The hallmark of human evolution is the explosive growth of the frontal cortex.
No one denies that understanding the brain is one of the two or three major challenges of science. Although research has been intense, bright and abundant, scientists, at least publicly, have not found the key to dissecting it.
We know today that the key to our mind is the connectivity between neurons, the geometry of its circuits.
We know the intricate mechanisms by which a neuron decides to send through its output the result of a complex calculation that has been done integrating the information of its 10,000 its input.
We understand the reinforcements of the synapses that underlie our memory, and we use high-level waves, resulting from the activity of millions of neurons, to diagnose mental illnesses and investigate the degree of consciousness in volunteers.
But we still don’t understand how the brain generates the mind.
Despite repeated and bold attempts to associate human specificity with one or another radically new piece of brain, with an original and unusual architecture in the history of the planet, the data stubbornly shows us that all our intended peculiarities began 600 million years ago with lesser species.
For example, it was the jellyfish, precisely, that invented the eyes. There is a gene called PAX6 that deals with designing the primitive eye and their connection with primitive neurons.
That same gene, which was initially discovered in the fly, is also responsible for the design of the human eye, and its slight mutations cause congenital diseases such as the absence of the iris and another dozen abnormalities in the development of the eye and its neurons.
In a deep genetic sense, our eyes and our visual brain originated in jellyfish 600 million years ago.
That is just the beginning of the long, long history of our connection with the origins of animal life.
From the optic lobe of primitive animals, which is precisely the domain of action of PAX6, comes our midbrain, essential for vision, hearing, body temperature regulation, movement control and the cycle of sleep and wakefulness.
From another part of the brain come our senses such as smell, which also anchors its origins in animal life. Then comes our cortex, the outermost layer of the brain, present in the most intelligent species like dolphins, whales, and elephants.
From the cortex and its associates, these evolutionary fruits of the ancient olfactory brain emanate all the amazing aptitudes of the human mind, everything that makes us so different and what we are so proud of.
That outer and unsightly layer of the brain generates – or, more accurately, embodies – our sensations of the outside world, our voluntary orders to move our mouth or arms, and a swarm of “association areas” where the senses, memories, and thoughts are integrated to produce a unique conscious scene, the fabric from which our experience is made.
The whole brain is an enigma, but if one had to choose a supreme problem in that jungle, that would be the mystery of consciousness. And there is a scientific story that needs to be told here.
For philosophers like Daniel Dennett, the problem of consciousness is inseparable from the enigma of the qualia: what we feel like the redness of the red color, the sweetness of a sweet, the sensation of pain that causes a toothache.
These philosophers believe that the riddle of qualia cannot be solved, or even addressed, by science, because those feelings are private and cannot be compared, learned or measured by external references.
This idea, however, contradicts the general principle that the mind is equivalent to the brain, as Alcmeón de Crotona advanced 2,500 years ago.
If everything that happens in our mind is the product of — or rather is identical to — the activity of certain neural circuits, consciousness cannot be an exception, or else we would return to irrational animism, to believe in a separate soul from the body; to ghosts and ectoplasms.
Scientists have then decided to skip the supposed riddle of the Qualia to concentrate on looking for the “neural correlates of consciousness,” that is, the minimum circuits sufficient for a conscious experience to occur. The strategy has been fruitful.
Let’s take the well-known effect of binocular rivalry. With a simple assembly, you can present an image to the left eye of a volunteer and another to the right eye.
You might think that the volunteer would see a shocking mixture of both sides, but if you ask him you will see that it is not so.
The two eyes compete to get their information to consciousness, hence “binocular rivalry”. What changes in the brain when consciousness flips from one face to the other?
Experiments of this type, combined with modern brain imaging techniques, such as functional magnetic resonance imaging (fMRI), point again and again to the “posterior hot zone.”
It is composed of circuits of three lobes, which are parts of the cerebral cortex: the temporal, located above the ears) the parietal placed just above the temporal, all over the head, and the occipital, a little above the neck.
This is in itself a surprise because most neuroscientists would have expected to find consciousness in the frontal lobes, the most anterior part of the cerebral cortex, and the one that has grown the most during human evolution. But it’s not like that. The consciousness resides in later areas of the brain that we share with the generality of mammals.
Another recent discovery is that the areas involved in consciousness – the posterior hot zone – are not those that receive direct signals from the eyes and other senses. What happens in those primary areas is not what the subject sees, or is aware of.
The consciousness is in areas that receive, elaborate and interconnect that primary information, both insight and in the other senses.
A traditional surgical practice offers us more valuable clues. When neurosurgeons have to remove a brain tumor or the tissues that cause very serious epileptic seizures, they take a very logical precaution first: with an open skull, they stimulate the neighboring areas with electrodes to see exactly where they are on the map of the cortex, and even where it is convenient to tamper with.
It was thus, in fact, as the motor homunculus was mapped, that deformed human figures that we have above the ear control all our voluntary movements. Stimulate here and the patient moves one leg; Stimulate there and move the middle finger of the left hand, or the tongue and lips.
When what is stimulated is the posterior hot zone, the patient experiences a whole range of sensations and feelings. You can see bright lights, deformed faces, and geometric shapes, or feel hallucinations in any sensory modality, or desire to move an arm.
In its normal form, this seems to be the material with which our consciousness is woven. When part of the hot zone is damaged by an illness or an accident, or removed by surgeons, the patient loses contents of consciousness.
They become unable to recognize the movement of any object or person, or the color of things, or remember faces that were familiar to them before.
Neuroscience, therefore, has not only demonstrated the hypothesis that the brain is the seat of the mind, but has also found the exact place where consciousness resides.
Understanding how that piece of brain works is a much more difficult matter, but the mere location of consciousness in the back of the cerebral cortex has a clear implication.
The hallmark of human evolution is the explosive growth of the frontal cortex. The posterior cortex, including the hot zone, we have inherited from our mammalian ancestors and beyond.
Understanding the brain is undoubtedly one of the biggest challenges facing current science. This is the most complex object we have news in the universe, and the task is formidable. But the reward will be great for research and thinking.