28.04.2010 00:00:00
Lefties were created to refute all the concepts that prevailed... in connection with the pathology and physiology of the two hemispheres
A. Subirana
Beginning short review the most fundamental provisions related to the problem of left-handedness, we note once again that “left-handedness” and “left-handedness” in a neuropsychological context are not always synonymous. These concepts merge and are interchangeable when we talk about natural (genetic) left-handedness. That is, in cases where the dominance (manual preference) of the left hand is determined by the genetically given, innate brain organization of a person. They are not synonymous when left-handedness is forced (which will be discussed further when discussing pathological left-handedness). What happens when a “right-handed” program and structure of the organization of brain activity is genetically specified, but one or another traumatic circumstance causes a more active use of the left hand.
This caveat is necessary to avoid terminological confusion. Indeed, further (except for those sections where pathological left-handedness is discussed), these two concepts are used specifically in relation to natural left-handedness, that is, as synonyms, since this book is primarily devoted to left-handedness as a natural phenomenon.
Left-handedness as an independent problem is the object of attention from medicine, neurobiology, psychology, and other scientific disciplines. This fact is due to the fact that the vast majority of theoretical ideas are based on the study of the right-handed population of humanity.
But, despite the fact that in recent decades there has been a radical turn towards the phenomenon of left-handedness within different approaches, it is apparently impossible to name a single finally resolved issue related to this phenomenon.
At the same time, an avalanche-like accumulation factual material clearly indicates the need for further developments in this direction. The central place here is occupied by a comprehensive consideration of the “right-handedness-left-handedness” model from the point of view of the mechanisms of functional asymmetry of the human brain that have developed in evolution.
Asymmetrical hand use (right- or left-sided manual preference) is a species-specific characteristic of humans. In animals this phenomenon is not specific. They may have a manual preference, but it is individual for a given individual, a moving indicator related to the nature of the task being solved. A review of data for the USSR, USA, Great Britain, Western Europe, Japan, and a number of nations in Asia and Africa indicates that approximately 80% of the world's population is right-handed. The incidence of left-handers ranges from 8% to 16-30%.
Such inconsistency of anthropoisometric data indicates, first of all, the heterogeneity of the studied populations in a number of important characteristics. For example, there is a significant “deficit” of left-handed people among the rural population compared to the urban population, which is explained by their migration and search for acceptable social and professional niches.
An accumulation of left-handed people was discovered among people associated with industrial production, among artists, mathematicians, artists, architects, accountants, athletes individual species sports, etc. The number of left-handers varies depending on the geographic region covered by anthropoisometric studies.
It is extremely important that over the past 50 years, the proportion of left-handed people in the European population has increased 3-4 times, which is rightly associated with the widespread cessation of retraining left-handed people, the creation of special technical devices, tools and means of activity specifically adapted to them.
This circumstance is undeniable, but at the same time, the accumulation of left-handed people in recent decades throughout the world is associated with a sharp increase in the number of children with congenital or acquired in early infancy brain insufficiency, which in a huge number of cases causes the occurrence of pathological left-handedness.
Let us return, however, to natural left-handers, recalling that their number throughout the history of mankind has been surprisingly constant: 20-25%. This invariability of indicators is the most compelling argument in favor of understanding the phenomenon of “left-handedness” as an evolutionary path of human development that is exceptional in its importance. If you like, an evolutionary experiment aimed at forming a brain frame fundamentally different from the “right-handed” subpopulation, designed to actualize specific, qualitatively different forms and levels of adaptation.
One of the most important points when assessing manual preference, the age of the subjects is traditionally considered; The child population contains fewer right-handers than adults - the number of right-handed people increases gradually. So the number of right-handers and left-handers (according to average data from a number of authors) is 52% and 47%, respectively, at the age of one year, by two years these figures change to 70% and 29%, and by seven years they reach 80% and 20%, respectively. Thus, the ratio of right-handers and left-handers in human population is a moving indicator depending on age, environmental, cultural and a number of other factors.
An analysis of the historical development of manual preference has allowed a number of authors to suggest that the percentage of left-handers in the human population has not changed from Australopithecus to the present day. And if in the pre-Paleolithic period the number of right- and left-handed people was approximately the same, then the development of tools and more highly organized skills led to clear manifestations of manual asymmetry.
The tendency towards the consolidation of right- and left-handedness as two basic species (truly human) categories in the course of historical development is confirmed modern research conducted on primitive peoples. When studying examples of fine art from Ancient Egypt to the present day, it turned out that in 76-80% of cases right-handed people are depicted in the paintings, and this situation has not changed over the past millennia,
The origin of left-handedness is widely debated. But for some reason, few people ask the questions: “Why did right-handedness arise, why are the overwhelming number of people right-handed?” This question is inexhaustible, but we will try to answer it in the most general terms. It seems that the answer to this should not be sought in the depths of psychology or philosophy; I'm afraid the reason here is much less romantic.
As you know, the blood supply to the left side of the human body (including its upper parts - the neck and head) is much more intense than the right. This is understandable, given the location of the heart, pancreas and spleen (our main citadels of life) on the left in most of humanity. Even at the beginning of the twentieth century, it was shown that in the embryogenesis of vertebrates (including humans), there is an accelerated development of the right half of the body. At the same time, the entire system of metabolic processes in the left part of the upper half of the body is formed earlier and more fully. And accordingly - the left cerebral hemisphere (hemisphere). This was associated with the specifics of the Earth's rotation. Thus, the designated homeostatic intensity was predetermined purely physiologically.
Here we cannot help but recall the hypothesis of M.I. Astvatsaturov, according to which right-handedness was established at the dawn of humanity precisely due to the fact that the left hand (due to its close innervation with the heart) was used immeasurably more sparingly. Firstly, due to the general undesirability of unnecessary trauma to the cardiac area for the body. Secondly, as static, covering (for example, with a shield) the left half of the body, which, as already noted, is replete with vitally dangerous (incompatible with life in case of defeat) bodily zones.
Thus, the evolutionary choice in favor of the left hemisphere of the brain, as dominant in the motor (and then speech, based on movement) sphere in the overwhelming majority of humanity, was apparently made in the direction from the body to mental activity, and not vice versa.
After this short digression, let's return to left-handedness. There are three main directions within which the problem of the emergence of this phenomenon is being developed: “genetic”, “cultural” and “pathological”.
Since 1871, when a high frequency of familial left-handedness was established among left-handed subjects, a model of the genetic determination of left-handedness has been discussed. The fact of inheritance of left-handedness was postulated, and a rule was formulated about the subordination of left-handedness to the recessive distribution according to Mendel, that is, 1:4. Genetic models are still the most common when discussing the origin of left-handedness.
Another group of scientists points to the possibility of not genetic, but cytoplasmic encoding of the gradient of manual asymmetry, putting forward the concept according to which both cerebral lateralization and manual preference are considered in a broad general biological aspect. It is assumed that brain development is influenced by the left-right gradient that exists in the Universe. This leads to earlier differentiation and maturation in ontogenesis of specific systems of the left hemisphere, which have an inhibitory effect on the right - as a result, dominance of the left hemisphere in speech and right-handedness arises. For reasons that are difficult to explain, which are only now beginning to be discussed in the depths of quantum physics, a certain cohort of representatives natural world shows the opposite picture. Left-handedness is one of the manifestations of this cosmic phenomenon.
Directly adjacent to the “genetic” direction are studies related to finding physiological and morphological indicators corresponding to right-handers and left-handers. The most important among them is the fact that the corpus callosum is significantly larger in left-handers. It has been established that in right-handers the speech zones of the left hemisphere are more developed than the corresponding ones in the right, while in 75% of left-handers they are approximately symmetrical. In right-handed people, the internal carotid artery on the left is larger, and the pressure in it is higher than in the right; For left-handed people the picture is the opposite. A similar morphological dissociation is observed in right-handers and left-handers in relation to the middle cerebral artery.
In a broad general biological aspect, the differences between right- and left-handed people are considered in connection with the peculiarities of the biochemical basis of their brain organization, and the question of the connection between left-handedness and specific biochemical, hormonal, and immune status is discussed. A number of researchers hypothesize that left-handedness is characterized not so much by features of morphological or neural organization, but by the comparative equivalence of the cerebral hemispheres due to the symmetrical activating influence on them from the brain stem structures, while right-handed people are asymmetrical in this regard.
This is the general tendency of studying the problem of left-handedness within the framework of “genetic” and general biological approaches. An alternative to the latter are theories based on the recognition of the determining role of cultural conditions in the formation of handedness. "Cultural-social" concepts, which arose in the last century, viewed left-handedness not as a result of the location of internal organs, blood supply or other physiological factors, but as a consequence of social pressure and training.
Along with the mentioned concepts of the origin of left-handedness, the idea of its pathological conditioning is widespread. This position is shared partially or completely by various authors. According to the extreme point of view, any manifestation of left-handedness is a consequence of birth trauma; this is one of the objective evidence of congenital encephalopathy. Such views are confirmed by the fact of a significant increase in the number of left-handers among twins, especially intrauterine development which predispose to the development of cerebral insufficiency.
An intermediate position is held by scientists, to which the author of this book belongs, who believe that only some left-handers are left-handed due to pre- and perinatal brain lesions, and the rest become so under the influence of genetic or other (natural and/or social) factors. It is clear that there may be combined variants of genetic left-handedness in a child with congenital brain pathology, but here I would like, without delving into neuropsychological subtleties, to confine myself to drawing a demarcation line between natural and pathological left-handedness.
What is pathological left-handedness or, more strictly formulated, pathological left-handedness? Indeed, using the term “left-handed” here is not entirely appropriate, since in all these cases it would be correct to use the phrase: “He is “left-handed” because he was unable, due to certain deviations, for example, in the development of his brain, to become right-handed.” In other words, he is not left-handed, but “not right-handed.”
Pathological left-handedness can occur due to “peripheral” and “central” insufficiency. Examples of a “peripheral” radical are such pathological conditions as right-sided torticollis, in which severe hypertonicity of the entire cervicothoracic region leads to complete or partial impossibility (or at least inconvenience) of active manipulation of the right hand. Naturally, since nature abhors a vacuum, the child in this situation is forced to actively use his left hand. The same situation can arise with congenital or acquired early age right hand injury.
The “central” origin of pathological left-handedness is associated with various pre- and/or perinatal functional or organic deficits of various brain structures. This may be a deficiency of the left hemisphere (as is known, which determines the manual activity of the right hand). But much more often in the modern child population, pathological (also known as forced, compensatory) left-handedness occurs in children as a result of hypertensive-hydrocephalic syndrome and deficiency of subcortical (especially stem) brain formations.
It should be noted once again that the compensatory, plastic capabilities of the child’s brain are so great that with competent neuropsychological correction aimed at eliminating (eliminating, leveling) cerebral dysontogenesis, a spontaneous change of the dominant hand often occurs.
This magical transformation is limited by the age of the child and is observed, as a rule, only up to 7-8 years of age. Let us emphasize that here we do not in any way mean “retraining” from the left hand to the right. The work is being carried out towards the formation and stabilization of adequate interhemispheric and subcortical-cortical relationships. Many of the correctional and habilitation programs developed in this direction are presented in the following chapters.
The same effect of “spontaneous” change of hand is observed in children with local lesions of the stem and midline structures of the brain. After surgical intervention carried out before the age of 7-8 years, many of them already within a few days experience a change in their dominant hand from the left to the right.
Thus, the study of the issue of the origin of left-handedness has so far proceeded in three main directions, developing within the framework of fundamentally different paradigms. The abundance of facts, sometimes contradicting each other, shows that each of these hypotheses requires further justification and development.
At the same time, it is obvious that the main provisions of the listed theories do not completely contradict each other. Moreover, they are largely complementary, which forms the basis for further interdisciplinary research, the need for which arises from a set of unresolved questions about the origin and nature of left-handedness.
Even in biblical times, it was noted that people differ in whether they prefer the right or left hand when performing various functions. Thus, the Book of Judges says that in 1406 BC. in the army of the sons of the tribe of Velyaminov, 700 left-handers were selected who “could throw stones from a sling and not miss.” In the Bible, for the first time, a negative attitude towards left-handed people was probably heard when describing the picture of the Last Judgment.
Since then, two lines can be traced in everyday and scientific attitudes towards left-handers. Let us remember Lefty Leskov and what was written in an ancient tome: “left-handed people inspire hostility, suspicion, the impression of the absence of any human virtues and skills; they are prone to practicing alchemy and witchcraft; apparently, they are messengers of the infernal world.” Let us add that many famous magicians and bearers of esoteric knowledge began training their apologists, forcing them to write, draw and perform a lot of everyday actions with both their right and left hands. They saw this as one of the necessary conditions for disclosure psychic abilities person.
The vast majority of studies of the specifics of mental activity of left-handed people are associated with the study of pathological phenomena. At the same time, it has been established that they are significantly more likely than right-handers to develop specific forms of dysontogenesis associated with insufficiency of speech, reading, writing, counting, optical-spatial, psycho motor functions etc.
The fact of the accumulation of left-handed people among patients with epilepsy, neuroses, alcoholism, and various types of substance abuse has been recognized; a connection is shown between the presence of left-handedness and immune disorders, chronic migraines, neuroendocrine pathology, a tendency towards the genealogical accumulation of Down syndrome, and early childhood autism. It is known that in left-handed people the course of psychopathological conditions changes significantly; they are prone to paradoxical reactions to various drugs. Extensive and extremely interesting material on this topic is presented in the famous works of T.A. Dobrokhotova and N.N. Bragina*.
When describing left-handers, one cannot help but emphasize a number of their specific features in comparison with right-handers . Left-handed people are unique emotional status, but also its vulnerability to various internal and external factors. Along with lagging behind right-handers in a number of parameters of mental development, left-handers exhibit a larger vocabulary, greater general awareness and erudition, and higher achievements in mathematics. Among them there are many artistically and artistically gifted.
When studying creativity (the ability to make creative decisions) thinking, its indicators turned out to be significantly higher in the population of left-handed people compared to right-handed people. Finally, one cannot fail to note the main “advantage” of true, natural left-handers - unusually high degree compensatory capabilities of their brain. This fact reveals itself most clearly when assessing the course and spontaneous positive dynamics of speech disorders with local brain lesions. It is also obvious when considering the extraordinary qualities of left-handed children (and right-handed children with family left-handedness) in terms of compensation for various kinds of dysontogenetic phenomena.
Analysis of the above data shows that left-handers represent a specific group in terms of their ontogeny, general patterns the course of mental activity in normal and pathological conditions. Many of the “pathological” traits attributed to them are associated with insufficient attention to the process of their upbringing and training. This circumstance leads to the conclusion about the need to develop special psychological and pedagogical approaches to left-handed children.
But this is possible only in the course of considering those basic mechanisms that predetermine the appearance of such an atypical and contradictory picture. Many authors regard it as evidence of a change in the nature of interhemispheric relationships in left-handers.
What are the meaningful mechanisms on which such a bright and unusual phenomenology is built? The answer to this question urgently required a comprehensive, systematic consideration of the structure of the brain organization of the mental activity of left-handers, its various types and forms of implementation. A unique opportunity on this path was provided by the method of neuropsychological analysis according to A.R. Luria.
The study was also initiated by the fact that in recent years a social order for the development of the topic “left-handedness” has clearly emerged. This is due, on the one hand, to the fact that left-handedness, like right-handedness, is one of the most important and constant characteristics individual psychological differences of people, which must be taken into account when solving a wide range of social and clinical problems. On the other hand, there is a widespread increase in attention to the health and education of left-handed children, in whom there are much more frequent indications of harm suffered during pregnancy and childbirth, as well as violations and distortions of the course of normal development.
In accordance with the fundamental views of Luriev's neuro-psychological science, in the work carried out (first with adult left-handers, and then with children), it was suggested that the basis on which the entire set of atypical manifestations of mental activity is built is their special type of brain organizations mental processes.
As an experimental model, traditional for neuropsychology, the facts of disturbances in mental processes in left-handers with local brain lesions were examined. Analysis of the results obtained allowed us to draw conclusions about fundamental differences cerebral organization of mental activity of right-handers and left-handers. What are they?
1. In right-handed people, as everyone knows, there is a clearly asymmetrical type of cerebral interhemispheric support of mental functions. The degree of their lateralization (representation in each hemisphere) is not the same: speech and somatognosis (perception of one’s own physicality) are represented in the brain of right-handed people in almost diametrically opposite ways, respectively, in the left and right hemispheres. And for example, auditory-speech memory and optical-spatial or constructive activity are more continuous.
But each hemisphere participates in the course of these processes in its own way, bringing its own individual, specific link only for it, its “individual talent.” And this talent is reproduced in every right-handed person; right-handed individuals constitute a fairly homogeneous, highly predictable group.
In left-handers, the cerebral representation of mental functions radically changes its character. They lose the distinctive features of hemispheric support and speech and non-speech processes, regardless of the degree of their lateralization. The cerebral interhemispheric organization of mental activity in left-handers becomes more symmetrical, ambilateral, more diffuse and, we emphasize, less ordered. Moreover, they represent (unlike right-handers) a surprisingly heterogeneous, low-predictable group.
In other words, right-handers, from the point of view of interhemispheric functional organization, are relatively “repeatable” from case to case, and left-handers have this property to a much lesser extent. In any case, during many years of work with left-handers, I was not able to see two subjects who were similar in terms of the cerebral organization of mental activity. This part of the human population is amazingly, rarely individualized and diverse. This applies equally to their following specific characteristics.
Only in left-handed people in adulthood is there a bilateral (in both hemispheres) representation of any psychological factor. A similar picture may occur in right-handed children, but not later than 2-3 years. Left-handers retain this unique, phenomenal quality throughout their lives. Which largely determines one of the basic prerequisites for their compensatory potential.
2. In left-handers, the intrahemispheric organization of mental processes also changes. In right-handed people, it is characterized by a fairly strict correlation of specific psychological links and factors with a specific zone of the anterior or posterior parts of the brain. In left-handers - intrahemispheric functional undifferentiation, diffuseness.
In other words, a certain area of the brain, which invariably actualizes its specific contribution to the course of the corresponding mental function in right-handers, may be associated with a completely different factor in left-handers. Figuratively speaking, for example, the temporal speech areas may not be included, as in right-handers, in providing speech activity, leaving this (in whole or in part) to other areas of the brain. At the same time, they can show their activity in that area of \u200b\u200bmental activity that is never associated with right-handers.
In a single picture of the mental activity of left-handers, such features and disorders can be simultaneously detected that are never found in right-handers simply due to the spatial distance, the functional distance of their brain organization. We emphasize that, similar to the interhemispheric functional specificity of left-handers, their intrahemispheric organization of mental processes is much less predictable and predictable compared to right-handers.
3. An exceptional feature of left-handers is the tendency towards relative functional separation, autonomy of the right and left hemispheres of the brain. The relative functional autonomy of the cerebral hemispheres reflects, along with the already indicated characteristics - functional interhemispheric ambilaterality, undifferentiation and intrahemispheric diffuseness, one of the most important components of the cerebral organization of the mental processes of left-handers.
This specificity of the brain functioning of left-handed people absolutely never occurs in right-handed people, except in cases of surgical or organic destruction of the corpus callosum, which is the central interhemispheric commissure. For left-handers, this is simply one of the features of the functioning of their brain.
4. A unique characteristic of the brain support for the mental activity of left-handers is the pattern of interaction between the cortical and subcortical systems of the brain that is specific to them. In right-handed people, they function, as a rule, in a sequential, reciprocal, asynchronous mode: greater activity of the subcortex invariably leads to less activity of the cortex (primarily its frontal regions) and vice versa. In left-handed people, a fairly frequent picture of simultaneous, synchronous involvement of the subcortical and cortical systems of the brain in ensuring one or another mental act can be observed.
Thus, the cerebral organization of mental processes in left-handers is a special specific system, the basis of which is the following basic parameters: functional ambilaterality, diffuseness and relative disunity, autonomy of the brain hemispheres, insufficient differentiation of subcortical-cortical relations.
Highlighting these characteristics allows us to take a new approach to a number of facts traditionally associated with the problem of left-handedness.
The high degree of friendly work of the cerebral hemispheres of right-handed people, their close functional interaction lead to reciprocal mutual influences between them in normal and pathological conditions. Many authors postulate the fact of the inhibitory influence of the left hemisphere of right-handed people on the right. Obviously, for the friendly functioning of the brain hemispheres, the left (due to its dominance in relation to the overwhelming majority of primordially human speech processes) has higher value. From the point of view of human adaptation in society, the same is true with regard to the dominance of cortical (especially frontal) brain structures over subcortical ones.
Taking into account the fact of the relative autonomy of the brain hemispheres of left-handed people, it can be assumed that in them the reciprocal inhibitory influence of the brain hemispheres is significantly eliminated. Thanks to this, the right hemisphere of left-handers should be more “free”, which allows it to more actively participate in the flow of various types mental activity. If such an assumption is correct, it should reveal itself when analyzing a number of phenomena that distinguish normal and pathological left-handed people.
Let us turn to one of the classic and generally accepted facts - the relatively unexpressed symptoms - of speech disorders and the high rate of their reverse development in left-handers. On the one hand, as has been shown, the right hemisphere in left-handers plays a significantly more active role in the course of verbal functions than right-handers. On the other hand, under conditions of functional disconnection, the right hemisphere does not experience an inhibitory influence from the left and can more intensively participate in the formation of compensatory mechanisms (it is possible that also at the biochemical level). Probably, the combination of these and other factors (especially unique subcortical-cortical relationships) contributes to the achievement of that level of compensatory capabilities of the brain of left-handed people, which leads to smoothness and rapid regression of speech symptoms.
Another example of the manifestation of the “liberated” right hemisphere and the synchronous work of the subcortical-cortical systems is the complex of emotional and personal characteristics of left-handers. Numerous studies conducted on right-handed people have convincingly proven that a person’s emotional life is under the control of the subcortical structures of the brain and the right hemisphere. At the same time, there are indications of the peculiarities of this aspect of mental life in left-handers compared to right-handers: a higher level of emotionality and neuroticism, a significant decrease in their extraversion and sociability.
We cannot exclude the possibility that it is the weakening of the reciprocal interaction of the hemispheres and subcortical-cortical systems, as well as the more diffuse functional organization of the brain of left-handers that is the cause of such phenomena as pronounced multimodal psychopathological symptoms, a tendency to paranormal phenomena, anticipation phenomena, etc.
Probably, it is precisely these factors that lead left-handed people to the creation of specific intermodal connections that contribute to their achievements in the field of painting and music, and determine their significantly significant accumulation among the artistically gifted. They are actualized in the fact that left-handers the best way adapt in individual, non-standardized activities, requiring not so much high conventionality (conventionality) as initiative and intuition.
From the point of view of the concept being developed, it seems possible to explain the fact that the percentage of left-handed people among patients with chronic substance abuse is significantly higher than that in the general population; at the same time, left-handedness is noted as a constitutional trait that predisposes to their occurrence. In conditions of hyperactivity of the right hemisphere and “liberated” subcortical structures associated with basic homeostatic, biochemical and emotional processes, in the absence of corrective control from the left hemisphere (especially its frontal parts), specific signs of the brain organization of left-handed people can act as predisposing factors to this pathology risk.
One of the most debated aspects of the problem of left-handedness is the recognition of its connection with various delays in mental development in childhood. Analysis of literary data, as well as the results of this study, indicate that one of the most significant causes of dysontogenesis may be the brain organization of left-handed people, the atypical (compared to right-handed) nature of its formation.
From the book: These Incredible Lefties: A practical guide for psychologists and parents.
Semenovich A.V.
Article courtesy of the site
Psychology books. Publishing house “Genesis”
Scientists' opinions about how and why left-handedness occurs are not only different, but also contradictory. But, despite a significant number of works devoted to this problem, it cannot be said that it has been fully studied and there is clear, evidentiary evidence of any one hypothesis. Among them there are curious and serious assumptions that have never been proven by anyone and those proven by many years of research.
Apparently, this is proof of the possibility of the existence of various causes leading to left-handedness.
Naturally, in theories of the origin of left-handedness, not only various factors are taken as a basis: sociocultural, pathological, genetic, but also their complex combination.
Many hypotheses of the last century explain the predominant use of the right hand, and left-handedness is considered as an exception.
According to one of these hypotheses, right-handedness is explained by the asymmetrical arrangement of internal organs, which shift the body’s center of gravity to the right. Necessary emphasis on left leg makes the right hand freer, it trains, its muscles become more developed, its movements become more precise.
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Even more popular was the so-called shield and sword theory, according to which the shield during battles should protect the heart and | therefore, the warrior holds the shield with his left (passive) hand, and holds the weapon in his right. Centuries of training of the right hand in countless battles determined the advantage of the right hand. Of course, we can now smile at the naivety, lack of evidence and inconsistency of these hypotheses, but for now we will not completely cross out the possibility of the influence of various environmental factors on the formation of left-handedness. Many researchers highlight social factors and, above all, the conditions of upbringing as the cause of left-handedness, appealing to Plato, who believed: “We use our hands like cripples, and the stupidity of mothers and nannies crippled us; equals-! The weight that nature observes when creating our limbs, we destroy through bad habit.”
One of the first theories of the emergence of left-handedness was the social theory of the English scientist S. Jackson, who considered left-handedness to be the result of an addiction. habits, the influence of social conditions of the environment (almost the theory of shield and sword!). However, Jackson did not believe that such a “trained” habit of working with the left hand was passed on from generation to generation.
Can we challenge this opinion? Of course, the purely social factor of the origin of left-handedness is easy to dispute, because Long-term studies of left-handedness in different countries and on different continents have proven that the percentage of left-handed and right-handed people in the human community does not change. It would hardly be possible with such a strong influence of the environment. However, we cannot completely reject such influence.
There are situations when right-handed people, under the influence of external circumstances, and not due to biological reasons, become left-handed. The most common causes of such left-handedness (sometimes called forced) are injuries to the right hand (fractures, dislocations, sprains) suffered in early childhood. Imagine a one and a half to two year old child with a broken arm. On the one hand, this is the age of very active hand movements, intensive formation of coherence, accuracy, on the other hand, attempts to draw something, add cubes, a pyramid, but the right hand is sick, incapacitated, and the left hand involuntarily becomes more active. If this period does not last long, then there is no reason to worry, but quite often there are several injuries in a row, and then the external environmental conditions take their toll. It becomes more convenient for a child to work with his left hand than with his right, and this is how forced left-handedness arises. There are forced left-handers among children with consequences of cerebral palsy. Neuromuscular regulation disorders and muscle tone on the right make movements with the left hand more convenient. The left hand trains more and develops better.
The influence of the environment may not be so severe, but it will give the same result in the formation of unforced left-handedness. The fact is that a child’s activity is formed by imitating the activities of adults or peers. This is how “non-violent” retraining arises and many left-handers surrounded by right-handers also begin to act more actively with their right hand.
We cannot exclude the possibility of the opposite situation, when a right-handed child finds himself in a left-handed environment.
He may have a left-handed father or mother or grandmother
or grandfather, brother or sister. And the most important thing is in the family
no one will be surprised by another left-hander, and he himself will
consider himself left-handed, unless he is very smart
lym. It should be noted that cases of forced change
hands are usually accompanied by neurotic and emo
national violations, as well as coordination
discomfort, difficulties in mastering complex engines
new skills.
So, external environment may influence and, under certain conditions, lead to the formation of left-handedness, but to be more precise, this is not true left-handedness, because handedness is complexly related to brain activity (but we will talk about this in a special chapter).
The close connection between handedness and the functional organization of the brain does not allow us to agree with the main conclusion of S. Jackson: if, under the influence of the environment, a certain variant of hand preference (right or left) can be formed, then both hands need to be trained equally. The desire to train both hands is the blue dream of sports trainers, alas, it turns out to be unattainable: and if a fencer or tennis player has a leading hand with his right hand, he works with his right hand, and is left-handed
Left precisely because right-handers and left-handers have their own special
digital nature of brain functioning.
Behind the social theory of the emergence of left-handedness there is a long trail of problems associated with retraining. The roots of the desire to retrain a left-handed person are that left-handedness was considered a bad habit, and bad habits not only can, but also must be eradicated.
For many decades, data have accumulated on a significant number of left-handers suffering from neuropsychiatric disorders, developmental delays,
destruction speech development. This gave reason to assume that a pathological genesis or pathological origin of left-handedness is possible.
Currently, there is a sufficient amount of evidence confirming the possibility of a pathological origin of left-handedness. Moreover, there are supporters of extreme views who claim that any left-handedness, even found in different generations left-handers in the family, has a pathological origin associated with brain dysfunction, and most of Such cases are explained by birth trauma. Indeed, studies conducted in different countries, including our country, show that among left-handed children the incidence of pathology in pregnancy and childbirth and birth injuries is very high. It cannot be ruled out that there is a genetically determined increased risk birth injuries in certain families. And then truly pathological left-handedness can manifest itself in different generations.
The likelihood of having a left-handed child increases the so-called birth stress. Childbirth stress is usually determined by the presence of at least two unfavorable factors, including low birth weight of the newborn, too fast or prolonged labor, the use of instruments during obstetrics, asphyxia of the newborn and a number of others.
But there are few supporters of such extreme views; most researchers are inclined to believe that pathological left-handedness does occur, and it is important to understand that in these cases left-handedness can be combined with a complex of disorders or delays in the development of speech, perception, and motor functions. At the same time, it is necessary to distinguish between left-handedness and these disorders, highlighting, perhaps, pathological factors associated with the course of pregnancy and childbirth as a single cause of both left-handedness and disorders in the development of the child.
The detection rate of a left-handed child in right-handed parents is only 2%, it increases to 17% if one parent is left-handed, and increases to 46% if both parents are left-handed. In the fifties of our century, these figures were clarified, but they do not differ much: 50% of children become left-handed if both parents are left-handed, 16.7% if one of the parents is left-handed, and 6.3% if there was no left-hander in the family.
However, confirmation of the possibility of the influence of genetic factors does not yet answer the question of how this happens. Genetic models that explain the intergenerational transmission of handedness should fill this gap. The estimated data produced by any genetic model must coincide with the practical data, and the higher the agreement, the more likely it is that such a model can be used to explain the inheritance of handedness.
One of the genetic theories of the origin of left-handedness, proposed by scientists D. Levy and T. Nagilaki, connects handedness and lateralization of speech functions and suggests the dependence of handedness on two genes, one of which determines speech lateralization, and the other determines the control of motor functions.
This model is based on the assumption that handedness in each specific case is associated with interhemispheric asymmetry and the type of motor control. In accordance with the model, most people are right-handed and have a speech center in the left hemisphere - this is true. Among left-handed people, there are two possible options for the location of speech centers, both in the left (the majority) and in the right hemisphere, which is also confirmed by specific results. It is known that each hemisphere controls the limbs of the opposite side of the body: the left - the right; side, right - left. However, in accordance with the model of D. Levy and T. Nagilaki, it was necessary to assume the possibility of controlling the right side of the body with the right I
hemisphere, and the left - the left. Until recently, the question of the possibility of controlling the hand not only by the opposite (contralateral) hemisphere was controversial, but electrophysiological data in recent years confirm the possibility of ipsilateral control of hand movements.
Left hemisphere dominance in hand and speech is considered by many authors as an indispensable condition for effective activity, and all discrepancies are explained by the result of pre- and perinatal lesions. However, there is no consensus among researchers on this issue either. It has been suggested that speech function and manual preference for one of the hands may not be related to each other, because the left hemisphere is usually dominant for language, regardless of motor lateralization. Further studies of the lateralization of speech and motor functions, to a certain extent confirming these provisions, at the same time convincingly showed the possibility of the existence of various options.
Another genetic model of American researchers N. Geschwind and A. Galaburda suggests the contribution of hereditary factors to the formation of lateralization depending on a person’s sex. The evidence is discussed that prenatal development factors are important in the origin of left-handedness, including the endocrine status of the mother and child, which determines the development of the right and left hemispheres of the brain during the period of intrauterine development.
A fairly clear genetic model of the inheritance of left-handedness, known as the “right shift” theory, was proposed by the English scientist M. Annette; according to this model, the “right shift” factor determines the probability of dominance of the left hemisphere for both speech and motor functions (i.e. determines right-handedness). Left-handedness is explained by the absence of a gene containing
What is needed is the right shift factor, and then an invariant random choice is possible, i.e. in fact, right-handedness is inherited, but left-handedness is not.
In accordance with the theory of M. Annette, left-handed people, ambidextrous people and right-handed people in the human population are distributed according to 4%, 33%, 66%. At the same time, almost all researchers note that left-handedness is not inherited as a simple trait.
There are variant theories that consider the complex combination of pathological and genetic factors underlying the manifestation of lateral preference factors.
Considering all these facts, apparently, the idea of heterogeneous (multiple causes) left-handedness would be more correct. Left-handedness, caused by various pathological factors, can be distinguished, as well as left-handedness, which is based on a mixed genesis. It is also impossible to exclude the possibility of a forced preference for the hand (forced left-handedness) due to the loss, defect of a limb, pathology of the peripheral part of the analyzing system, and other reasons.
In fact, three main factors can be identified that determine preference for the left hand: genetic laterality, or genetic hereditary basis; pathological laterality, largely due to pre- and perinatal disorders in brain development; forced laterality. Moreover, it is possible various options combinations of these factors, which significantly complicates the identification of the nature of their origin and requires the development of special diagnostic methods, causes and degree of hand preference.
So, we don’t yet know why most people on earth have a more dexterous, stronger and more accurate right hand and why only part of it is their mirror image.
We do not know why the number of left-handers does not decrease or increase, what factors determined such a high stability of the ratio of left-handers and right-handers, regardless of space and time, which means we are still far from solving the mystery of the origin of left-handedness.
THEORIES OF THE APPEARANCE OF LEFT-HANDERS
In general, there are many theories regarding the origin of left-handedness. From semi-fictional to completely scientific. Right-handedness was explained by the right-sided location of the liver, which shifts the center of gravity of the body, and left-handedness by the heart, forcing the warrior to hold. Some (the majority) believe that left-handedness develops during pregnancy. Others are of the opinion that brain development and the determination of the dominant hemisphere are strongly influenced. There are hypotheses that the number of left-handed people fluctuates depending on historical period. Some experts tend to see left-handedness as a consequence of birth trauma and even pathology.
GENETIC THEORIES
These theories are associated with the existence of a right shift gene, that is, right-handedness. This gene is inherited in a certain way, while the left shift gene appears as a random variant. Even the position of the fetus during pregnancy can play a role.
Previously, the theory generally accepted by right-handed society sounded like this: - these are freaks, a deviation from the norm. They need to be retrained. Left-handedness is a disease, like six-fingeredness, it is inherited - it needs to be eradicated. During the formation of man or later, nature played with genes for the purpose of evolution and gave birth to left-handed people. It was a mistake of nature, but the gene turned out to be tenacious. It lay low, passing on through a generation or even two, but not allowing itself to be destroyed.
If the mirror arrangement of internal organs is observed in one person out of ten thousand, then people who write with their left hand and play tennis with it are quite common - 8-15 percent of them. total number homo sapiens. Dr. Christopher Makienas from University College London suggested that a process occurs in the human body that has two stages. Our body has a gene that allows us to distinguish left from right - a kind of mark. This tag is then read by another gene, not yet discovered by scientists. It is he who determines whether a person will be left-handed or right-handed.
If we are born with one variant of the latter gene, we become right-handed. But a significant part of people have another version of it, it reads the mark in such a way that they become left-handed.
Recently, scientists from Oxford University managed to discover the gene that makes its owner left-handed. In addition to left-handedness, the gene carries a slightly increased risk of schizophrenia in its carrier.
Gene codenamed LRRTM1, in all likelihood, plays a key role in the distribution of functions such as speech and emotion between the hemispheres.
The structure of the brain is character. In right-handed people, the left hemisphere controls speech, and the right hemisphere controls emotions. Left-handers are distinguished by a mirror distribution of normal functions, and it is the gene found that is such a “switch.”
According to the leader of the study, Dr. Clyde Franks from the University of Oxford, understanding the genetic nature of the distribution of functions between the hemispheres will help to trace the development and structure of functional asymmetry of the brain.
The division into right-handers and left-handers is a unique feature of humans. It is curious that only half of left-handers are genetic left-handed, and 50 percent are compensatory, that is, they became left-handed as a result of damage to the left lobe of the brain
And in addition to this, perhaps the most incredible and interesting theory - it explains the origin, paranormal and simply outstanding abilities of left-handers and the mechanism of transmission of the gene for left-handedness - " A parallel world". It was expressed by Peter Chereda and is as follows:
"Once upon a time, an expedition from parallel universe. The aliens, who looked no different from people, also had extraordinary psychic abilities; they had the gift of foresight, were able to instantly assess the situation, and make decisions with lightning speed and correctly. Our ancestors perceived them as gods who descended from the sky. Later, earthly women gave birth to children from these “gods”. And a cycle began - “divine mirror genes” bestow amazing abilities and talents on their descendants. According to the hypothesis of Peter Chereda, almost all brilliant people in distant relatives have aliens from the “mirror space”.
LEFT-HANDED
THEORIES OF THE APPEARANCE OF LEFT-HANDERS
In general, there are many theories regarding the origin of left-handedness. From semi-fictional to completely scientific. Right-handedness was explained by the right-sided location of the liver, shifting the center of gravity of the body, and left-handedness by the heart, forcing the warrior to hold a shield in his left hand and a sword in his right. Some (the majority) believe that left-handedness develops during pregnancy. Others are of the opinion that the development of the brain and the determination of the dominant hemisphere are strongly influenced by the nature of the course of labor. There are hypotheses that the number of left-handed people fluctuates depending on the historical period. Some experts tend to see left-handedness as a consequence of birth trauma and even pathology.
GENETIC THEORIES
These theories are associated with the existence of a right shift gene, that is, right-handedness. This gene is inherited in a certain way, while the left shift gene appears as a random variant. Even the position of the fetus during pregnancy can play a role.
Previously, the theory generally accepted by right-handed society sounded like this: left-handers are freaks, a deviation from the norm. They need to be retrained. Left-handedness is a disease, like six-fingeredness, it is inherited - it needs to be eradicated. During the formation of man or later, nature played with genes for the purpose of evolution and gave birth to left-handed people. It was a mistake of nature, but the gene turned out to be tenacious. It lay low, passing on through a generation or even two, but not allowing itself to be destroyed.
If the mirror arrangement of internal organs is observed in one person out of ten thousand, then people who write with their left hand and play tennis with it are quite common - 8-15 percent of the total number of homo sapiens. Dr. Christopher Makienas from University College London suggested that a process occurs in the human body that has two stages. Our body has a gene that allows us to distinguish left from right - a kind of mark. This tag is then read by another gene, not yet discovered by scientists. It is he who determines whether a person will be left-handed or right-handed.
If we are born with one variant of the latter gene, we become right-handed. But a significant part of people have another version of it, it reads the mark in such a way that they become left-handed.
Recently, scientists from Oxford University were able to discover a gene that makes its owner left-handed. In addition to left-handedness, the gene carries a slightly increased risk of schizophrenia in its carrier.
The gene, codenamed LRRTM1, appears to play a key role in the distribution of functions such as speech and emotion between the hemispheres.
The structure of the brain is asymmetrical. In right-handed people, the left hemisphere controls speech, and the right hemisphere controls emotions. Left-handers are distinguished by a mirror distribution of normal functions, and it is the gene found that is such a “switch.”
According to the leader of the study, Dr. Clyde Franks from the University of Oxford, understanding the genetic nature of the distribution of functions between the hemispheres will help to trace the development and structure of functional asymmetry of the brain.
The division into right-handers and left-handers is a unique feature of humans. It is curious that only half of left-handers are genetic left-handed, and 50 percent are compensatory, that is, they became left-handed as a result of damage to the left lobe of the brain
And in addition to this, perhaps the most incredible and interesting theory - it explains the origin, paranormal and simply outstanding abilities of left-handers and the mechanism of transmission of the gene for left-handedness - “Parallel World”. It was expressed by Peter Chereda and is as follows:
“Once upon a time, an expedition from a parallel universe visited our planet. The aliens, who looked no different from people, also had extraordinary psychic abilities, they had the gift of foresight, were able to instantly assess the situation, make decisions with lightning speed and correctly. Our ancestors adopted them like gods who descended from the sky. Later, earthly women gave birth to children from these “gods.” And a cycle began - “divine mirror genes” bestow their descendants with amazing abilities and talents. According to the hypothesis of Peter Chereda, almost all brilliant people have aliens in their distant relatives from "mirror space".
The origin of left-handedness is traditionally associated with the action of three groups of factors: environmental (including cultural), genetic and pathological. In accordance with the first, environmental conditions, social traditions and the education system determine the preferential choice of the leading hand. Until recently, it was believed that left-handed people accounted for an average of 5% of the population. However, there are some differences in the incidence of left-handedness in different regions.
According to some data, the frequency of occurrence of the “lateral phenotype”, assessed by the nature of sensorimotor asymmetries and interhemispheric
EEG differences reveal a connection with the characteristics of environmental conditions. Thus, among the indigenous population of the northeast of Russia (Eskimos, Chukchi, Koryaks, etc.), the “right hemisphere phenotype” is much more common, which is characterized by the predominance of right hemisphere functions in the interaction of the hemispheres. It is assumed that the increase in the proportion of left-handers and ambidextrous people in northern populations indicates their more optimal adaptation to life in those conditions. Perhaps the large proportion of left-handers among these population groups is partly due to the fact that the pressure of cultural traditions aimed at the predominant use of the right hand in this region has always been weaker than in central Russia. The point of view is also expressed that the “right-hemisphere phenotype” is generally more characteristic of populations close to nature and less involved in scientific and technological progress.
Among schoolchildren in China and Thailand, only 3.5 and 0.7%, respectively, use their left hand to write. At the same time, 6.5% of children from Eastern countries studying in US schools (where the pressure to use the right hand is weakened) prefer the left hand. Among Japanese schoolchildren, 7.2% are not right-handed, and if we take into account those who are overeducated, this figure increases to 11%. Until recently, in many countries there was a practice of retraining left-handed children. As the pressure from the environment weakens, the number of people writing with their left hand increases noticeably, up to 10-12%.
The significant role of environmental experience in determining handedness seemed to be supported by the fact that infants did not show a dominant hand. However, it is known that the relatively late manifestation of a trait does not mean its environmental conditioning. In studying the handedness of adopted children adopted in infancy, it was also shown that adoptive parents (as opposed to biological ones) have little influence on the establishment of the dominant hand in children.
One approach to solving this problem is to develop specific genetic models that explain the possibility of transmission of handedness from generation to generation. Experimental data for constructing genetic models are obtained mainly from family studies of handedness, including studies of adopted children, as well as from twin studies.
The first family studies of handedness suggested that segregation for this trait occurs according to Mendel's law. As a result, a simple genetic model was proposed, according to which it was assumed that handedness is determined by the action of one gene, which has two different forms (two alleles): one allele R is dominant, encoding right-handedness, the second l is recessive, encoding left-handedness. A child who has inherited the R alleles from both parents will be right-handed, as will a child with the Rl genotype (R from one parent, l from the other). Children with genotype ll who inherited the l allele from both parents will be left-handed.
This model, however, cannot explain the fact that, according to various sources, from 45 to 54% of children of two left-handed parents are right-handed. The model predicts that all children of such parents should be left-handed, since the only allele that left-handed parents can pass on to their children is the l allele. Attempts were made to improve this model by introducing the concept of penetrance. In particular, it was proposed to consider that the manifestation of a recessive gene depends on random factors, and possibly on the action of other genes.
A more complex model was proposed by J. Levy and T. Nagilaki. They proposed that handedness is a function of two genes. One gene with two alleles determines the hemisphere that will control speech and the dominant hand. The L allele determines the localization of speech centers in the left hemisphere and is dominant, and the l allele determines the localization of speech centers in the right hemisphere and is recessive. The second gene determines which hand the speech hemisphere will control - on its side (ipsilateral) or the opposite (contralateral). Contralateral control is encoded by the dominant allele C, and ipsilateral control by the recessive allele c. An individual with the LlCC genotype, for example, will be right-handed with a speech center in the left hemisphere. An individual with the Llcc genotype will also have speech centers in the left hemisphere, but he will be left-handed. This model suggests that handedness of a particular person is associated with the nature of his interhemispheric asymmetry and type of motor control. A key assumption of this model was that the position of the hand when writing (straight or everted/inverted) can serve as a phenotypic manifestation of a specific genotype. Individuals who use an inverted hand position when writing, from the authors’ point of view, do not have a decussation of the pyramidal tract, i.e. They are characterized by an ipsilateral type of organization of motor control. On the contrary, individuals with a normal (straight) hand position when writing have a decussation in the structure of the pyramidal tract, which leads to ipsilateral control.
IN last years the model of J. Levy and T. Nagilaki has been repeatedly criticized. It has been shown to conflict with some facts. For example, clinical data obtained from 131 patients do not give reason to believe that there is a connection between the position of the hand when writing and the localization of the center of speech, determined by the Wada test. It is also known that many left-handers can use both straight and inverted hand positions for writing. In addition, the model has not received adequate statistical support in family studies.
To accurately establish the specialization of the hemispheres in relation to speech, the Wada method is used - selective “hemisphere anesthesia”. In one
a solution of a sleeping pill (amytal sodium) is injected from the carotid arteries in the neck (left or right). Each carotid artery supplies blood to only one hemisphere, therefore, with the blood flow, the sleeping pill enters the corresponding hemisphere and exerts its effect on it. The greatest changes in speech activity are observed when the hemisphere in which the center of speech is localized is affected.
In addition, other models of the heritability of left-handedness have been proposed (Table 16.1).
The famous English psychologist M. Annette has been developing an original genetic model of lateralization for twenty years, which is fundamentally different from the model of Levy and Nagilaki. She hypothesized that most people have a gene called the right shift factor (rs+). If a person has this gene, then he is predisposed to become right-handed with left-hemisphere localization of speech centers. In its absence and the presence of a pair of its recessive alleles (rs-), a person can become either right-handed or left-handed, depending on the circumstances (for example, on the conditions of intrauterine development).
In accordance with M. Annette’s hypothesis, in the human population there is a balanced polymorphism associated with the distribution of the dominant “right-sided shift” gene and its recessive allele. Moreover, from her point of view, the influence of these genes extends not only to handedness (right-handedness or left-handedness), but also to cerebral dominance, i.e. general dominance of one or another cerebral hemisphere. She believes that the choice of the preferred hand is only one of the results of cerebral lateralization, and since it is impossible to assess the degree of lateralization directly, the easiest way is to use indirect methods, among which the leading place is occupied by the assessment of handedness.
M. Annette has developed a unique test to assess the degree of use of the hand - the peg shifting test. In this test, the subject must transfer pegs from one cell to another, and the cells are located on two parallel panels. Since he performs this task for a time and alternately with his left and right hands, by comparing the results, it is possible to evaluate differences in the effectiveness of action with one and the other hand . Thus, the indicator of manual asymmetry here is the test completion time: the dominant hand works faster. Using mainly this test, she conducted numerous studies of manual asymmetry in children and adolescents with speech disorders (in particular, dyslexia) and in healthy ones in connection with the success of performing different tests on mental development. These studies allowed M. Annette to make some very significant additions to her theory.
Table 16.1 Models of heritability of left-handedness (according to various authors)
|
Model type |
Basic |
Options |
|
|
publications |
phenotypes |
||
|
genetic |
allele R - right-handed |
right-handed (geno- |
|
|
single-locus |
types RR, Rl) |
||
|
(dialle- |
allele l - left-handedness |
left-handed (geno- |
|
|
genetic |
allele L - speech center |
eight phenotypic |
|
|
T. Nagilaki |
bilocus |
in the left hemisphere |
Pov, I distinguish- |
|
quadruple- |
allele l - in the right |
local |
|
|
allele C - counterlate- |
tions of the center |
||
|
ral control ru- |
speech leading |
||
|
hand and position |
|||
|
allele c - ipsilate- |
|||
|
genetic |
allele rs+ - shift la- |
at rs+ rs+ (geno- |
|
|
single-locus |
teralization to the right |
types: rs+ rs-) |
|
|
diallelic |
allele rs-------situational- |
right-handed indie |
|
|
ny determination of ru- |
|||
|
bones under the influence |
with genotype |
||
|
epigenetic factors |
rs-rs- how right- |
||
|
hands and left |
|||
|
hands (proportions |
|||
|
may vary) |
|||
|
I. McManus |
genetic |
allele D - right-handed |
|
|
single-locus |
|||
|
diallelic |
allele C - situational |
||
|
handedness definition |
individuals with leading |
||
|
under the influence of epigene- |
cabbage soup with your right hand |
||
|
tic factors |
|||
|
allele D is not dominant |
|||
|
ten in relation to |
|||
|
K. Leland and |
genetic transmission |
handiness of the individual |
|
|
others (K. Laland |
cultural |
happens according to the model |
defined: |
|
I. McManus |
1) its genotype; |
||
|
cultural influences |
2) phenotype ru- |
||
|
displace genetic |
the bones of his parents |
||
|
disposition in this or |
|||
|
in a different direction |
It is assumed that the right shift gene leads to the development of asymmetry, inhibiting in early ontogenesis the development of, firstly, the temporal region (planum temporale) in the right hemisphere, and secondly, the left posterior parietal region of the cortex in the left hemisphere. Thanks to this, the left temporal region gains the opportunity to primarily participate in the processes of phonological processing, and the right posterior parietal region - in the processes of visual-spatial analysis.
In accordance with such ideas, homo- and heterozygous states of the right shift gene can lead to significant changes in the functioning of each hemisphere, and heterozygotes for this gene will have the greatest advantages in the cognitive sphere (Table 16.2). Homozygotes for the dominant gene (rs+) will experience a particularly strong shift effect,
Table 16.2
Predicted effects of lateralization based on the theory of “right-sided shift” [by: 185]
|
Levels of Analysis |
|||
|
1. Genetic: |
|||
|
prevalence |
|||
|
(in the population) |
|||
|
2. Cerebral: |
|||
|
left advantage |
absent |
moderate |
|
|
POTTUPTYAPIA |
|||
|
deficit of functions of the right |
absent |
moderate |
|
|
second hemisphere |
|||
|
3. Cognitive: |
|||
|
probability of violations |
present |
absent |
absent |
|
phonological spe- |
|||
|
properties |
|||
|
probability of violations |
absent |
moderate |
|
|
visual-spatial |
|||
|
natural abilities |
|||
|
4. Behavioral: |
|||
|
right-wing dominance |
absent |
moderate |
|
|
percentage of people I write- |
|||
|
left-handed* |
|||
the consequence of which at the behavioral level will be weakness of the left hand, probably combined with some impairment of the functions of the right hemisphere (for example, spatial abilities). The presence of the rs+ gene, from Annette’s point of view, generally has a negative effect on spatial abilities. Homozygotes for the recessive gene (rs-), on the contrary, form a risk group with regard to the development of speech skills, in particular phonological ones. Experimental evidence of the advantage of heterozygotes has been established in the study of certain academic achievements.
M. Annette's theory has become widely known and is the subject of serious debate. For example, in 1995, an entire issue of the journal (Current Psychology of Cognition. V. 14. No. 5) was devoted to its collective discussion. Moreover, the theory has both supporters and critics.
The target of criticism is the main positions of the theory: the idea of balanced polymorphism and the advantage of heterozygotes for the right shift gene. For example, F. Breeden, considering the estimated proportions of distribution in the population of the rs+ and rs- genes, which, according to Annette, are manifested in the characteristics of cognitive deficits, notes the discrepancy between the postulated theory and the actual facts of disorders in cognitive functioning. He, like I. McManus et al., considers it appropriate to distinguish two dimensions of lateralization - direction and degree - and is of the view that these dimensions may have different genetic determination.
Along with Annette's theory, other genetic models of lateralization are being developed. These include, for example, the genetic model of I. McManus. His model, however, in many respects is very close to Annette’s model; McManus postulates the existence of one gene D, which determines right-handedness, the second gene C determines not left-handedness, but a situational variant of the formation of the leading hand. In addition, the existence of a modifier gene localized on the X chromosome and influencing sex differences in left-right-handedness is assumed. Unlike Annette, McManus does not deal with features of cognitive functioning, i.e. it does not extend the actions of genes D and C to cognitive functions.
As already noted, it has been suggested that manual asymmetry may be of a polygenic nature. However, there are no polygenic models of lateralization that have been developed in detail like M. Annette’s theory [review: 196]. For example, according to one of them, it is assumed that a moderate degree of lateralization is a species-specific norm, while it is assumed that polygenic homozygosity leads to destabilization of development, which manifests itself in a significant number of developmental disorders, including schizophrenia, autism, and some physical disorders.
anomalies, including extreme shifts in either direction from the species-specific predetermined moderate right-handedness. However, from these positions it is difficult to explain how genetically determined mechanisms of lateralization arose in anthropogenesis. In the context of simple genetic models, this issue is easier to resolve, since one or two new genes can be assumed to change or appear in evolution.
It is recognized that functional asymmetry of the brain, like left-hemispheric dominance, is a unique, specific feature of the human brain that arose in anthropogenesis in connection with the advent of speech and right-handedness. We can talk about the predominance of the left hemisphere and its control of motor functions in animals. However, lateral control of motor functions in animals is dynamic, and its lateralization can shift depending on the function being served. According to some assumptions, the tendency to preferentially use the right hand arose in primitive populations, when their members did not yet have a genetically determined superiority in the use of one or another hand. The evolutionary events that determined the appearance of the leading hand apparently occurred after the separation of man as a species from his closest relative, the chimpanzee.
As follows from the above, despite a significant number of studies in this area, there is currently no generally accepted genetic model that explains the phenomena of lateralization of the hand and speech centers. There are also facts that are difficult to explain by all genetic models. We are talking, first of all, about the approximately equal similarity of MZ and DZ twins in terms of manual asymmetry [review 196]. These data are often used as evidence of the absence of genetic determination of handedness. However, as D. Bishop notes, the possibility of situational determination of handedness under the influence of epigenetic factors (Annett and McManus models) allows us to reject this objection. There is another interesting observation regarding the heritability of the dominant hand in twins. Parents who are not clearly right-handed are more likely to give birth to twins.
M. Annette, in order to explain the lack of significant differences between MZ and DZ twins in terms of manual asymmetry, also admits that the manifestation of the right-sided shift gene in twins compared to singletons is reduced by 33-50%.
Against this highly controversial background, a number of criteria have been formulated that genetic models of handedness must satisfy. In accordance with these criteria, the model should explain the following facts: 1) the ratio of handedness of parents and children in the following proportions: approximately 90, 80 and 60% of children with parental combinations right/right, right/left, left/left should be right-handed; 2) inability to predict
a child's handedness based on the handedness of his siblings, as well as approximately equal degrees of concordance between MZ and DZ twins in handedness; 3) the existence of differences in the frequencies of handedness in different geographical regions, which can be traced very clearly, although all human communities are predominantly right-handed. In addition, the model should explain what mechanisms underlie the establishment of handedness in ontogeny. The last point cannot be justified within the framework of purely genetic models of handedness. At the same time, environmental models of handedness cannot explain the first two provisions.
Another, recently formulated approach is associated with the proposal to introduce a genotype-cultural, co-evolutionary model for the establishment of handedness. It is based on three provisions: 1) the existing diversity in hand dominance is not determined by genetic diversity; 2) individual differences between people on this basis arise as a result of the interaction of cultural influences and development conditions; 3) genetic influences are preserved, since handedness acts as an optional (optional) trait acquired during ontogenesis. Thus, the authors of the model assume that variations in handedness are not determined solely by genetic factors; in other words, all individuals, regardless of origin, have a common genotype that determines the preferential use of the right hand. However, since this tendency does not clearly predict right-handedness, some part of the population becomes left-handed. Parents play a decisive role here: they significantly increase their children’s tendency to be right-handed (if they are both right-handed) or no less significantly reduce it if they are left-handed. The authors of the model limit themselves to analyzing handedness only, without touching on the problems of hemispheric specialization and the role of genotype-cultural factors in its formation.
In conclusion, it should be said that current models of functional lateralization concern only the establishment of the dominant hand and, to some extent, hemispheric dominance. At the same time, they completely do not take into account such a phenomenon as the profile of lateral organization, and do not include in the analysis all possible aspects of lateralization of functions. In this regard, we can add one more requirement to a full-fledged genetic model of lateralization: it must provide the ability to describe and explain all aspects of functional lateralization and specialization of the brain.
FEATURES OF FUNCTIONAL ASIM
In terms of asymmetry, twins differ significantly from other representatives of the population. For example, among twins there are often mirror pairs in which one twin is right-handed and the other is left-handed.
It is widely believed that left-handedness is more common among twins than in the general population, however, a specific comparison of the incidence of left-handedness among twins and singletons found only a slight trend towards a predominance of left-handedness in the twin population.
Differences between twins are also noted in the asymmetry of the eyes, legs and a number of morphological characteristics (for example, the direction of curl of the hair on the back of the head, dermatoglyphic patterns on the fingers and palms of the hands).
Studies of hand asymmetry in twins are quite numerous, and, as a rule, approximately the same similarity between MZ and DZ twins in terms of manual asymmetry is noted. According to various studies, 22.5-24.6% of MZ twins and 19.3-30.7% of DZ twins show discordance (mismatch) in handedness [review 196, 334]. There are only a few works on the asymmetry of other psychophysiological functions: these are studies of auditory asymmetry based on dichotic listening data and the asymmetry of some visual functions. Moreover, both contain indications of the dissimilarity of MZ twins in terms of the phenomena being studied.
It is possible that the absence of significant differences between MZ and DZ twins when studying body asymmetries is due to the methodological features of assessing asymmetries as such. As a rule, in groups of DZ and MZ twins, similarity is assessed on the side of dominance of a trait or function. The quantitative expression of asymmetry is not always taken into account. Meanwhile, as noted, the direction of asymmetry and its intensity are most likely relatively independent traits that may have different sources of genetic determination.
The use of the twin method to study hereditary and environmental influences on the formation of functional asymmetries may be more productive if twins are compared not only by side of preference or direction of asymmetry, but also by the quantitative expression of the trait on both sides (provided that the trait being studied can be measured) , as well as by the degree of intensity of asymmetry, independent of the side of preference. According to some ideas, the genotype controls not only and not so much the direction of asymmetry as such, but the degree of its severity.
Various attempts have been made to explain the emergence of asymmetry-discordant twin pairs. For example, it was hypothesized that the inversion of asymmetry in pairs of MZ twins, called mirroring, is associated with the characteristics of their embryonic development, namely with the stage at which the embryo was divided into two independent organisms.
If division occurs by early stages development, when there is no differentiation into the right and left halves of the embryo, then there will be no signs of mirroring. If the separation occurs later, at the stage when such differentiation has already occurred, one can expect the appearance of mirror MZ pairs. This hypothesis is supported by the fact that in conjoined twins, who are considered to have formed at relatively late stages of embryonic development, specularity is much more common, and cases of extreme expression of specularity (inversion of the location of internal organs) are possible. However, this hypothesis does not explain why among MZ twins there are pairs that are mirror in some characteristics and identical in others. In addition, Newman's hypothesis cannot explain the existence of discordant pairs among DZ twins. And among MZ twins, mirror pairs are quite rare: according to twin statistics, the number of MZ twins who separated after the formation of the first amniotic sac (8-9 days of embryonic development), when lateralization of characteristics is already possible, is only 34% of the total number pairs of MZ twins.
Other possible reason the appearance of pairs discordant for asymmetry - a greater susceptibility of twins to the action of pathogenetic environmental factors, which can have different effects on each of the partners. Such factors are likely to equally increase the number of discordant pairs among both DZ and MZ twins. The possibility of the emergence of asymmetry-discordant couples due to uneven intrauterine conditions and birth stress is recognized by many authors.
Among the pathogenic intrauterine factors affecting the overall development of twins is also called delayed maturation. A single fetus has a better intrauterine position; twins, both MZ and DZ, experience greater limitations in both intrauterine space and resources. In late pregnancy, these restrictions can lead to significant delays in maturation. Indeed, there is evidence that in the period from 19 to 32 weeks, the development of some grooves and convolutions on the surface of the cerebral cortex in twins is delayed by 2-3 weeks compared to singletons. This delay in the maturation of the cerebral cortex creates unfavorable conditions that, in part, may explain delayed speech development and the appearance of left-handedness among twins of both types.
Thus, the occurrence of pairs of twins discordant for asymmetry can be caused by various reasons. In addition to the inequality of conditions of intrauterine development and childbirth, which are equally possible in pairs of twins of both types, in pairs of MZ twins there are cases of actual mirroring due to the specificity of their formation from one embryo.
A recognized method for studying functional asymmetry of the human brain is the study of manifestations of asymmetry in the bioelectrical activity of the cerebral cortex (EEG and EP). A systematic study of indicators of interhemispheric asymmetry of the background EEG was carried out by T.A. Meshkova [see Ch. XIII]. She provides the following observations regarding asymmetrical manifestations in the EEG of adult twins.
1) Partners are characterized by varying degrees of EEG asymmetry. If one twin has pronounced asymmetry, then the EEG of the second, as a rule, is symmetrical. There are no actual mirror pairs, i.e. partners with pronounced asymmetry of the opposite sign. There are pairs of DZ twins, discordant in hand dominance, in which the right-hander is characterized by pronounced and even increased asymmetry of the EEG of the temporal leads, while the EEG of the left-hander is symmetrical.
2) Significant asymmetry of the EEG of the temporal leads was recorded with increased activation on the left.
3) Genetic factors are more pronounced in the right hemisphere, and environmental factors - in the left.
4) The intensity of EEG asymmetry is determined by environmental factors.
It is characteristic that all of the listed features are most clearly detected when analyzing the EEG of the temporal regions or are partially true for the EEG of other symmetrically located zones.
In a study conducted by T.A. Meshkova and colleagues, in twins aged 7-8 years (28 pairs of MZ and 22 pairs of DZ), the heritability of manual asymmetry and the degree of asymmetry of the background EEG were simultaneously studied. In children, as well as in adult twins, in terms of indicators of manual asymmetry (it was assessed using several quantitative tests) and indicators of EEG asymmetry, no significant differences were found in pairs of MZ and DZ twins. The authors come to the conclusion that hereditary factors do not affect the intensity of lateralization either in motor skills or in the bioelectrical activity of the brain.
Thus, the study of asymmetries in pairs of MZ and DZ twins does not make it possible to resolve the question of what contribution genotype factors make to the formation of interindividual variability in asymmetry indicators.
At the same time, a comparative study of the nature of interindividual variability in EEG indicators of the left and right hemispheres, taken independently of each other, was carried out. The data obtained indicate that the individual EEG characteristics of the left and right hemispheres depend to varying degrees on genotype factors. Moreover, this is true both for relatively elementary EEG indicators (amplitude and alpha rhythm index), and for more complex ones, such as the EEG periodicity coefficient (the ratio of the power of the periodic to the random component), the dispersion of instantaneous amplitude values, and the coefficient of local nonstationarity. Based on the totality of all data from T.A. Meshkova came to the conclusion that there are significant interhemispheric differences in the degree of genetic influence on the listed indicators, and there is a greater susceptibility to environmental factors of some EEG parameters of the left hemisphere compared to the right, especially in terms of EEG indicators of the temporal zones.
When studying the genetic determination of visual evoked potentials to stimuli different shapes and content, significant interhemispheric differences were established: in the temporal zone of the left hemisphere, the variability of responses to most stimuli was determined mainly by environmental influences; in the temporal zone of the right, on the contrary, the variability of similar responses was determined by the influences of the genotype.
T.A. Meshkova and her colleagues also established some other facts indicating a relatively greater contribution of the right hemisphere to the interindividual variance in motor skills and EEG indicators. In other words, the intensity of asymmetry both in motor skills (in terms of the use of the left hand) and in the EEG is formed mainly due to individual differences in the functioning of the right hemisphere. Some gender differences were also established, in particular, for example, right-handed girls had more pronounced EEG asymmetry. In addition, it turned out that children with higher intelligence scores had more developed motor skills of the left hand. Drawing on data on the greater similarity of people in the anatomical features of the left hemisphere, T.A. Meshkova and her colleagues made the following assumption. The left hemisphere, due to the localization of speech centers and the leading hand in it, is more susceptible to the unifying influences of the environment and therefore is functionally more uniform in all people. The right hemisphere, more than the left, determines the natural individuality of a person, associated with his biological, including hereditary, characteristics.
Functional asymmetry and specialization of the hemispheres is a unique feature of the human brain that arose in anthropogenesis in connection with the advent of speech and right-handedness. In ontogenesis, the specialization of the hemispheres is formed gradually and reaches its definitive form only in the period of maturity. There are large individual differences in the asymmetries of paired organs, which allow us to speak about types, or stable profiles, of the lateral organization of the individual. Genetic studies of asymmetry examine the heritability of manual asymmetry and, to some extent, hemispheric dominance. There are a number of genetic models describing the possible inheritance of the dominant hand, but none is generally accepted. Criteria are formulated that must be satisfied by a genetic model that adequately describes the type of inheritance of the leading hand.
MZ and DZ twins, as a rule, show a similar degree of concordance in terms of functional asymmetry. Some MZ twins are characterized by the phenomenon of mirroring in paired characteristics. At the same time, studies of the heritability of electrophysiological parameters of the left and right hemispheres performed on twins give reason to assume a greater contribution of the genotype to the variability of indicators of the functioning of the right hemisphere.
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