The six layers of neocortex

The thing that makes us humans intelligent is a huge neocortex. It is the most recently developed area of the brain and it has a very interesting structure. The neocortex is a huge sheet build from six distinct layers folded in the brain many times. It takes a lot of space inside human skull, and the density of neurons per cubic measure is in order of magnitude smaller than in the cerebellum. The human brain is a wonder. Why does it even make sense?

The brain developed in 3 phases

It is instrumental to think that the brain developed in three phases. This model is called “triune brain”.  It is just a model. Recent studies based on paleontological data or comparative anatomical evidence strongly suggest that the neocortex was already present in the earliest emerging mammals.

First comes the primitive brain we share with reptiles (called reptilian brain), including basal ganglia structures like the thalamus and cerebellum. It is very complex, tightly packed with specialized neurons, and effectively dealing with all body functions.

Then there is the limbic system. It is shared by all mammals. It serves emotions and it modulates memory and learning, including face recognition (in amygdala). Some of the structures involved are the hippocampus and amygdala. When discussing memory, we usually refer to this particular brain.

The rest of the brain, and most of its volume is dedicated to the neocortex. Humans have a huge neocortex. It is associated with humans, but it existed before.

The geography of the neocortex is divided into lobes, which are further subdivided into cortical fields. The neocortex of early mammals had something like two cortical fields. Primates have significantly more complex neocortex: more cortical fields with complex connectivity. Possibly these structures were required to jump safely on tree branches. We may never know.

The human brain is x3 larger than the chimpanzee brain.  Moreover, a part of the brain called the cerebral cortex – which plays a key role in memory, attention, awareness and thought – contains twice as many cells in humans as the same region in chimpanzees. Networks of brain cells in the cerebral cortex also behave differently in the two species. So to study what it means to be human neurologically, we need to study the neocortex.

Neocortex developed from reptillian brain part called pallium. Birds are also smart and they have a smaller dedicated brain parts with functionality similar to neocortex. Honestly, I do not understand avian brain morphology.

The geography of neocortex

As a metaphor, I would address the human brain geographically. We can discuss countries and continents, which are the lobes and cortical fields. Not all the countries are mapped, and the continents may drift. Does it really matter that the frontal lobes include

Alternatively we can start digging through the six layers of neocortex, trying to understand the shape and the function. This is similar to digging into the earth core. We see that as we dig deeper we see different things.

We can also build super-fast tunnels, super-highways connecting various areas. In the brain there are such structures: associative connection connecting various brain region. They are white, colored with a protein accellerating neural signals up to x100.

Let us start digging

The upper layer of neocordex, Layer I, touches the membrane surrounding the brain also called pia mater. It contains several kinds of neurons, mostly dealing with GABA neurotransmitter. If it was a chip, we would talk about power supplies. Some of the neurons in this layer are superficial, and others go deep.

Layer II contains pyramidal neurons. These neurons are also found in the spine. Basically they collect sensory information in other formats and generate neural exitation. In a chip these would be A/D units or input voltage converters. Their ability to integrate information is limited.

Layer III is built of vertical neurons, similar to skip connections in the neural networks. They allow integration of unprocessed inputs with overly processed results of deeper layers.

Layer IV is the main processing layer. It contains several kinds of processing cells.  This layer integrates information and generates a different level of granularity or spatial resolution.

In medical  language Layer IV separates between supragranular layers (layers I-III), and infragranular layers (layers V and VI).

Layer V internal pyramidal layer, contains large pyramidal neurons, that interface senso motoric centers in basal ganglia. This layer contains also giant pyramidal cells called Betz cells, going to the spinal cord.

Layer VI, the polymorphic or multiform layer, is basically a converter, establishing column-by-column communication between neocortex and the thalamus.

(If you want to memorize, consider computer chip metaphor as a logical marker)

The secret of the IVth layer

OK, so we understand that the main magic happens in the IVth layer of the neocortex. What is its structure?

Layer IV, the internal granular layer, contains different types of stellate and pyramidal cells, and is the main target of thalamocortical afferents from thalamus type C neurons (core-type ) as well as intra-hemispheric corticocortical afferents.

What does that even mean? Basically it means that the layer integrates information all over the neocortex. While other layers extablish vertical connectivity, this particular layer is busy with horisontal connectivity through the cortical field, as well as associative connections between the cortical fields.

Type C neurons are gray as they are not accelerated and they have short axons. They are the computation units of our neural network. Corticocortical afferents can be compared with cache memory units, and occasionally we can poke them with advanced microscopes.

The IVth layer is well hidden between the preprocessing and postprocessing units, and it is translating dense low-quality information into sparse high-quality outputs.

Just how large is neocortex?

In the human brain it is between 2 and 4 millimetres thick, and makes up 40 per cent of the brain’s mass. 90 per cent of the cerebral cortex is the six-layered neocortex with the other 10 per cent made up of allocortex. There are between 14 and 16 billion neurons in the cortex, and these are organized radially in cortical columns, and minicolumns, in the horizontally organized layers of the cortex. 

Neurons within the cortical minicolumns”receive common inputs, have common outputs, are interconnected, and may well constitute a fundamental computational unit of the cerebral cortex”. Minicolumns comprise perhaps 80–120 neurons.

So basically we are talking about 150 million independant microprocessor cores intricately interconnected with each other. For comparison, Titan-V GPU has 5120 cuda cores. But the GPU clock is 1455MHz, while our brain’s clock is probably below 100Hz. If our brain was significantly faster, it would probably heat up and explode.

(Numerical memorization is fun. Give it a try)

Lobes and fields


(The brain is a special sort of mental palace once you learn to know it)

Frontal lobe

The frontal lobe contains most of the dopamine-delicate neurons in the cerebral cortex. The dopamine system is associated with reward, attention, short-term memory tasks, planning, and motivation. Dopamine tends to limit and select sensory information arriving from the thalamus to the forebrain. 

We love to talk about it. There are four principal gyri in the frontal lobe. The entirety of the frontal cortex can be considered the “action cortex”. It deals with commands for vision (for example Frontal eye fields control saccading), language (for example Broca area), motoric processing, declarative memories. The fronal lobe is huge, well-mapped and we love to talk about it.

Parietal lobe

The parietal lobe integrates sensory information among various modalities, including spatial sense and navigation (proprioception), the main sensory receptive area for the sense of touch (mechanoreception) in the somatosensory cortex which is just posterior to the central sulcus in the postcentral gyrus, and the dorsal stream of the visual system. The major sensory inputs from the skin (touch, temperature, and pain receptors), relay through the thalamus to the parietal lobe.

Several areas of the parietal lobe are important in language processing. The somatosensory cortex can be illustrated as a distorted figure — the homunculus (Latin: “little man”), in which the body parts are rendered according to how much of the somatosensory cortex is devoted to them.

This is a great metaphor. All of our body sensors are mapped to one brain part and the information is fused between sensors for further processing. The structure of this brain part is a mirror of our body structure.

Occipital lobe

The occipital lobe is divided into several functional visual areas. Each visual area contains a full map of the visual world.  The first functional area is the primary visual cortex. It contains a low-level description of the local orientation, spatial-frequency and color properties within small receptive fields. 

The ventral stream is known for the processing the “what” in vision, while the dorsal stream handles the “where/how.” This is because the ventral stream provides important information for the identification of stimuli that are stored in memory. With this information in memory, the dorsal stream is able to focus on motor actions in response to the outside stimuli.

Recent studies have shown that specific neurological findings have affected idiopathic occipital lobe epilepsies. Occipital lobe seizures are triggered by a flash, or a visual image that contains multiple colors.

Basically we have a huge visual coprocessor, that is so complex that it can collapse from overstimulation. It transcodes visual information from high-resolution noisy data into low-resolution well-integrated knowledge represenation.

Temporal lobe

The temporal lobe is involved in processing sensory input into derived meanings for the appropriate retention of visual memories, language comprehension, and emotion association. Within the temporal lobe is an area of the brain called the hippocampus which is associated with forming new memories and learning new things.

Here we generate memories and deal with time and logic… Somehow this area is sensitive to rhythms and music. A problem here may cause hallucination, mood swings, seizures. It is one of the most protected areas of our brain.

Limbic lobe

As I wrote before, this is not a part of neocortex, but a part of limbic system connected to neocortex. Our emotions color the way we process reality, modulating processes in each of the neocortex columns.

Why does that matter?

The brain is very complex, but the neocortex itself is morphologically simple. There is no one specific area responsible for visual processing and learning. Each new memory results from complex responses in all brain lobes.  The largest part of the neocortex does not even deal with processing: it deal with connectivity to other brain areas. Interestingly, our GPU chips and our brain neocortex are somewhat similar in their morphology.




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