“Psychic cells” were silly in 1909. And they are still silly, regardless of what you call them.

Brodmann-areas

Here is Korbinian Brodmann (of cortical Brodmann area fame) writing about a trend towards assigning functional roles to single neurons based on anatomical type, back in 1909:

“There has been occasional talk of “sensory cells” located in particular regions, or of sensitive or sensorial “special cells”. People have invented acoustic or optic special cells and even a “memory” (*12) cell, and have not shied away from the fantastic “psychic cell”. Apart from the fact that such so-called “special cells” have only been described in young or foetal brain with the Golgi method and mainly only in animals, and therefore lack confirmation in the adult human brain, and quite apart from the fact that no attempt has been made to determine the precise regional location of the zone within which such cells appear exclusively, it seems to me that to pose this problem is wrong.” [emphasis added]

And here is a news item from a couple of years ago:

BigNeuron

Psychic cells indeed! Or perhaps we should call them zombie cells.

(Zombie concepts keep coming back from the dead to eat our brains. Other examples include ‘selfish genes’ and ‘pleasure molecules’.)

Which is the most evolutionarily advanced part of the human brain?

This is a potentially controversial issue, since there is no consensus yet on the evolution of the brain, beyond a very coarse-grained chronology. Broadly speaking, neocortical areas are new, hence the term “neo-cortex”. But among cortical areas, there is still some disagreement about which areas emerged most recently in primates.

Based on what we know about development in the womb, along with structural findings, my labmates, who are neuroanatomists, suggest that the “eulaminate” areas — the ones that have sharply defined layers — may be the most recent, evolutionarily, compared to the “agranular” and “dysgranular” cortices, which have less sharply defined layers. These less sharply defined areas are also labeled as “limbic”.

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How to navigate on Planet Brain

I was asked the following question on Quora: “How do you most easily memorize Brodmann’s areas?”. The question details added the following comment: “Brodmann area 7 is honestly where the numbering starts to seem really arbitrary.” Here’s how I responded:

Yup. The Brodmann numbering system for cortical areas is arbitrary. If you find a mnemonic, do let us know!

I’m a computational modeler working in an anatomy lab, so I confront the deficits in my anatomical knowledge on a daily basis! I can barely remember the handful of Brodmann areas relevant to my project, let alone the full list! I have a diagram of the areas taped up next to my monitor. 🙂

Neuroanatomists become familiar with the brain’s geography over years and years of “travel” through the brain. Think of it like this: what they’re doing is like navigating a city that doesn’t have a neat New York -style city block structure with sensibly numbered streets and avenues. Boston, where I live, is largely lacking in regularity, so one really has to use landmarks — like the Charles River, the Citgo sign, or the Prudential Center. The landmarks for neuroanatomists are sulci and gyri. Over time they learn the Brodmann area numbers. Only instead of a 2D city, neuroanatomists are mapping a 3D planet!


Over the years my lab — the Neural Systems Laboratory at Boston University — has developed a structural model that explains cortical areas and their interconnections in terms of cytoarchitectonic features. They don’t have a naming/addressing system, but at least they provide a way to make sense of the forest of areas!

Fig 1. Schematic representation of four broad cortical types. Agranular and dysgranular cortices are of the limbic type. Figure from [1].

The structural model [1,2] is based on the observation that the 6-layer nature of isocortex is not uniform, but varies systematically. The simplest parts of the cortex are the “limbic” cortices, which include posterior orbitofrontal and anterior cingulate cortices. Limbic cortices have around 4 distinct layers. The most differentiated parts of the cortex are the “eulaminate” cortices, which include primary sensory areas, and some (but not all!) parts of the prefrontal cortex, such as dorsolateral prefrontal cortex. Eulaminate cortices have 6 easily distinguished layers. [See Fig 1]. Interestingly, there is some evidence that the simplest cortices are phylogenetically oldest, and that the most differentiated are most recent.

Fig 2. Schematic representation of cortico-cortical projections. Figure from [2].

Every functional cortical hierarchy* consists of a spectrum of cortices from limbic to eulaminate areas. Areas which are similar tend to be more strongly connected to each other, with many layers linking to each other in a way that can be described as “columnar”, “lateral” or “symmetric”. Dissimilar areas are generally more weakly connected, and have an “asymmetric” laminar pattern of connections, in which projections from a less differentiated area to a more differentiated area originate in deep layers (5 and 6), and terminate in superficial layers (1,2 and 3). Projections from a more differentiated area to a less differentiated area have the opposite pattern: they originate in superficial layers (2 and 3), and terminate in deep layers (4,5 and 6). [See Fig 2.]

 For more on the details of the model, check out the references [1,2]. My boss, Helen Barbas, just submitted a short review about the structural model. When it is out I will append it to this answer.

To return to the city analogy, the structural model tells us that we can infer the (transportation/social/cultural?) links between pairs of neighborhoods based on what the two neighborhoods look like. If the structural model were true for cities, then neighborhoods that have similar houses and street layouts would be more closely linked that dissimilar neighborhoods. Similar neighborhoods would have one type of linkage (the “symmetric” type), whereas dissimilar neighborhoods would have another (the “asymmetric” type).

References

[1] Dombrowski SM, Hilgetag CC, Barbas H (2001) Quantitative architecture distinguishes prefrontal cortical systems in the rhesus monkey. Cereb Cortex 11: 975-988.

[2] Barbas H, Rempel-Clower N (1997) Cortical structure predicts the pattern of corticocortical connections. Cereb Cortex 7: 635-646.

Notes

* Heterarchy might be a better description than hierarchy.

Here’s a link to the Quora answer: How do you most easily memorize Brodmann’s areas?