Explaining the Cerebral Cortex

I was asked by some of the administrators of the Frontiers Journal to write a short article explaining how the cerebral cortex works.  I was hoping that this would be the first of a series of articles trying to explain, to a younger audience (8-12 years), the workings of the brain and how the same relates to autism.   The effort was dropped by the Frontiers Journal but hopefully I will be able to pursue the same in a series of blogs at Cortical Chauvinism.

Abstract

What allows you to understand the text that you are reading? What generates our judgment, morals, and intellect?  That job belongs to the cerebral cortex, a part of the nervous system that covers the brain like the rind of a fruit.  Its jelly-like consistency betrays the presence of billions of cells whose ultimate goal is to characterize information derived from within ourselves as well as from our immediate environment.  The versatility of the cerebral cortex for analyzing information is due, in part, to its modular organization.  The ability of cortical modules to connect and combine their actions helps explain how the cerebral cortex works under normal conditions and how it fails to do so in some medical disorders.

Introduction

The young woman laid unresponsive in her hospital bed. Although she should have been a healthy 18-year-old girl, she now found herself unconscious and unable to control the movements of her extremities.  Before hospitalization, she had been restricting the amount of food that she ate and misusing laxatives.  Her eating behavior proved unhealthy, weakening the muscles of her heart and causing it to momentarily stop beating.  The lack of blood circulation resulted in widespread brain damage, specially to its outer rim or cortex.  Although seemingly unaware of her environment and mute, she could open her eyes, momentarily follow moving objects in her room, and swallow food.

Clinical observations of patients with brain lesions allow us to characterize the functions of a given area by comparing performance before and after injury.  In the case of our patient, the cerebral cortex appears responsible for processing Information related to thinking, making us aware of our environment, and the production of language.  Without the cerebral cortex a person would be unable to purposefully respond to either visual, auditory or painful stimuli.

Trying to analyze how the cerebral cortex allows us to interact with the environment is a difficult task. Scientists have attempted to simplify this endeavor by dividing the cerebral cortex into ever smaller pieces.  They hope that by putting the pieces together, they will learn how the brain works (textbox 1).  In the case of the cerebral cortex, dissecting the structure into microscopic pieces reveals the presence of billions of cells whose combined actions account for our thinking process.  The specialized brain cell that receives, processes, and transmits information is called a neuron.  People tend to think of the neuron as the representative cell of the brain. However, it is important to note that neurons vary among themselves in regards to size, shape, location, and function.  The large variability denotes the existence of multiple types of neurons rather than a gradual transition of the same cell type into a spectrum.

humpty dumpty

Textbox 1.  A nursery rhyme by Mother Goose goes,

“Humpty Dumpty sat on a wall,

Humpty Dumpty had a great fall;

All the king’s horses and all the king’s men

Couldn’t put Humpty together again.”

Scientists usually dissect large structures into smaller ones.  Humpty Dumpty broke into many pieces when he fell off a wall.  Scientists are like the king’s men attempting to put Humpty Dumpty back together again. They hope that in the process of putting the pieces together they will learn how the larger structure worked. In science this is known as the reductionist approach.

Over the decades, the term neuron has served to describe an idealized type of cell.  According to this notion, stimuli are conducted in only one direction from one end of the cell to another.  However, the presence of many exceptions to this rule has proven to be a major headache to scientists.  We can’t generalize the functions of a neuron to every nerve cell of the brain.  Furthermore, the role of the neuron as the sole cell capable of information processing has been challenged.  We now know that there are other supporting cells whose joint actions provide for the processing of information within the brain.

Neurons do not work in isolation but are dependent on other neurons for their continued survival.  Attempts at growing neurons within a culture plate in the laboratory will fail unless a critical number of these cells are present and able to establish close contact with each other.  Animals whose genes have been manipulated so as to prevent neurons from establishing contact with each other, exhibit massive cell death.  Such animals usually die early on in life. This interdependence of neurons requires that multiple cells work together in order to carry out their function.

Modules

Neurons usually establish between 1,000 to 10,000 connections with other neurons.  This vast number of connections necessitates that neurons working together be adjacent to each other.  The economy gained by wiring closely adjacent units helps define the existence of cortical modules.  These units are characterized by a high amount of connectivity within themselves but, by comparison, reduced connectivity between individual modules.  Vernon Mountcastle called these units of information processing minicolumns. “Mini”, because they were microscopic and “columns” because they spanned the radial width of the cerebral cortex.  You can think of modules or minicolumns as the microprocessor of a computer.

subpial transection

Note: Uncontrolled electrical activity of neurons (seizures) can lead to changes in behavior, movement and level of consciousness. Depending on where in the brain the uncontrolled activity happens, a child may lose the ability to speak. This rare childhood disorder is known as the Landau Kleffner Syndrome.  The standard treatment for this condition is the use of anti-seizure medications.  When this therapeutic approach is ineffective, surgical intervention is an option.  The surgical technique in this instance uses the fact that the cortex is divided into modules each one defined by its connectivity properties.  In these cases, multiple small cuts through the width of the cerebral cortex helps restrict the spread of seizures (connectivity between modules) while leaving unimpaired the function of the individual cortical modules (connectivity within modules).  In effect, even though multiple slices of cortex are done during the operation, the language of the patient is preserved.

Microprocessors are miniaturized computers capable of processing many different types of information.  These microprocessors have posts in standard locations which allow them to receive information from multiple areas and to provide appropriate responses.  Within the cerebral cortex, the connectivity of its modules is similarly stereotyped.  Information going to the cerebral cortex enters and makes first contact in its middle layers. From here onward, stimuli are processed by going up and down the different layers of the cortex.  On their route, connections establish a repeating circuit that is similar in all cortical areas examined. In this regard, it may be helpful to think of the organization of the cortex in analogy to the 3 different divisions of a letter sorting tray in an office: the central layers acting as an inbox for messages, the superficial layers for messages being sent to other offices or parts of the building, and the deeper layers being the place for outgoing messages.

The advantages of organizing the cerebral cortex into modules seems obvious when we discover similar structures in numerous other species.  We have a limited number of genes coding for the development of the brain.  Having a vast number of possible information processing units in the cerebral cortex, each one varying from one another and across locations, would have required many more genes than are presently available.  The presence of independent but repeating units solves this problem.  Furthermore, modularity has expedited the processing of information by limiting or codifying its circuitry.  The resultant structure removes redundant loopholes and allows cells to specialize and increase their productivity.  In this regard, the advantages offered by modular assemblies of the cerebral cortex are similar to the organizational benefits observed in line assemblies, e.g., car manufacturing plants.  This organizational scheme allows engineers to easily restructure and balance assembly lines in order to optimize productivity.  In the brain, the restructuring process that optimizes function is called “neuroplasticity”.

In modern day computers, mathematical and logical processes can be implemented by electronic circuits.  Some of these circuits are more versatile than others.  Those that are called “universal” allow us to implement any required programming, no matter how complex they may be.  In similar fashion, the standardized connectivity of cortical modules allows us to process information regardless of its origin, this accounts for the neuroplasticity of the brain.  A person who is born blind may have no use for that part of the cortex that processes visual stimuli.  However, the brain dislikes having a portion of its cerebral cortex not being utilized.  Studies now show, that tactile sensation, as used in reading Braille, innervate the available visual area of these individuals.  In a similar way, a person who is born deaf has his/her auditory cortex modified to process visual information.  The overall expansion of available visual cortex provides the individual with superior peripheral vision and motion detection. In a certain way, you could say that the brain plays a balancing game when faced with adversity.  In some cases, although experiencing neurological deficits as a result of a brain lesion, the versatility ingrained in modular connectivity allows for the enhancement of the remaining functions.

power company

Note:  Electricity is generated in power plants and distributed by cables that connect individual homes to a generator usually miles away from the property.  The generator is a device that converts mechanical energy into electricity.  Generators produce an oscillating or alternating current which can be accessed through the electrical outlets of the house.  In the United States, this makes available a current having 110 volts with a periodicity of 60 Hertz (hertz= cycles per second). Even though the electrical outlets are of uniform construction, the electricity available in the outlet is capable of powering appliances of many different functions, e.g., TV, radio, microwave oven.  In a similar way, modules within the brain are able to process a variety of information stemming from different parts of the nervous system.

Computers

The brain is often compared to a computer.  However, rather than a modern computer driven by miniaturized electronic components, the brain is more akin to an old analog computer of the type used in the 1960’s and 70’s.  These were bulky machines that generated a lot of noise and heat.  If you ever opened one of them you would find many boxes and a large number of cables.  The boxes were mechanical relays that opened and closed circuits.  A lot of the “intelligence” provided by the analog computer had to do with the way the different relay boxes were connected together.  This necessitated a lot of cables.  One aspect of the design of a computer therefore involved ways of avoiding clutter by bundling cables together and planning their routes.  In the brain, modules of the cerebral cortex, much like the relay boxes, serve to process information.  However, a lot of capabilities of these modules depend on how they are connected together and the efficiency of their routing.

The brain exists within a confined space.  The skull provides protection against trauma but limits the amount of available space to the structures that it holds inside.  This premium for space necessitates that the brain, much like the analog computer, avoids clutter and organize its connections.  It may seem that having short connections would help to organize how modules transmit their information while simultaneously minimizing the energy expenditure entailed by longer connections. However, short connection would involve many jumps which would delay getting messages to their final destination.  Furthermore, a mistake in processing a stimulus at one of its initial stages could be magnified by subsequent handling at intermediate posts.  It is for this reason that the best way to transmit information combines both short and long jumps.

Connectivity in the brain is similar to that observed in airline hubs.  The resultant pattern is called a “small network”.  In this way, airlines concentrate traffic by going in short jumps to a way station that can transfer the traveler to their final destination.  In the brain, the resultant ratio of long to short connections adds a particular style of our personality to the way we think.  Researchers believe that a person’s thinking abilities, to be either extremely concrete or abstract, depends on the way these modules are connected together.  Lesions tend to transform the ordered connectivity into a plate of spaghetti, often with deleterious consequences.  These lesions can then affect not only our neurologic functions (e.g., motor strength, sensations, reflexes) but also aspects of our personality.

small world network

Note:  A small network maximizes the possibility of reaching a larger hub of connectivity in one or a few small steps.  This type of connectivity is seen in social groups, in business companies, earth sciences (e.g., seismic network in Southern California), and in many other real-life organizations.

Conclusions

To a large extent, the modular arrangement of the cerebral cortex accounts for our ability to do parallel processing.  Modules of the cerebral cortex may thus independently analyze different aspects of an object before tying everything together into a perceptual whole.  In this regard, modules receiving visual information individually analyze color, motion, shape and depth before consciously thinking of an object as a rose, book or airplane.  This specialization argues that, although of similar construction, modules are not exact replicas of each other.

Disease can affect modules by targeting elements within a given area and causing clinical manifestations, e.g., paralysis of the face or limb, pain sensation in an area of skin.  However, disease may also target generalities of its modular organization or affect widespread areas of the cerebral cortex.  The symptoms provided by such lesions (e.g., intellectual disability, lapse in judgement) may escape attempts by a physician to localize the lesion to a particular area of the cerebral cortex.

Glossary

Generator– A device that transforms mechanical energy into electricity so as to be used by an external circuit.  The mechanical energy is usually produced by moving a wheel or rotor typically fitted with vanes, e.g., steam or water turbine.

Microprocessor– This is a miniaturized computer that can be programmed to control electrical circuits.  It is found in a variety of appliances including our cloth washers, microwave ovens, TV sets, and DVD players.

Minicolumn– An ecosystem of nerve cells and their projections working together to process information within the cerebral cortex.

Module–  An independent and replicable unit that functions as part of a larger whole.

Neuron– The nerve cell that carries electrical impulses.

Neuroplasticity– The ability of the nervous system to reorganize itself in order to maximize function. Neuroplasticity is a natural process during brain development but it can also be seen after injuries.

Small world network– A type of connectivity map that maximizes information transfer.   Such networks have both clusters of short connections and centers providing for longer projections.  Modular arrangements are well poised to take advantage of this type of connectivity.

Spectrum– When used in a classifications scheme, a spectrum makes reference to a continuum of characteristics that exist between different extremes, e.g., the spectrum of colors in a rainbow.

For additional reading material see:

The cerebral cortex and autism: https://corticalchauvinism.com/2013/01/26/the-cerebral-cortex-and-autism/

An introduction to the brain: https://corticalchauvinism.com/2019/03/25/an-introduction-to-the-brain/

Our latest book: Autism Updated: Symptoms, Treatments and Controversies https://www.amazon.com/dp/1079144102?ref_=pe_3052080_397514860

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