How does an image of the world reach our consciousness, and how does it reach the deeper layers of the psyche? The rules that govern the “grey tissue between your ears” are explained by Dr Paweł Boguszewski, a researcher at the Nencki Institute of Experimental Biology.
It was evening. Lying on the couch in front of the television, Klara surfed channels mindlessly. Suddenly there appeared on the screen the face of a well-known politician, twisted into a grimace. The man was shouting something from a lectern, shaking his fist, threatening something. “What nonsense,” Klara murmured, disgusted, stabbing her thumb down on the remote to switch off the set.
This scene plays out every day in thousands of homes around the world. Nothing special. But for a neuroscientist these few seconds are a fascinating event, showing what a wonderful work of evolution the human mind is – along with the whole nervous system. Between the appearance of the threatening face on the screen and the set being switched off, extraordinary things happen in Klara’s nervous system, and most of all in her brain. Let’s try to look at each of these phenomena in order.
In complexity, strength
The mind is the most complicated system we know. It’s made up of about 86 billion neurons, cells that specialize in processing and transmitting information. They carry sensory impressions from the periphery of our bodies to the centre, perform complex analyses and comparisons, record information for future use and send commands to the muscles.
Each neuron is exceptional, changing under the influence of experience, and a complex machine in and of itself. Its characteristic attribute is a huge number of branches, of which the shorter ones (the dendrites) are used to receive information, and the longer (the axons) to send it. The longest human axons reach from the head to the feet. Neurons always work in groups, smaller or larger – networks that can do much more than individual cells.
And just a few more words about how the brain processes information. Inside your brain, information occurs in the form of electrical and chemical impulses, very weak charges – small flows of charged icons across the cell membrane. Through the dendrites, the nerve cell receives charges from nearby neurons. These charges add up, and when they reach a certain threshold, the cell sends a sharp electrical impulse along its axon. That reaches other nerve cells or muscles, even distant ones. So the principle is very simple. But when we multiply it across tens of billions of neurons and hundreds of billions of synaptic connections, we get a machine with incredible capabilities. The uniqueness of the human brain lies in its complexity.
Now that we know all this, let’s return to Klara, and start our adventure – along with the information in her brain – from the very beginning.
Through the ports of the senses
The sensory organs translate physical phenomena – light, sound, the presence of chemical substances, pressure – into the language of the nervous system, meaning electrical impulses. In the case of the visual system, electromagnetic waves of a certain frequency, meaning visual light, enter the eye through the cornea and the lens, generating a reverse image of the world on its rear wall, the retina. This layer is covered in cells that detect the presence of light – they change light energy into electrical impulses. They can do this thanks to the presence of pigments, which are specialized proteins.
This mechanism allows us to see not only light, but also the entire image. The scene displayed on the television screen appears on Klara’s retina, causing a complex biochemical reaction. But this is only the start of a long road, because although we look with our eyes, we see with our brains. The axons of the retinal cells, entwined in a bundle, make up the optic nerve, along which information runs from the eye to the mind.
The thalamus, a suspicious neighbourhood
The image of the threatening politician rushes along Klara’s optic nerve. First it reaches the thalamus – an evolutionarily old region, tucked away inside the brain. If we imagine the brain as a city, the thalamus is a dodgy neighbourhood around a train station. This is where the optic nerve completes its run; it’s also where impulses arrive from our other senses (aside from smell).
For Klara to see something consciously, the information has to wander through other parts of the brain, but already in the thalamus certain important attributes of the image may be discovered, such as the grimace of rage on the boss’s face. This type of stimulus is exceptionally important and can trigger our emotions completely automatically. The thalamus is a neighbourhood that doesn’t like dangerous newcomers. When one appears, the information is transmitted to the amygdala – another deep structure, responsible for triggering the fear reaction. This shortcut is what awakens the emotions before the image even makes it to our consciousness.
But we’ll come back to the amygdala and the emotions. For now, we’ll follow the path of conscious seeing. In the thalamus, the optic nerve transmits the information from the eye to the neurons, whose axons lead to the external layer of the brain, meaning to the cortex, and specifically to the occipital lobe, to the region designated V1 (the primary visual cortex). And here there occurs something fundamental for human vision: information about the million patches of light that arrived from the eye is translated into information about lines. Because the vision system doesn’t construct an image from points, but from lines, which are detected by the cells in the V1 region.
This is also where the two main paths of vision begin: the first, known as ‘where’, which runs in the direction of the top of the head, and discovers movement and the spatial arrangement of elements of the image. The second is the ‘what’ path, leading from the occiput toward the temple and recognizing the objects we’re looking at. The data about the lines extracted from the image are sent in both of these directions for further examination.
In the case of the politician’s face, the ‘what’ path is more important. The successive stages of this path detect objects layered on top of each other, describe colours. Finally, the information arrives at the fold of the cortex in the temporal lobe known as the fusiform gyrus. When the signal gets there, Klara sees the face consciously. But for her to understand that it’s a face she knows, the information has to go further toward the front of the temporal lobe, where the so-called grandmother cells are found. They react to a particular individual, regardless of whether we see them, hear them or recognize them by other characteristics.
So we’ve got it. The face from the television finally reached Klara’s conscious, and is recognized as belonging to a politician she doesn’t like. Now the appropriate authorities will decide what to do with it.
Emotions, emotions everywhere
From the beginning, the entire event is accompanied by emotions. Fear at the sight of the politician’s threatening expression and gestures. Unease in reaction to the stream of words flowing from his mouth. And also relief after the television is switched off.
Fear and arousal in response to angry facial expressions is an automatic reaction, involuntary and culturally universal: we don’t need to learn it, because it’s deeply encoded in our DNA. It allows fast, non-verbal transmission of information within a group: if one individual expresses emotions, they obviously have important reasons for that, and other members of the herd should react to it, looking for the reason or just fleeing at once. Thanks to this mechanism, it’s easier for people and other social animals to act as a group. On the other hand, it’s a powerful tool allowing us to be directed against our will.
Responsibility for these primal emotions lies with a structure mentioned earlier: the amygdala. This relatively small region is located deep in the temporal lobes, on both sides of the brain. Its basic role is to give meaning to emotional stimuli. In the brain, the amygdala plays the role of a sentry. As soon as it sees or hears something suspicious, it sounds the alarm. Unfortunately, it’s certainly an over-cautious sentry. In the case of the slightest doubt, it prefers to raise a hue and cry rather than ignore the threat. That’s where we get most of our problems with excessive fear in daily life.
The feeling of distaste and disgust is the responsibility of another region, the insular cortex, hidden between the frontal and temporal lobes. The case of disgust is extremely intriguing, because it shows how evolution uses existing mechanisms and applies them to new situations. The insular cortex is the sensory cortex for taste – that’s where the mind builds impressions based on nervous impulses from the taste buds. It’s also where disgust arises – the initial reaction to distasteful, potentially toxic food, but also to the smell of substances that threaten us with infection, such as excrement. What’s more, this same region is also aroused when we see blood, or the faces of other people twisted in disgust. And at the mere thought of something repulsive, even if it’s a morally repulsive act. Many factors, and a single reaction – initially resulting from a very biological need. And here too is the source of Klara’s disgust at the lies flowing from the screen.
And the relief when the tube is switched off? Here the so-called reward system has acted. These are structures of the brain whose stimulation we experience as pleasure. They’re the carrot element in our lives, while the amygdala is the stick.
These emotions definitely affected our heroine’s decision.
Time to do something
Taking a decision is a complex process, and it requires the cooperation of many neural networks. The most important ones are located in the prefrontal cortex: the region of the brain that’s located furthest to the front. You can find it with great precision by banging your head on your desk. The prefrontal cortex is engaged in numerous tasks and functions that we identify with the higher mind functions, such as – in addition to decision-making – moral assessment, adjusting our behaviour to situations, planning, social behaviours, control of impulses and drives, and many more.
The importance of this part of the brain was made clear to Phineas Gage, or rather to the scientists researching his case. In 1848 this model citizen, an outstanding labour foreman and exemplary father, accidentally caused an explosion, which drove a metal rod through his skull and deprived him of his prefrontal lobes. Still, he lived, which seemed to be a miracle, and after convalescence he returned to work. But deep changes in his personality soon manifested themselves: he became mercurial and vulgar, and was unable to foresee the consequences of his actions.
Gage’s case was a milestone in the development of neurology, but primarily a bucket of cold water for many of his contemporaries: nobody expected that you could lose your morality by losing a few neurons, and that phenomena which we considered cultural or related to religious systems were closely connected with brain biology.
Getting back to Klara: it’s in her prefrontal cortex that the decision was made to turn off the TV. Now it was time to put this plan into action.
In muscles, strength
To affect the outside world, the brain sends signals to the muscles. That’s the only way. Moving, sitting still, manipulating objects, smiling, speaking or holding back the words pressing on your lips – all of this is based on directing your muscles the right way. So just as the brain needs the sensory organs to take in the world, so it needs the motor system to interact with it.
When the decision is made in Klara’s prefrontal cortex to press the button on the remote, her brain has a whole range of actions ahead of it. First, the specialized motor areas plan the execution of the task. For that, they need to have a full report about the body’s condition. It’s obviously good to know where the hands and the thumbs are before we start using them, and whether the remote is in our hand. This report is provided by the sensory cortex, collecting – from the muscles, joints, tendons and cutaneous receptors – data on position and acting forces. After adding information from our vision, we have a ready representation of the body.
Before the motor cortex sends impulses to the muscles ordering them to contract, other areas of the brain – the premotor cortex and the supplementary motor area – change our intention into what exactly the body is supposed to do. In the end, in wanting to press the button, we don’t think about which groups of muscles we need to contract, and which to relax. It’s these regions of the brain that do that. They’re active even when were only thinking about movement or observing the movement of other people. This allows us to train mentally and to understand the intentions of other individuals. (But unfortunately, we don’t burn any more calories just by thinking.)
The sequence, arranged and sent from the motor cortex, is still subject to modifications and oversight. At a minimum, the cerebellum, a small structure at the back of the brain, makes sure everything’s happening according to the plan, and makes adjustments on the fly. Without it, our movement would be uncertain and imprecise, our speech slurred and our vision blurred. If this image reminds you of something, it should: the cerebellum is the first area to be influenced by alcohol.
The impulses from the brain run along the spinal cord, transferring on the synaptic connections to the motoneurons, which directly innervate the muscles. During this transfer, the impulse can still be modified (e.g. under the influence of reflexes). For example, if the remote turned out to be a red-hot piece of iron that started to burn our hand, rather than pressing the button we would reflexively drop it.
But this time the remote didn’t burn her, the operation was a success, and Klara sighed with relief. The cat also felt great relief, and tucked itself into her lap.
This scene of a few seconds, completely trivial, turned out to be a field for numerous neuroscientific deliberations, and allowed us to take a journey through the human brain. But even so it was just a side trip, because many phenomena weren’t mentioned, including working and long-term memory, other senses and emotions, reflexes, speech, attention, consciousness. This shows what a complicated structure the brain is, and how we don’t appreciate its extraordinary capabilities, which it shows even in ordinary, everyday life. We take them as obvious – until something starts to break down.
Klara is without doubt a human, part of the species Homo sapiens – ‘wise man’. On the issue of human origins, Darwin’s theory is still in fine form. Science is constantly perfecting it, adding new mechanisms, but the basic concept remains the same. As humans we are the product of evolution, made up of similar elements to those of all living creatures, and our uniqueness comes from the great complexity of our nervous system. Meaning that we, our nature, our inner experiences, thoughts: it’s nothing other than the flow of billions of subtle currents through the greyish tissue that resides between our ears. For me, that’s a reason for pride, not a degradation of humanity.
Translated from the Polish by Nathaniel Espino