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Medical Cerebral Cortex

  1. May 6, 2009 #1
    Why does the cerebral cortex require a large surface area (demonstrated by the large number of folds and creases in it)? Surely the brain simply requires as big a volume as possible, to fit in more cells, as there is nothing outside the brain it needs to interact with.
     
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  3. May 6, 2009 #2

    Moonbear

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    That is correct, it's not so much the surface area as the large overall size, and even more important than that are the connections between and among cells. As brains get larger and more developed, they get more bends and folds in them, and sulci and gyri are just more examples of these bends and folds. Though, the cells do use cerebrospinal fluid (CSF) as a place to get nutrients, communicate with one another, and expel waste, so the "bumpy" surface does increase exposure to CSF. In that regard, surface area does become important. If the brain were one big solid mass and didn't have grooves and holes (i.e., ventricles), the CSF would be very slow to penetrate and cells deep inside might die before nutrients got to them.
     
  4. May 6, 2009 #3
    I didn't know that- I thought that the CSF just cushioned the brain.
     
  5. May 7, 2009 #4
    There's a long history of that in neuroscience. Anytime we see something in the brain that we don't understand, the first explanation is always "it's just there for cushioning". Frequently this turns out to be wrong.
     
  6. May 8, 2009 #5
    I was doing a bit of research in this area.
    It said that most of the brain's 'processing' cells are on the cortex, due to the way it evolved from much smaller animals, and the reason the cortex has a large surface area is because it has to fit as many of these in as possible. This also explains why, in MRI scans, you can see that there are the same folds on the inside of the cerebrum. What do you think
     
  7. May 9, 2009 #6
    According to the wiki article the cortex has six layers of different kinds of neurons:

    http://en.wikipedia.org/wiki/Cerebral_cortex
    So, if we take a brain and lay it out flat we'd have a sheet of a certain thickness to accomodate the six layers, and of a certain area. Do we make a more powerful processor by adding more thickness to each layer, or by adding more area to the whole? The argument that the human brain is a better processor is based on the latter idea: more neurons side by side make a better processor; greater square footage of its cortex. A smooth brain with no sulci and gyri could still have six layers of the different kinds of neurons, but the total square footage of the cortex would all be much smaller in area: you wouldn't have as many neurons to work with.

    So, apparently we're fine with the six layers at the thickness they presently have. A better brain would have more room to add neurons side by side: more surface area.
     
  8. May 9, 2009 #7

    atyy

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    Lissencephaly is bad in people, but I don't know if the lack of folds in the cortex is bad per se, or whether something bad causes the lack of folds.

    There are mammals in which lissencephaly is normal, including some primates.
     
  9. May 9, 2009 #8

    atyy

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    BTW, there is a funny article that appeared in "Science" some years back asking "Is your brain really necessary?". Massive hydrocephaly (not directly related to lissencephaly, just that in both conditions one has less brain) usually has in obvious behavioural effects. However, there are interesting cases eg. in which the patient did not even know he had massive hydrocephaly, finished a maths degree at some British university, and only found out later that he had essentially no brain after he went for a scan for some other minor thing.

    However, most people I've told the story to say they knew there was always something strange about mathematicians! :smile:
     
  10. May 9, 2009 #9

    atyy

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    I'm not really sure this is true - mice have thinner cortices than cats (1 versus 2 mm), and the thickness of each layer is different in different cortical areas. Nonetheless, there is an important, but unproven, idea that a fundamental computational unit in the cortex is the "column", and that columns in different cortical areas are variations of some "canonical microcircuit". Hence to get more computational power, one keeps the columnar architechture intact, and replicates it by expanding surface area.

    http://www.archive.org/details/redwood_center_inaugural_symposium_08
     
    Last edited by a moderator: Apr 24, 2017
  11. May 10, 2009 #10
    Yes, you are right: the number of layers in a column seems to get larger with more "advanced" animals:
    http://en.wikipedia.org/wiki/Cortical_column
    So, that is clearly essential. But getting more columns in, as you said, means adding more surface area.

    This story about finding out you don't have a cerebrum is an urban legend, and I first read it in one of Stephen King's novels, way back.

    A person can, in fact, live without one cerebral hemisphere, but there is always contra-lateral paralysis, and loss of all the functions of the missing hemisphere. Individual hemispheres can be selectively put to sleep in a WADA test. A guy I know on an Epilepsy forum had this done in preparation for surgery and he described how fascinating and frightening it was, especially when the left side was knocked out and he was reduced to a language-less, symbol-less perceiver.

    In a similar experience following a stroke, Jill Taylor describes life without a left hemisphere:

    https://www.amazon.com/My-Stroke-Insight-Scientists-Personal/dp/0670020745
     
    Last edited by a moderator: May 4, 2017
  12. May 10, 2009 #11

    atyy

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  13. May 10, 2009 #12
    This is not the Stephen King story, in which a guy goes to the doctor for headaches and a brain scan shows the guy just has a sort of brain stem flopping around in his cranium with no cerebral lobes at all.

    Regardless, these cases of massively enlarged ventricles are mighty peculiar. It's ascribed to "Berker 1985" but no journal is given. I would certainly like to read the original paper. Despite preserved intellect, and their own lack of awareness of any problem, did these people have no deficits or problems that were obvious to other people? If not, then I'd have to wonder if it was really hydrocephalus or a genetic mutation for really big ventricles.
     
  14. May 10, 2009 #13
    That article, I see, is online:

    www.toriah.org/articles/Lewin-1980.pdf [/URL]

    "There's a young student at this uni-
    versity," says Lorber, "who has an IQ
    of 126, has gained a first-class honors de-
    gree in mathematics, and is socially com-
    pletely normal. And yet the boy has vir-
    tually no brain."

    The story about the student with a 126 I.Q. and hardly any cortex might be a hoax by that Lorber to make his point. He admits to exaggeration and his colleages characterize him as "not very scientific".

    In the intro to "Phantoms In The Brain" Ramachandran confesses that, along with changing names to protect patient's identity he also "compiles" patients from the literature. This Lorber sounds to me like he might be doing something similar, but more fictional, to get attention for his ideas about recovery from hydrocephalus.
     
    Last edited by a moderator: Apr 24, 2017
  15. May 10, 2009 #14

    atyy

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    Google books doesn't let me read the relevant page of the bibliography either, and the article doesn't seem to be on PubMed. However, this PubMed citation might provide some clue:

    Dev Med Child Neurol. 1992 Jul;34(7):623-32. Reciprocal neurological developments of twins discordant for hydrocephalus. Berker E, Goldstein G, Lorber J, Priestley B, Smith A.

    Studies of 10 sets of twins discordant for hydrocephalus in early life revealed striking differences in degree and nature of development of verbal vs. non-verbal cognitive functions, birth order, and hand and eye preference. Despite similar (four dizygotic pairs) or identical (six monozygotic pairs) genetic endowment and grossly similar intra- and extra-uterine environmental and socio-economic influences, the consistency of the differences between the hydrocephalic children and their seemingly normal twins indicate systematic differences in pre-, peri- and/or early postnatal organization and development of hemispheric function. Follow-up studies also documented development of above-average intelligence, despite drastically reduced cerebral mantle size in hydrocephalus of early onset. The atypical patterns of development of the non-hydrocephalic twins also confirm previously described qualifications reported in studies of the significance of genetic vs. environmental factors in twins.
     
  16. May 10, 2009 #15
    Less is more?
     
  17. May 10, 2009 #16

    atyy

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    I wish! On the other hand, I'm quite happy being stupid. :smile:

    You are right to want good documentation for such things, but suppose Lorber isn't making stuff up, the reason I am not that surprised (after the first 5 minutes - the first time I saw Berker present his stuff, I thought, "Oh my god, I've been interpreting MR scans wrongly this whole time" - but no, I hadn't, and Berker's story was that this guy really had almost no brain!) is that there is more plasticity in younger people, and presumably such plasticity is sometimes able to overcome even gross anatomical defects. There's also plasticity in adults, but it's activated under less general conditions. In young people with "lazy eye", the more easily activated plasticity is actually bad, so they have to wear an eye patch prevent the plasticity. They can stop wearing the eye patch as adults once the plasticity is not so easily activated. I think one of the most fascinating lines of research is trying to harnass or activate the brain's plasticity so that it cures itself - we have many clinical case studies from stroke etc, but there is no underlying neuronal model that really predicts what sort of plasticity is or isn't possible.
     
  18. May 10, 2009 #17
    You have seen him give a presentation? Did he have any video of the guy? I have a hard time believing someone like this could present as normal. The "intelligence" isn't so much of a stretch, given autistic savants, but it's hard to believe he would look and act like any normal Joe: plasticity must have limits.
     
  19. May 10, 2009 #18

    atyy

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    I heard Berker gave this presentation: http://web.mit.edu/8.581/berker/ennis.html. He showed a video of the guy talking and running etc - seemed completely normal - then he showed the MR. Of course I wouldn't know if the video and MR did not come from the same guy, but at present, I believe that Berker was being honest. If I remember correctly, Berker did say that the guy has neurological deficits, but they can be picked up only by running the full range of tests.
     
    Last edited by a moderator: Apr 24, 2017
  20. May 10, 2009 #19
    Did the MR look to you as if the guy had almost no brain?
     
    Last edited by a moderator: Apr 24, 2017
  21. May 10, 2009 #20

    atyy

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    Yes, when I saw the MR that was supposed to be from the guy in the video, my first thought was that I had been reading MRs completely wrong (ie. mistaking brain for fluid)! I'm not a professional, but had looked at MRs before that just for fun, and thought I could at least tell brain from ventricle! Amazingly, there was nothing wrong with my rudimentary MR reading skills, and the guy really had (almost) no brain.
     
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