The unique human ability to speak and think, object perception (gnosis) and actions with objects (praxis), as well as their disorders in focal brain lesions have always been in the field of view of neurologists. With the improvement of ideas about the structural and functional organization of the brain, the laws of nervous activity, the socio-biological nature of man, the laws of functioning of the human psyche, language, ideas about gnosis, praxis, speech and thinking became more complicated, were rebuilt and received coverage in related scientific disciplines, primarily in physiology, psychology and linguistics. The clinical semiotics of agnosia, apraxia and aphasia, respectively, expanded, and the principles of classification of these disorders, the significance of the corresponding syndromes for topical and nosological diagnostics, and the appropriate methods for studying patients have been changed.
The ability of a person to speak, think, and perform purposeful activity is primarily provided by a specially developed cortex of the large brain. Until relatively recently, the term “higher cortical functions” was used in neurology, although it was already clear that the activity of the cerebral cortex and, consequently, the functions of speech and thinking are impossible without the activity of stem-subcortical structures. Then, the term “higher brain functions” has been introduced. The concept of “cognitive functions” came to us from the English-language literature around the 1990s.
The higher mental functions have the same hierarchical structure, as well as other functions of the CNS (sensory, motor, autonomic, etc.). Functional systems include projection zones of the cortex or specific nuclear zones of analyzers, where information is directly received, secondary associative areas of the cortex or specific periphery of analyzers that analyze and recognize the received information, and tertiary associative areas of the cortex or overlapping zones of analyzers, where simultaneous analysis of information received through different channels occurs. At the same time, it is necessary to take into account the different functional significance of their elements. For their normal formation and development, the integrity of the morphological substrate, the brain, is necessary.
The higher mental functions are thought to be based on the conditioned reflex mechanism formed on the basis of innate unconditional reflexes and explaining the individual variability of higher mental functions. A person also has the ability to be creative, the ability to navigate in a new situation, plan and forecast their actions, adapt to life in society.
By origin, the higher mental functions are socio-historical, and in the individual development of a child, their formation occurs after their birth and only under the influence of general cultural and linguistic social environment.
A person has been mastering language and culture for many years, entering into complicated types of activities (emotional communication with adults, playing with objects, social role-playing, school education, industrial and professional activity, aesthetic creativity, etc.); therefore, the normal structure of higher mental functions for a modern adult is formed over time. Findings considered abnormal in adults often act as a stage in the development of higher mental functions in children. Thus, in preschoolers, an age-related incompetence of spatial orientations or phonemic analysis of words may be observed, which in schoolchildren and especially in adults indicates a focal pathology of the cerebral cortex. In the assessment of the state of higher mental functions, the age of the subject, their education and social functions performed by their should be considered.
Higher mental functions form one of the most complex functional systems of the body. When diagnosing disorders of higher mental functions, it is necessary to locate focal brain lesion that led to a particular pathology using a system analysis of syndromes of pathology of higher mental functions. The foundations of system analysis were laid by the great Soviet scientist A. R. Luria. The systematic analysis of higher mental functions implies, firstly, rejection of the ideas of both narrow localization, which implies the presence of close connections between a specific function and a certain area of the cortex, and the equivalence of cortical areas in the implementation of all functions. Secondly, the system analysis of higher mental functions requires the use of special neuropsychological methods of examination.
Nowadays, the problem of disorders of higher mental functions is one of the most relevant. These disorders lead to a decrease in the quality of life, disruption of social and professional activities of a person, and, in some cases, to disability and the development of complete dependence on others.
In the process of development of higher mental functions, the formation of interhemispheric asymmetry occurs. In right-handed people, the left hemisphere (dominant) is associated with abstract thinking and speech, as well as with logical and analytical functions, which are speech-mediated. The dominant hemisphere is also involved in the formation of the most complex motor acts, the assessment of time relations. In right-handed people, the right hemisphere (subdominant) is associated with specific thinking, providing general, visual and spatial perception. The subdominant hemisphere also provides emotional reactions, the ability to perceive intonations of speech, perception and differentiation of non-speech stimuli, in particular, music.
Functional differences in the hemispheres and related functions are the basis for the occurrence of various clinical syndromes that develop when they are affected.
The following functions are most significant in clinical practice:
- aphasia;
- apraxia;
- agnosia;
- amnesia.
Aphasia is a speech disorder with a complete or partial loss of the ability to understand someone else’s speech or use speech to express oneself’s thoughts due to a damage to the cortex of the dominant (left in right-handed people) cerebral hemisphere without disorders of the articulatory apparatus and hearing. Speech is the most complex form of mental activity, acting as an independent function and as a connecting link of all mental processes. Speech is closely related to thinking, memory and perception.
There are areas of the dominant cerebral hemisphere, which can give rise to aphasia when damaged:
- premotor — posterior part of the inferior frontal gyrus, cortical areas 44, 45;
- postcentral — lower part of the cortical areas 1, 2, 5, 7 and partially of the area 40;
- superior temporal gyrus — cortical area 22;
- inferior parietal lobule — cortical areas 3, 40;
- posterior temporal lobe — cortical area 37;
- prefrontal — anterior parts of the frontal lobe, cortical areas 9, 10, 11, 46.