Author: Amadeo Muntané Sánchez
Consciousness is the state of knowledge of oneself and one's environment by which the individual performs his or her perceptual, intellectual, affective and motor functions. From a neurological point of view, consciousness manifests itself in action through brain activity and is considered as a complex of information units that have their material basis in the brain.
In the central nervous system there are neurons involved and neurobiological mechanisms that are related to consciousness. The so-called reticular activating system is known to control the activity of the central nervous system, which includes wakefulness and sleep. This system includes Structures such as the brain stem, where the reticular formation is located, which is a set of nerve nuclei formed by neurons of different shapes and dimensions, the thalamus and the cerebral cortex. The thalamus is a brain structure that receives and integrates the information that subsequently reaches the cerebral cortex via the thalamo-cortical circuits. Consciousness represents the activity of the entire cerebral cortex, i.e. it should not be understood by focusing on one brain region without considering the relationship of this region with the others, therefore consciousness is neurophysiologically related to the cortical brain areas of association.1.
One of the difficulties we encounter in the study of consciousness is its intrinsic subjective character. A person knows that he or she is conscious, and on the other hand others verify that this is the case, because the individual has the capacity to respond appropriately to environmental stimuli. The human being when conscious and mentally normal can exchange with other individuals different elements of subject social, linguistic, ideological, sentimental, etc., however the loss of consciousness can prevent this exchange to a greater or lesser extent.
Under normal conditions, in order to be able to exercise his freedom, his volitional, intellectual, emotional and mental activity at final , as well as to be aware of perception through the senses and sensory organs, the human being must be conscious, i.e. the "I" manifests itself in this state.
Searle says that "consciousness refers to a state of "awareness" that begins when we awaken from sleep and continues throughout the day until we go back to sleep, die, or in other words when we become unconscious. Dreams are also a form of consciousness, although in many ways it is very different from normal states of alertness."2 . Basically, the mechanism of sleep production results from a decrease in the excitability of the reticular system by hypnogenic centres located in the hypothalamus, brainstem and cerebellum as well as changes in the biochemical state of the neurons of this system, since there are molecules that are related to sleep such as serotonin and noradrenaline. This sleep-wake cycle is a physiologically occurring phenomenon and is necessary for the normal functioning of the nervous system 1.
From a philosophical point of view it has been said that consciousness is a phenomenon that is always in the present, it does not change, that is why it perceives time, i.e. the change that affects the processes of the physical world, although this activity requires not only the conscious present but also the relation of the past to the future, which is typical of consciousness in conjunction with report and other cognitive functions. In the perception of time we have on the one hand that we would not know anything about time if we were not part of the changing world, on the other hand, if we were only changeable, we would not know how to recognise past events as past. If we could not evidence the passing of the external and of our corporeality to a reality that does not pass or pass, that is, that is, that is not affected by physical change, we would not be aware of time. The perception of time is timeless and non-physical. This leads us to think that there is immateriality in the process of consciousness3.
Gerald Edelman distinguishes between two types of consciousness: primary consciousness and higher order consciousness. Primary consciousness would consist of certain phenomenal experiences such as mental images that would be linked to the immediate present. Here there is no possibility of recognising a past or a future. Edelman conceptualises primary consciousness as the conjunction of different perceptions at a given moment, which the subject lives or experiences as a scene. This does not mean that there is "a place" in the brain where the perceptions are brought together and the scene is formed, but rather that the scene is an emergent product of the functioning of the brain that cannot be reduced to any of its components. In this sense it can be argued that consciousness is not something to be had but is constructed moment by moment. Higher-order consciousness involves the subject's recognition of its own activity, as well as the possibility of visualising a past, a present and a future. From a functional and structural point of view, primary consciousness is necessary for higher-order consciousness. The neurobiological components of primary consciousness are present and their functioning is part of the nervous elements operating in higher consciousness. In that sense, human beings with higher consciousness do not experience primary consciousness alone, nor the opposite 2 .
Genesis of consciousness
Activation at the level of the reticular system in the brainstem generates nerve impulses that are transmitted to the cerebral cortex via the thalamus and allow us to experience consciousness. This activation can be motivated by sensory and sensory stimuli that originate impulses in the cerebral cortex itself as well as stimuli that can originate in the cingulum, hippocampus, hypothalamus and basal ganglia. How is it possible that the ionic changes that occur in the membranes of nerve cells and the biochemical phenomena of nerve impulses give rise to consciousness with all that it represents? There is no neuroscientific explanation final for how consciousness is produced, although there are anatomical parts of the brain involved in the elaboration of consciousness. Zagmutt comments that "to formulate an explanatory theory of consciousness would be tantamount to unravelling the greatest mystery of the human and biological sciences. Unfortunately, we are not yet in a position to arrive at such a theoretical formulation" 2.
One of the first to locate the problem of the immaterial and immortal soul in consciousness was the interactionist dualist John Eccles, even assigning it a place in the brain, as Descartes had done earlier with the soul. The place from which, according to the metaphor he used, the pilot steers the human body *(4).
This award Nobel Prize winner was well aware of the advances of cognitive science in explaining the operations of the mind. However, Eccles declared the subjective awareness we have of our mental operations to be inexplicable. This would be the mysterious essence of the spiritual soul. Thus Eccles' dualism was opposed to any scientific attempt to reduce consciousness to neural activity 4 .
Chalmers agrees with Eccles in also affirming the irreducibility of consciousness to neural activities 4 . Chalmers comments: "against reductionism I will argue that the tools of neurology cannot provide a complete explanation of conscious experience, although they have much to offer" *(5). "However, explaining subjective consciousness is the "hard problem", for even if we can locate and describe the groups of neurons that receive or compose sensations, it will always remain difficult to explain why and how that activation comes to produce the subjective experience we have of colours, sounds, tastes, etc., as well as of our inner world, feelings, etc. "4 . 4 For Chalmers, the science of consciousness must aim to find a parallel between two sets of data: one that of the phenomena observed and described by neurologists from the outside, and, on the other hand, that of our experiences, which are only the subject of first-person descriptions. This parallelism is described in his third principle for an informational theory of consciousness, the principle of the double aspect: "there is a direct isomorphism between certain physically embodied informational spaces and certain phenomenal informational spaces. We can find the same abstract information recorded in physical processing and in conscious experience" 4 . Chalmers also assigns to the science of consciousness the task of accounting for how certain micro Structures and micro neural dynamics can produce macro structural effects and macro neural dynamics. But unlike emergentist theories, he maintains the irreducible distinction between these collective macro dynamics and subjective experience, so that one cannot speak of the production of consciousness as defined by emergentists 4 .
Damasio, from a naturalistic and biological theory, criticises this separability between brain and consciousness. John Searle is an illustrious representative of what he calls "biological naturalism" and defends the biological character of mind and consciousness. According to these authors, mind and consciousness can only be understood and simulated if we include the underlying phenomena, i.e. their biological or somatic basis 4 . Damasio says that "we have not yet resolved numerous details concerning the molecular function of neurons and circuits; nor have we succeeded in understanding the behaviour of populations of neurons in the framework of a particular brain region; and we still have a poor understanding of large-scale systems, i.e., those involving multiple brain regions. I believe that the mind is biological in nature and that there will come a time when we will be able to describe it through biological and mental expressions" *(6).
Francis Crick and Christof Koch have proposed that consciousness would depend on synchronised electrochemical discharges of cortical neurons at a frequency of 40 Hz. Daniel D. Dennet postulates that there is a combination of numerous independent processes that produce a coherent response to a perceived event 5 .
Roger Penrose is one of today's most original and creative thinkers. He is one of the most important physicists to work on General Relativity since Einstein. For Penrose there must be something of a non-computable nature in the physical laws to come. This argument is based on Gödel's now famous theorem, which implies that the formal indemonstrability of a certain mathematical proposition is a sign that it is in fact true. Penrose concludes that our thinking, at least our mathematical thinking, has non-computable components. If one admits that there are non-computable physical processes, one has to see how the brain could make use of them. First of all, Penrose believes that there is a direct relationship between this non-computability and the bridge between the quantum level and the classical level, which in turn is related to the quantum measurement process. Therefore, one would have to look for a place in the brain that can harness quantum coherence effects to couple them to the neural activity observed on a large scale in the brain. The most likely place seems to be the microtubules of Stuart Hameroff and colleagues at the University of Arizona, which are part of the cell cytoskeleton 5 .
Philosopher Joseph Levine says that there is a gap in the explanation linking physical neural processes and consciousness. Moreover, physicist Steven Weinberg claims that despite the power of physical theory, the existence of consciousness does not seem to follow from its laws. Surely in the end it will be possible to explain the neural correlates of consciousness, but that is not explaining consciousness itself 5.
Physicist and theologian John Polkinghorne in his book Science and Theology comments: "Consciousness seems to be a phenomenon so different from other perceptible phenomena in the physical world that it must be something very special. As to its physical organisation, I can discern clearly that it is a matter of traditional ideas of physics organised into more complex systems. But there must be something else, something whose nature is completely different from the other things that are important in the way the world works. Something which, even if used occasionally, has such a refined organisation that it takes advantage of the organisation of states and channels it with the goal to make us function, but which is very rarely harnessed in physical phenomena in a useful way" *(7).
Consciousness allows mental processes to be experienced in an instant in which everything is perceived as a unified experience. There is an assembly in which the visual entrance , the auditory area, the tactile and painful sensitivity receptors, the olfactory pathway, the mechanisms for checking the space in which the subject moves, the report, the understanding, the volitional acts, the attention and the emotions all come into play. And all these are represented in different areas of the cerebral cortex. How is it possible for all these mental phenomena to come together in an instant?
Dr. Róger Sperry won the award Nobel Prize in 1981 for his studies on the specialised functions of the human brain. He performed a callosotomy on patients suffering from uncontrollable epileptic seizures, i.e. surgery to separate the two cerebral hemispheres by cutting the nerve fibres in the corpus callosum that connected them. After surgery, one hemisphere appeared to participate in different experiences from the other hemisphere. The skill of speaking, walking and eating were not impaired, but higher brain functions were impaired. Interhemispheric coordination and dynamics were impaired: contradictions in bimanual actions occurred; by hiding the hands from view, one hand could be unaware of what the other hand was doing *(8).
There is evidence that, after surgical separation of the cerebral hemispheres, learning and report are not affected and each hemisphere allows independent sensing and perceiving. Further studies have shown that the right hemisphere is involved with non-verbal expression, intuition, spontaneous, reciting poems, melody of songs, colour discrimination, matching objects with pictures, matching words with meaning, drawing and manipulating objects, expression through face, voice, body gestures, responding to demonstrated instructions, visualisation, remembering faces-shapes-melodies-complex pictures-stories-emotional events, daydreaming, imagining, creating and discovering. On the other hand the left hemisphere is related to verbal expression, using words to name-describe-define, associating colours with objects, thinking with words, using symbols to name things, spelling words, organising, expression through language, responding to verbal instructions, calculation and mathematical analysis, remembering names, facts, days and complex motor sequences.
In summary, it can be said that the right hemisphere develops those functions that require a synthetic intellectual vision of many things at the same time, and the left hemisphere develops those functions that require analytical and elementalist thinking. The connection of both hemispheres allows for a globalising, systematic and continuous function, which runs practically simultaneously*(9).
The neurophysiologist Rodolfo Llinás states that the thalamus, which is connected to different regions of the cerebral cortex, maintains a continuous "dialogue" between its neurons and the neurons of the cerebral cortex, in such a way that an oscillation is produced that expands and is transmitted by means of a "sweep" from the frontal cortex to the occipital cortex every 12.5 milliseconds. This dynamic is based on the action potentials and the transition from the polarised to the depolarised state of millions of neurons that are triggered into action in this time. This means that the experiences of reality are integrated in this very short lapse of time in the frontal cortex, the parietal cortex, the occipital cortex, etc. Llinás postulates that this sweep is what allows us to have all these polysensory experiences unified and what gives us the sensation of continuity and unity of the external world. There are measurements made with the so-called magnetoencephalograph which can record the magnetic fields of nerve cells, which are very weak. It has the advantage of being able to make recordings of greater depth than the electroencephalogram, since what it records are the voltage fluctuations due to the electrical currents flowing through the membranes of the neurons. This apparatus has shown that the minimum time interval in which two events in the external world can be perceived, which is called the psycho-physical quantum, lasts 12.5 milliseconds*(10).
Consciousness is one of the most complex topics in the study of brain physiology.
Despite the different theories that have been presented above, the precise neuronal mechanisms that take place in the process of consciousness have not yet been explained. It is a verifiable fact that there is an anatomical and neurobiological substrate for its development, which is supported by the fact that encephalic injuries such as cranioencephalic trauma, haemorrhages, cerebral infarcts, tumours or toxins can give rise to a disorder of consciousness due to the affectation of the neuronal Structures and/or neurochemical function. However, although the concurrence of nervous tissue is necessary in the elaboration of consciousness, there are authors whose opinion emphasises that this activity cannot be reduced to neuronal function alone.
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