Part Of: Neuroanatomy sequence
Followup To: The Thalamocortical Plasma Globe
Content Summary: 1100 words, 11 min read

Cortical Area & The Obstetric Dilemma

Last time, we learned that the brain is organized like a plasma globe: a sphere within a sphere. Today, we’ll be exploring a technique for reasoning about the cerebral cortex, or “outer sphere”. A few things you should know about this organ:

• It weighs about a pound.
• Stretched flat, it would cover an area of about 2.5 square feet.
• It is about six millimeters thick.
• It houses 20 billion neurons.

Does your neocortex have the most neurons? No, that title goes to the cerebellum, whose 100 billion neurons coordinates complex movements. But your brain must do a lot more than motor fine-tuning. Your brain perceives its environment, identifies objects, sets goals, makes decisions, feels emotions, and experiences consciousness. Where does your brain perform these tasks?  Primarily in the neocortex! Loosely speaking, the outer sphere does much of the “heavy lifting” for your brain.

Why is the human brain so wrinkled? After all, not all species have brains with this shape:

Consider the following evolutionary pressures, together known as the obstetric dilemma.

• A bigger neocortex does more work, and homo sapiens make its living (ecologically speaking) by intelligence. Thus, natural selection will select towards increased cortical area.
• Compared to other animals, human childbirth is a uniquely dangerous affair. Why? Brain size. Thus, natural selection will select away from increased brain volume.

As any microchip designer can tell you, wrinkles are a way to increase surface area, while holding volume constant!

Flattening The Lobes

And now, a short story. Cognitive neuroscience is rife with Latin terms for cortical areas. There are hundreds of them: “anterior cingulate cortex”, “fusiform gyrus”, “temporo-parietal junction”, etc. Over the past few years, as I consumed more of the field, I had slowly acclimated to hypotheses regarding the functions and interrelations for such areas. But given my lack of robust anatomical intuitions, these Latin names were just linguistic markers; I lacked an appreciation for geography.

While 3D models of different locations were mildly helpful, the pieces really fell into place when I discovered cortical flat maps.

While the neocortex is like a sheet, in some ways it is more accurate to imagine two sheets. Most people know that the brain has two hemispheres, but fewer know that these hemisphere’s are not (directly) connected? The two halves of your brain do talk to one another, but via a subterranean tunnel known as the corpus callosum.

Okay, here it is: a flat map of the human brain.

The brain is often divided into four to six different sections, or lobes. My map colors each lobe, illustrating the relationships between 3D brain and 2D map.

Here are my five lobes:

1. Occipital (red, back of the brain). This lobe is primarily responsible for vision.
2. Parietal (yellow, top of the brain). This lobe is primarily responsible for touch.
3. Temporal (green, sides of the brain). This lobe is primarily responsible for concepts.
4. Frontal Lobe (blue, front of the brain). This lobe is primarily responsible for personality.
5. Limbic Lobe (purple, underneath the brain). This lobe is primarily responsible for emotion.

Flat maps are an underappreciated resource. But I’ll return to this point some other day. In the meantime, I should mention that these flat maps are imprecise elaborations of the flat maps created by the Gallant Lab. That is, my creations strike me a bit like this:

I am planning to ultimately construct more precise flat maps using tools like Freesurfer.  But in the meantime, I’ll stick with my quick-and-easy 16th century cartographic approach. 🙂 

Primary vs Association Cortex

Let’s turn now to perception. In Tunneling Into The Soup, we discussed how sense organs (e.g., the rods and cones in your eye) translate the physical reality outside your body into neuron-compatible signals that your brain consumes. To make this intuitively compelling, I employed the following metaphor:

Your eye transmits data to your brain via information highways known as the optic nerves. The traffic of these highways – sense data – captures only a subset of physical reality; call this subset an umwelt. Such highways drain their contents onto landing sites on the cortex known as primary areas.

Aristotle is wrong: you have more than five senses. Balance and body alignment are senses in just the same way as sight, hearing, touch, taste, and smell. I am currently aware of nine different types of sense, or sensory modalities.  But of these, three sense modalities are particularly important (and consume more space!) in the human brain. They are:

1. Body sense-data. This area includes touch, and other senses associated with body sensation and position.
2. Visual sense-data. Nerves that leave your eyes deliver their data here.
3. Audio sense-data. Nerves that leave your ears deliver their data here.

These three primary areas of your cortex are marked in dark green:

Non-primary areas of the cortex (areas that are not dumping grounds) are called association areas. Association cortex is associated with two distinct functions:

1. Non-modal computations like goal generation.
2. Multi-modal computations such as “hand-eye coordination”.

Evolutionary Considerations

How does the human brain compare to that of other species? Consider again the mouse brain. It is obviously smaller than the human variety, but is the ratio of primary-vs-association cortex preserved?

It turns out that the answer is no. Because primary sensory processing is more immediately useful to survival, the neocortex of the mouse is actually dominated (>50%) by primary areas.

This biological fact reminds me of the (amusing) Expanding Earth conspiracy theory. In contrast to plate tectonics, the Expanding Earth claim is that continent size has remained constant, whereas the oceans have been expanding. Despite being complete bunk, it does provide a colorful metaphor to our genetic distinctions from the mouse. Our species has invested more heavily in association cortex (oceans) than primary cortex (continents).

Have humans invested equally in every part of the association cortex? No: the hominid line shows pronounced (6x) gains in the prefrontal cortex, and comparatively restrained growth elsewhere. We thus have reason to believe that most uniquely human behaviors must be supported by the prefrontal cortex. To be continued.

Takeaways

• The outer sphere of your neural plasma globe – your neocortex – is responsible for nearly all of your higher thought.
• Your neocortex covers an area of about 2.5 feet. Flat maps of the neocortex allow us to intuitively visualize the brain.
• Your neocortex contains primary (direct sensory input) and association (abstract computations) areas.
• Humans have more association areas, especially in the prefrontal cortex. This hints at a basis for our unique abilities.