A Grey Anatomy: Mapping the Cortex

This is the first of a series of occasional postings drilling down deeper into the structure of the brain.

Show Me the Map

In chapter 2 of The Master and His Emissary, Iain McGilchrist sets out to describe what the left and right hemispheres of the brain do. It generally reads as a long list of ‘such-and-such a behavior is to be found in this particular lobe/cortex/gyrus of the left/right hemisphere’. The chapter reads in a similar way to how a South Londoner, countering the North London dominance, might argue about how wonderful and underestimated it is South of the river. What McGilchrist is trying to do here is to provide scientific weight to his argument (of the importance of the role of the often-underestimated right hemisphere), and so distance himself from accusations of the usual left brain/right brain quackery. In the context of the argument, the many of the particular details are not relevant. It is the weight of the citations that is important.

Iain McGilchrist’s ‘The Master and His Emissary’, Yale Press

But the basic facts are interesting nonetheless and the lay reader may well be interested as in those particulars of where in the brain particular seem top be performed. My initial frustration reading the chapter was basically ‘just show me a map’, but I realized this wouldn’t really help. It was as if the South Londoner was making his case to a visitor from Mars, who had no concept of what a city even is. The Martian might hear repeated reference to words like ‘park’, ‘road’ and ‘hill’ and hence deduce they were particular geographical artifacts, so that Richmond Park is a particular instance of a park. Similarly, the casual reader may infer from references to ‘anterior cingulate gyrus’ and ‘superior temporal gyrus’ and so on that a gyrus is a particular brain feature, without any idea of what a gyrus actually is. ‘It’s all Greek to me’ – in this case, literally, since the medical terms are built up from Greek word roots.

Some basic orientation around the brain (actually, just the cerebrum is covered here) is needed in order to know what the ‘geographical’ features on a map of the brain are. So here is a basic introduction, to lay the foundations before wandering around Wikipedia to be able to then take over.

Orientation

Firstly, the basic anatomical North/South/East/West directions:

  • dorsal (in the context of the brain, this is the top) vs ventral (“belly”, bottom of brain). Imagine the dorsal spine running from the nose, over the head and down the back.
  • anterior (front) vs posterior (back).
  • distal (distant to) vs proximal (close to body); of limbs, for example.
  • superior (upper) vs inferior (lower).
  • rostral (towards the nose/“beak”) vs caudal (towards the end of the spine/“tail”). In human brain terms, rostral corresponds to anterior and caudal corresponds to inferior.
  • lateral (towards the outside) vs medial (towards the middle plane; in the context of the brain, towards the plane that dissects the brain into left and right hemispheres).

And these can be combined e.g. dorsolateral: at the top, just to one side.

Physical location: Overall

Imagine the developing embryo as a tadpole; its central nervous system (CNS) comprises, from head to tail:

  • the forebrain: uppermost; this will develop into parts of the brain including the cerebrum, thalamus and hypothalamus.
  • the midbrain
  • the hindbrain: lowermost; this will include the cerebellum (“little brain”).
  • the spine.

(And beyond this, the peripheral nervous system (PNS) spreads nerves around the body.)

The cerebrum (from which: cerebral) is the major part of the forebrain, comprising:

  • the cerebral cortex: the ‘grey matter’ (in contrast to the white, below).
  • the white matter, and
  • the basal ganglia (at the base of the forebrain).

As a simple analogy, think of the cerebrum as a strange variation on the theme of cauliflower cheese:

  • two soggy cauliflowers, squashed together (representing the white matter and basal ganglia).
  • processed cheese sheets (representing the cortex “bark”/”rind”/”shell”) are wrapped around the two cauliflowers. In small mammals, the cortex is thin – like cling film – such that the brain then has a smooth surface. But in humans, the cortex is thick (2-4mm thick) and large (the total surface area of the human cerebral cortex is 2,500 cm^2, equivalent to a 50cm x 50cm sheet, representing a large proportion of the total brain volume). It manages to be so large by folding into the gaps between the florets of the cauliflower, such that more than two-thirds of it is buried in these folds, typically 2cm deep.

The two halves of the cauliflower are the two hemispheres, which are divided by the medial longitudinal fissure on the sagittal plane.

Physical location: Lobes, Gyri and Sulci

source: Wikipedia

Looking from the outside, each hemisphere is divided into 4 major lobes:

  • Frontal lobe (at the front; 41%)
  • Parietal lobe (on the top; 19%)
  • Temporal lobe (at the side; 22%)
  • Occipital lobe (at the back; 18%)

The lobes are named after the various bones in of the skull which cover them. I presume the correspondence between lobes and bones is not coincidental. The percentages refer to the relative volumes as a proportion of all 4 lobes.

All lobes have the characteristic wrinkles of the human brain, which create definable regions for reference, named after the various:

  • gyri (“ridges”, singular “gyrus”), and
  • sulci (“furrows”, singular “sulcus”), typically 2cm deep.

The furrows between lobes are generally deeper than other sulci and often called fissures and may themselves have sulci. Such fissures present of the outside of the brain are:

  • the central sulcus: separates the frontal and parietal lobes.
  • the lateral sulcus: separates the temporal from frontal and parietal lobes.
  • the parieto-occipital sulcus: separates the parietal and occipital lobes.
  • the small preoccipital notch: essentially the temporal/occipital fissure.

Tucked behind the lateral sulcus is a small ‘lobule’ – the Insula.

Image: Click here: Gray’s transverse cross-section view of the hemisphere (source: Wikipedia) shows the insula tucked within the lateral sulcus between the temporal and frontal/parietal lobesing. (It also shows the many gyri and sulci of the lobes, and the white matter of the corpus callosum, connecting across to the other hemisphere, and of the base of the peduncle down to the lower brain.)

Looking at the brain from the sagittal plane (with the other hemisphere removed), those 4 lobes cannot be distinguished, but an additional lobe at the bottom is sometimes distinguished: the Limbic lobe.

The Corpus Callosum and other Commissures

As well as going down to lower parts of the brain, the ‘cauliflower’ white matter also connects the two hemispheres together: commissures are the connections (bundles of fibres) between the 2 halves of the brain. The corpus callosum (‘tough body’; also referred to as the callosal commissure) is the largest and most well-known. It is as if the cauliflower stems are split with one half of each bent to connect the two hemispheres (with the walnut-sized thalamus wedged within the split). Its approx 6cm2 cross-sectional area is squashed flattish and wraps around the front, top and rear of the left and right lateral ventricles where the hemispheres join. These lateral ventricles are reservoirs of cerebro-spinal fluid in which the brain and spine ‘float’ to help protect it from its bony packaging. Between the two lateral ventricles, where the hemispheres join is a thin membrane called the septum lucidum (c.f. the septum that separates the nostrils).

The corpus callosum is separated into various regions, in order from front over top to back:

  • Rostrum: “beak” pointing backwards and down, underneath the front of the ventricles.
  • Genu: Basically connecting the two frontal lobes. The connecting fibres are thinner here than elsewhere (implying slower communication than other corpus callosum connections).
  • Truncus: Basically connecting the two parietal lobes.
  • Splenium: Basically connecting the two occipital lobes.

3 of the 4 other commissures connect the halves in the lower parts of the brain, below the cerebrum. The remaining commissure of interest here is the anterior commissure. At the front, underneath the ventricles, it is about 1/10th the size of the corpus callosum and basically connects the two temporal lobes.

Image: Click here: Gray’s medial view of the brain (source: Wikipedia), showing the parts of the corpus callosum and the anterior commissure.

Functional Location: The One and Many Cortex

When we hear reference to things like the ‘Primary Motor Cortex’ and ‘Somatosensory Cortex’, we might be inclined to asked ‘what is a cortex?’ But the cortex refers to the whole of the grey matter. The ‘Primary Motor Cortex’ would be better described as the ‘Primary Motor function part of the cortex’. Cortex as an indicator of location points to an area, irrespective of how that part of the cortical sheet is folded up to fit into our heads. Prominent ‘cortexes’ with crude function are:

  • decision-making: in the orbitofrontal cortex (OFC) in the frontal lobe.
  • complex language processing: in the prefrontal cortex (PFC) in the frontal lobe.
  • risk/fear: ventromedial prefrontal cortex in the frontal lobe.
  • planning, organization: dorsolateral prefrontal cortex (DLPFC) in the frontal lobe.
  • movement: in the primary motor cortex and premotor cortex in the frontal lobe.
  • touch and proprioception: in the primary somatosensory cortex (S1) and secondary somatosensory cortex (S2) in the parietal lobe.
  • planned movements: in the posterior parietal cortex in the parietal lobe.
  • vision: in the primary visual cortex (V1) in the occipital lobe. There are other ‘visual cortical areas’ associated with vision with designations but not names e.g. V2, V3, V4.
  • hearing: in the primary auditory cortex in the temporal lobe.
  • body regulation: in the anterior cingulate cortex in the limbic ‘lobe’.
  • human awareness: in the posterior cingulate cortex in the limbic ‘lobe’.

In many cases, knowledge of the name and lobe of the cortex gives a good idea of the approximate location of the function, e.g. the entorhinal cortex and perirhinal cortex are near the rhinal gyrus.

Area Designations: Brodmann Postcodes

Just as a geographic location can be described in one of:

  • physical terms (e.g. Ludgate Hill),
  • political terms (‘St Paul’s’)
  • or a postal area designation (‘London EC4’),

a location of the cortex can be described in one of:

  • a physical location (‘precentral gyrus’),
  • a functional location (‘primary motor cortex’)
  • or an arbitrary location designation (Brodmann Area 4)

In both examples, there need be no agreement between the boundaries of these different terms; a functional area does not need to coincide with a physical area.

The Brodmann Wikipedia page maps the Brodmann areas (numbered between 1 and 52) to approximate physical or functional locations.

Expanding the Cortex

It is difficult to visualise the shape of the cortex with it in its normal, compact, wrinkled state. Some videos of transformations of the human cortex by Marty Sereno and others at UCSD help in this regard. Note: The videos linked to below here are all extremely short; if viewing through Windows Media Player, I suggest you slow them down and put them on repeat play. Pictured here are still images captured from these videos to help see details.

The first video shows an expansion of the cortex from its normal state to create a larger structure with a smooth surface. The view of the hemisphere is lateral (from the outside side of the head). Areas that are gyri before such expansion are coloured green; areas that are sulci/fissures are coloured red. The still pictures are from near the start and near the end of this expansion.

Lateral (side) view of cortex after slight expansion

Lateral (side) view of expanded cortex

It is still possible to discern the horn of the temporal lobe and the (red) central sulcus (separating frontal and parietal lobes) from this expanded view, and imagine how it could be folded back in by pressing in on the red parts. The second and third videos show this same expansion from a medial (from the inside side of the head, with the other hemisphere taken away) and ventral (from the underside of the brain) views respectively, this time with gyri coloured light grey and sulci coloured dark grey (but the occipital lobe with different colouring).

Medial view of the cortex

Medial view of expanded cortex

Ventral view (view from underside) of the cortex

It is disappointing (for me, as it was particular what I was wanting to see) that these views don’t indicate where and how the white matter of the basal ganglia and commissures break through the cortex.

In the fourth video, Sereno transforms the irregularly-shaped expanded cortex into an ellipsoid.

Medial view expanded and transformed to ellipsoid

This then allows a coordinate system to be imposed on the cortex, as shown in the fifth video.

Ellipsoid grid imposed onto expanded cortex, lateral view

The grid appears far from regular when the cortex is in its original compact form.

Grid imposed onto collapsed cortex, lateral view

The final Sereno video shows the morphing between two different people’s brains (‘Subject A’ and ‘Subject B’). Because of the different shape of the brain and skull of two individuals, the same cortex coordinate (which may or may not map to the same function, which is an entirely different matter), may get folded up to be on a gyrus in one brain but a sulcus in another…

Subject ‘A’ gyri/sulci pattern

Subject ‘B’ gyri/sulci pattern

Show Me the Globe

Two-dimensional maps only ever provide a distorted view of reality; a globe often provides a far better insight into the shape and size of Earth’s features. A 3-D model of the brain will be better than Gray’s drawings, but it may be difficult to see what’s going on in the folded-up cortex unless we can take that model apart. Expanding the cortex is one way around that.

If we want some sense of how close two places in the world are, the best information to have is their longitude and latitude coordinates. Knowledge of what countries the places are in (or were in pre-1914 or pre-1989) may be useful but is not going to be as good.

Similarly, if we want to map brain functions onto a location in a generic human brain, and map white matter connections between parts of the cortex, it would seem more appropriate to refer to a longitude/latitude coordinate system on an idealized ellipsoid rather than to ridges and furrows of the folded-up cortex.

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