B1, Eph-A5–6) …”

    I don’t know enough about what they expected to find to be able to judge the amount of surprise this unexpected response engenders.

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