Methodological aspects of studying the mechanisms of consciousness

Highlights

• The relationship between the mind and brain remains an extremely important area of neuroscience research.

• The existence of a number of neurological data have not yet led to the creation theory of the regulation of consciousness.

• Neurobiological data related to the biological mechanisms of the regulation of consciousness was thus summarized.

Abstract

There are at least two approaches to the definition of consciousness. In the first case, certain aspects of consciousness, called qualia, are considered inaccessible for research from a third person and can only be described through subjective experience. This approach is inextricably linked with the so-called "hard problem of consciousness", that is, the question of why consciousness has qualia or how any physical changes in the environment can generate subjective experience. With this approach, some aspects of consciousness, by definition, cannot be explained on the basis of external observations and, therefore, are outside the scope of scientific research. In the second case, a priori constraints do not constrain the field of scientific investigation, and the best explanation of the experience in the first person is included as a possible subject of empirical research. Historically, in the study of cause-and-effect relationships in biology, it was customary to distinguish between proximate causation and ultimate causation existing in biological systems. Immediate causes are based on the immediate influencing factors [1]. Proximate causation has evolutionary explanations. When studying biological systems themselves, such an approach is undoubtedly justified, but it often seems insufficient when studying the interaction of consciousness and the brain [2], [3]. Current scientific communities proceed from the assumption that the physical substrate for the generation of consciousness is a neural network that unites various types of neurons located in various brain structures. Many neuroscientists attach a key role in this process to the cortical and thalamocortical neural networks. This question is directly related to experimental and clinical research in the field of disorder of consciousness. Progress in this area of medicine depends on advances in neuroscience in this area and is also a powerful source of empirical information. In this area of consciousness research, a large amount of experimental data has been accumulated, and in this review an attempt was made to generalize and systematize.

Introduction

Consciousness research has always faced serious difficulties. The very definition of consciousness is rather vague and includes many interdisciplinary aspects related to psychology and philosophy. There is no generally accepted definition of consciousness, but most modern researchers agree that consciousness in its simplest form is “a feeling or awareness of internal or external existence” [4], [5], [6]. Until about the 20th century, the concept of consciousness was defined as "inner life", the introspection of personal thoughts, imagination and will. Today, most scientists include in the concept of consciousness individual experience, cognition and perception. It is also generally accepted that there may be different levels or orders of consciousness, as well as different types of consciousness, but assessing the level of consciousness is a non-trivial task. A separate question is whether only humans possess consciousness, or at least some animals also possess consciousness.

The founder of the concept of neuro-darwinism, Gerald Edelman, believed that it is necessary to distinguish between primary consciousness and higher (superior) consciousness [7]. Primary consciousness is a multimodular structure that brings together various sources of information. In contrast to the primary, higher consciousness develops in parallel with the acquisition of language, and continuously structures the experience gained by the individual. While primary consciousness links memory to current perception, higher consciousness directs the synthesis between memory patterns distributed over time. Edelman's model implies that the integrative activities required for higher consciousness arise at the border between the thalamocortical system and other areas of the brain. It is an autonomous dynamic nucleus that cannot be identified either as the brain as a whole or as a specific part of the brain [7], [8].

Numerous studies, including with the use of modern imaging technologies, have been undertaken to answer the question "what happens in the brain when the awareness of the surrounding world disappears?" So, in a recent study, a change in the consciousness of an experiment was carried out with a simultaneous analysis of brain activity using positron emission tomography (PET, PET). Measurements of brain activity were carried out during wakefulness, under the influence of various anesthetics, as well as during non-rapid eye movements (non-REM) sleep [6]. The deep stage of general anesthesia, when the subsequent report on mental content did not contain any signs of awareness of the surrounding world, was considered as a confirmed lack of consciousness [6]. The level of consciousness of the test subjects was regulated by propofol or dexmedetomidine with a stepwise increase in the concentration of the drug [6].

Decades of research have led to a certain consensus based on the fact that consciousness, like numerous forms of complex behavior, are network-level phenomena, that is, a phenomenological product of a neural network [9], [10]. Highly organized social behavior requires high-level integration of various sensory inputs, synchronization of many motor patterns, and coordination of their activities within large-scale neural networks. Consequently, the accumulation of data on the functioning of neural networks contributes to the formation of a number of testable hypotheses regarding the neural foundations of cognitive functions, memory, and motivated behavior.

Currently, the study of consciousness consists of three main directions of levels: the search for neural correlates of consciousness, the mechanisms of functioning of these correlates and the reproducibility of the hypotheses put forward [11].

This manuscript is intended for neuroscientists, psychologists-clinicians, as well as other scientists involved in the field of relationship between consciousness and its brain substrate. Our aim was to more accurately formulate the main conclusions reached by experimental neurobiology while investigating the problem of the regulation of the mechanisms of consciousness by the human brain. It was used data from the last quarter of the previous century up to the present time.