Cerebral Cortex

jamesb-uk

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.

Moonbear

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.

jamesb-uk

Quote by Moonbear

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.

I didn't know that- I thought that the CSF just cushioned the brain.

Cincinnatus

Quote by jamesb-uk

I didn't know that- I thought that the CSF just cushioned the brain.

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.

jamesb-uk

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

zoobyshoe

Quote by jamesb-uk

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

According to the wiki article the cortex has six layers of different kinds of neurons:

After the work of Korbinian Brodmann (1909), the neurons of the cerebral cortex are grouped into six main layers, from outside (pial surface) to inside (white matter):

1. The molecular layer I, which contains few scattered neurons and consists mainly of extensions of apical dendrites and horizontally-oriented axons, as well as glial cells[4]. Some Cajal-Retzius and spiny stellate neurons can be found here.
2. The external granular layer II, which contains small pyramidal neurons and numerous stellate neurons
3. The external pyramidal layer III, which contains predominantly small and medium-size pyramidal neurons, as well as non-pyramidal neurons with vertically-oriented intracortical axons; layers I through III are the main target of interhemispheric corticocortical afferents, and layer III is the principal source of corticocortical efferents
4. The internal granular layer IV, which contains different types of stellate and pyramidal neurons, and is the main target of thalamocortical afferents as well as intra-hemispheric corticocortical afferents
5. The internal pyramidal layer V, which contains large pyramidal neurons (such as the Betz cells in the primary motor cortex); it is the principal source of subcortical efferents
6. The multiform layer VI, which contains few large pyramidal neurons and many small spindle-like pyramidal and multiform neurons; layer VI sends efferent fibers to the thalamus, establishing a very precise reciprocal interconnection between the cortex and the thalamus (Creutzfeldt, 1995).

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.

atyy

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.

atyy

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!

atyy

Quote by zoobyshoe

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.

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/redwo...l_symposium_08

zoobyshoe

Quote by atyy

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.

Yes, you are right: the number of layers in a column seems to get larger with more "advanced" animals:

The dolphin cortical column is composed of 5 layers, the reptilian cortex is composed of three.

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.

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!

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:

Taylor: When the cells in my left brain became nonfunctional because they were swimming in a pool of blood, they lost their ability to inhibit the cells in my right hemisphere. In my right brain, I shifted into the consciousness of the present moment. I was in the right here, right now awareness, with no memories of my past and no perception of the future. The beauty of La-la land (my right hemisphere experience of the present moment) was that everything was an explosion of magnificent stimulation and I dwelled in a space of euphoria. This is great way to exist if you don't have to communicate with the external world or care whether or not you have the capacity to learn. I found that in order for me to be able to learn anything, however, I had to take information from the last moment and apply it to the present moment. When my left hemisphere was completely nonfunctional early on, it was impossible for me to learn, which was okay with me, but I am sure it was frustrating for those around me. A simple example of this was trying to put on my shoes and socks. I eventually became physically capable of putting my shoes and socks on, but I had no ability to understand why I would have to put my socks on before my shoes. To me they were simply independent actions that were not related and I did not have the cognitive ability to figure out the appropriate sequencing of the events. Over time, I regained the ability to weave moments back together to create an expanse of time, and with this ability came the ability to learn methodically again. Life in La-la land will always be just a thought away, but I am truly grateful for the ability to think with linearity once again.

http://www.amazon.com/My-Stroke-Insi.../dp/0670020745

atyy

Quote by zoobyshoe

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.

In addition to "Is your brain really necessary?", you can also try the Berker citation here (ignore the rest of the book): http://books.google.com/books?id=hAe...over#PPA234,M1.

zoobyshoe

Quote by atyy

In addition to "Is your brain really necessary?", you can also try the Berker citation here (ignore the rest of the book): http://books.google.com/books?id=hAe...over#PPA234,M1.

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.

zoobyshoe

That article, I see, is online:

www.toriah.org/articles/Lewin-1980.pdf

"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.

atyy

Quote by zoobyshoe

It's ascribed to "Berker 1985" but no journal is given. I would certainly like to read the original paper.

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.

zoobyshoe

Quote by atyy

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:

" Follow-up studies also documented development of above-average intelligence, despite drastically reduced cerebral mantle size in hydrocephalus of early onset."

Less is more?

atyy

Quote by zoobyshoe

Less is more?

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

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.

zoobyshoe

Quote by atyy

- 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!)

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.

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jamesb-uk	Cerebral Cortex Tweet 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.

atyy Recognitions: Science Advisor	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.