by Lyudmila ZHAVORONKOVA, Dr. Sc. (Biol.), Leading Researcher, Laboratory of General and Clinical Neurophysiology, Institute of Higher Nervous Activity and Neurophysiology, Russian Academy of Sciences
Why the number of left-handers is so great among artists, musicians - representatives of the so-called creative professions, while, say, pilots are as a rule right-handers? Why do functions impaired after craniocerebral injuries more rapidly restore in left-handers? What are similarities and differences between people with different individual asymmetry profiles? What are the mechanisms of formation of these peculiarities? The answers to these questions are interesting not only to neurophysiologists, but to specialists of many other spheres of knowledge as well.
Paul Broca, a French anatomist and anthropologist, showed for the first time in 1861 that injury to the left brain hemisphere leads to aphasia (loss of speech), while injury to the right hemisphere is not associated with this abnormality. These data underlie a concept, which existed in science for a long time: the left hemisphere is the leading one in right-handers, while the functional role of the right one is secondary. However, as facts were accumulated, it became clear that the right hemisphere performed many important functions as well. For left-handers the right hemisphere was considered to be predominant for speech, but this opinion proved to be erroneous: the speech representation in individuals with this type of asymmetry can be left-hemispheric and even bilateral (mixed).
According to various data, the number of right-handers in human population varies within 80 - 95 percent, while the rest are left-handers and ambidexters (people with equal potentialities of both hands). Judging by statistical data, the number of left-handers, for example in Europe, increased 3 - 4-fold during the recent half-century, which is justly attributed to overall cessation of their re-training and creation of adapted technological devices and instruments for them. "Left-handedness" becomes an object of increasingly focused attention of neurophysiologists, psychologists, and sociologists. These data
EEG coherence spectra in symmetrical regions of the brain at rest in a right-hander.
are in general essential for everyday life-for upbringing and training of a child, choice of occupation. Manifestations of special artistic and music gifts, actor's capabilities, and high level of general information, often associated with achievements in mathematics, architecture, and other sciences, were revealed in left-handers.
Increasing interest in studies of brain organization in left-handers (this term includes the definition of not only the leading hand, but of other organs of motion and senses) is determined, among other things, by the requirements of medical practice. Psychoneurological disorders in left-handers, similar to those in right-handers, manifest themselves in dissimilar symptoms. Some data indicate that left-handers are at a higher risk of epilepsy, schizophrenia, etc.; they are more sensitive to external and internal factors.
Numerous data accumulated by the present time indicate that in all of us, irrespective of the asymmetry profile, one brain hemisphere differs from the other by a number of morphological, functional, and biochemical parameters reflected in the peculiarities of mental activity. A common feature of all Homo sapiens is that brain asymmetry progressively increases with years in children, thus determining their normal mental development, reaches the peak by middle age, when brain work is the most effective, and is leveled in old age, its productivity decreasing with aging. Let us note that due to rapid development of modern methods of investigation, such as positron-emission tomography, etc., we now better understand the mechanisms of these intricate processes.
The anatomical differences between the two brain hemispheres are better studied in detail in right-handers. It was found that many structures of the left hemisphere are characterized by greater size in comparison with the symmetrical compartments of the right one. The most expressed (7-fold!) neuroanatomical asymmetry was detected in the acoustic cortex. Similar differences are also characteristic of the posterior part of the postcentral gyruses. It was reported that the left occipital pole is longer and often reaches beyond the midline in comparison with the right pole, while the right hemisphere is wider than the left one in the central and frontal compartments. Differences in the blood supply to the hemispheres can be evaluated at least by the size of the left internal carotid artery and arterial pressure in it: the figures are higher. The same is true, according to available data, for the volume of the left lateral ventricle and the content of the gray matter in proportional comparison with the white matter in the left hemisphere, though the right hemisphere is heavier. The cortical areas, involved in the realization of the speech signals, are more extensively represented in the left hemisphere.
The neuroanatomical differences of the brain of right-handers are characteristic of not only "speech" zones,
EEG coherence spectra in symmetrical regions of the brain at rest in a left-hander.
but of other structures as well-visual, posterior associative (parietal), which are significantly larger in the right hemisphere than in the left one. On the other hand, these comparisons indicate that this asymmetry is not modal specific ("speech" vs. "visual" hemisphere). It is most likely that the primary projection zones of both systems are concentrated mainly in the left brain hemisphere, while the cortical areas of intermodal associative zones are located mainly in the right hemisphere.
These features create prerequisites for unequal functional potentialities of the hemispheres: short axon* bonds predominate in the left hemisphere, particularly in the primary projection areas of the cortex, while the bonds between the regions predominate in the right hemisphere. Hence, the left hemisphere in the right-handers possesses structural basis for "local" processing of a stimulus, while the right hemisphere has better potentialities for complex information processing. Therefore, two basic features are important in the picture of neuroanatomical asymmetry of the brain in right-handers: the sensory and motor regions are better represented in the left hemisphere, while the associative zones are characteristic of the right one; intracortical bonds predominate in the left hemisphere, while intercortical in the right one.
These differences form the base for the differentiated role of the hemispheres in the use of cognitive strategies. The left hemisphere plays the leading role in solution of linguistic, verbal problems, provides realized psychomotor activity, speech, and thinking and memory based on it. The right hemisphere dominates in the solution of spatial constructive tasks and is responsible for the psychosensory sphere: perception of the world and oneself in the world, emotional experience, memorization of events in the form of sensory images with accurate reflection of space and time during which the events took place and their image formed in the mind of the individual.
The data on neuroanatomical organization of the brain in left-handers are scanty, but even they give a picture of its lesser asymmetry in comparison with the right-handers. Sylvius sulcus** is located symmetrically in both hemispheres in 71 percent of left-handers, though some data indicate a greater size of the acoustic cortex in the right hemisphere in comparison with the left one. The internal carotid artery and blood pressure are greater in the right hemisphere in many left-handers (remember that the situation is different in right-handers). However, the neuroanatomical asymmetry of the brain in left-handers is not a "mirror" reflection of that described in the right-handers and the variants are much more numerous.
* Axon is a neuron process conducting nerve pulses from the cell body to innervated organs or other nerve cells. - Ed.
** Sylvius sulcus is the lateral sulcus of brain hemispheres. - Ed.
Analysis of the capacity of each of the hemispheres to perform the functions of the contralateral one showed that impairment of the left hemisphere at the early stages of ontogenesis in right-handers does not lead to development of aphasia, and speech develops due to intactness of the right hemisphere. At the same time, disorders in nonverbal forms of behavior, emerging after impairment of the right hemisphere, are not compensated for by functioning of the intact left hemisphere. Left-handers are characterized by lack of clear-cut correlation between the side of brain injury and the emergence of local disorders, which indicates more diffuse (in comparison with right-handers) representation of verbal and constructive functions.
At present we have data indicating that the differences between the hemispheres, observed during performance of cognitive tasks both in right- and left-handers, are due to differences not only in the cortical systems of the right and left hemispheres, but also in the subcortical structures. Based on the clinical findings, it was suggested that the realization of mental processes in the left hemisphere of right-handers was transferred into the cortex in the course of evolution, while the subcortical formations started acting as pacemakers*. This did not happen in the right hemisphere, and the same processes ran in the presence of significant activity of the deep formations, for example, diencephalic**, having closer functional relationship with it. The data on the left-handers indicate that functional differences in the subcortical structures are less developed in them in comparison with the right-handers.
Different theories were suggested on the mechanisms of inter-hemispheric interactions, but, as a rule, they concern the right-handers. W. Denenberg, a German neurophysiologist, suggested in the early 1990s that interactions between the hemispheres were possible in simultaneous realization of three processes: activation, inhibition, and correlation (speaking about correlation, this scientist meant that both hemispheres were components of a regulatory system working as the negative feedback mechanism).
The model of inter-hemispheric interactions, formulated in the 1980s by Vitaly Bianki, Dr. Sc. (Biol.) (Leningrad State University), is also based on three main principles of brain activity: asymmetry, dominant, and complementarity. The model implies the participation of both horizontal and vertical streams of stimulation and inhibition, forming the hemispheric asymmetry, - presumably, they are different in right- and left-handers. Numerous observations indicate that systems and processes, essential for realization of such functions as speech, visual spatial and motor functions, are formed and retained in one hemisphere and suppressed in the other in the course of the brain maturing in right-handers. In left-handers the development of mechanisms of inter-hemispheric interactions is atypical with sufficiently independent and stable formation of motor and speech digrams*** in the right and left hemispheres during the ontogenesis.
"Intercourse" between the hemispheres is realized through numerous commissures (adhesions), specifically, corpus callosum**** (dissection of corpus callosum impairs the sensorimotor, tactile, and other information exchange). By the way, corpus callosum in left-handers is larger than that in right-handers, and this, no doubt, plays the key role in the inter-hemispheric organization of mental processes in the former and is essential for the character of manifestation of pathological symptoms.
Many facts about one more aspect of brain asymmetry - neurochemical - were accumulated during recent decades. The biochemical substances, including the mediators, are asymmetrically distributed in the right and left hemispheric structures of right-handers. For example, the content of dopamine***** is greater in the left-hemispheric basal nuclei****** in comparison with the right hemisphere. Asymmetry of this hormone level in various brain structures increases with brain maturing. Analysis of enzymatic activities in the cortical matter of right-handers showed high levels of acetylcholine******* in the precentral and frontal cortex of the left hemisphere in comparison with the right one. By contrast, neurochemical studies of serotonin******** metabolites distribution showed that they more often predominated in the right hemisphere, particularly in the mediofrontal compartments of the brain. These facts should be borne in mind by physicians prescribing drugs modulating the mediator systems of the brain for more effective recovery of right or left hemispheric functions.
Let us add here that the same drugs, for example, anesthetics, can cause different therapeutic effects in lefthanders in comparison with right-handers, which is presumably due to the former's different neurochemical asymmetry of the brain and also requires special attention of practitioners.
Analysis of distribution of neurohormones and neuropeptides and their receptors revealed differences in the brain hemispheres of right- and left-handers and more incident (2.5 times) diseases of immune nature in lefthanders. This prompted putting forward an immune theory of the origin of "left-handedness" (along with genetic, culturological, and pathological theories).
* Pacemakers are neurons spontaneously generating pulses. - Ed.
** "Diencephalic" means referring to the diencephalon, - Ed.
*** Engram is a hypothetical structure keeping the memory trace. - Ed.
**** Corpus callosum is a nerve fiber cord connecting the left and right brain hemispheres. - Ed.
***** Dopamine is a nervous system mediator, a neurohormone; biochemical precursor of norepinephrine and epinephrine. - Ed.
****** Basal nuclei are anatomically independent group of paired subcortical structures; facilitate the motions triggered by the cortex and suppress the unnecessary concomitant ones. - Ed.
******* Acetylcholine is a mediator (transmitter) of nervous stimulation. Released into the blood, it decreases its pressure, decelerates heartbeat, etc. - Ed.
******** Serotonin is a mediator of nervous pulse conduction through the synapse. Cerebral neurons utilizing it are involved in regulation of many behavioral forms, sleeping and thermoregulation processes. etc. - Ed.
Time course of the mean EEG coherence levels and motor responses to an acoustic stimulus in a right- and left-hander during falling asleep and awakening.
Electrophysiological studies appreciably supplement the above-described picture, as analysis of the main electroencephalogram (EEG) rhythms, recorded in the cortex (like the peak of an iceberg, it reflects the deep basic neuroanatomical and biochemical processes in the brain), is one of approaches to studies of inter-hemispheric asymmetry. Comparison of the amplitudes and frequencies of the main normal human EEG rhythm (alpha-rhythm*) revealed that its amplitude in right-handers in a state of quiet consciousness was less in the occipital regions of the left (dominant) hemisphere. Its amplitude decreased and frequency increased in various types of mental activity, these parameters depending on the type of the task. Specific asymmetry of this rhythm depends on the preferred strategy, type of the task to be performed (verbal or spatial constructive), emotional factor, degree of anxiety of the examined subject, etc.
Increase in the number of recorded potentials and ranges of EEG rhythms analysed (slow: delta and theta and fast: beta and gamma) is expected to provide more ample information on the involvement of appropriate brain structures in certain cognitive processes. Our many-year studies showed that coherent** analysis of rhythmic components of EEG, reflecting the efficiency of relationships between various regions of the brain and widely used both in our country and abroad, is the most informative for analysis of the hemispheric function.
Our analysis proved that the hemispheric differences between right- and left-handers were the most demonstrative in evaluation of intracortical interactions in cortical regions linked with the location of speech zones. The coherency values were higher in the centro-frontal regions of the right hemisphere in both right- and lefthanders. This can be attributed to a greater intensity of cerebral bloodflow and metabolic processes in the centro-frontal compartments of the right-hemispheric cor-
* Spectral analysis of cerebral potentials is an important part of their investigation; different components of the spectrum are named delta (0.5 - 4 Hz), theta 1 (4 - 6), theta (6 - 8), alpha (8 - 13), beta 1 (13 - 20), beta 2 (20 - 40 Hz), gamma waves (including the beta 2 rhythm frequency and higher). - Ed.
** Coherency is a course of several oscillatory or wave processes coordinated in time. - Ed.
Changes in EEG rhythm ranges in symmetrical frontal and central compartments of the brain in right- and left-handers.
tex and right hippocampus*, which reflects not so much the intracortical, as mainly cortico-subcortical interactions. Impairment of the above inter-hemispheric balance can be associated with emotional imbalance, as a result of dysfunction of the median structures and right hemisphere linked with the formation of negative, philogenetically more ancient basic human emotions.
Clear-cut differences in EEG organization in right- and left-handers were detected in functional exercises, for example, in exercises performed by the leading hand. Right-handers exhibited a local elevation of EEG coherency in the symmetrical central regions of the cortex for alpha- and beta rhythm ranges (zone of the motor analyzer cortical projection) in parallel with its reduction in the rest of compartments, including the symmetrical regions of the right hemisphere. Left-handers were characterized by universal changes in coherency for the majority of cortical regions and rhythm ranges. These features presumably reflect the formation of local systems involved in motor regulation in right-handers and of diffuse systems in left-handers.
Oppositely directed changes in alpha- and theta-range coherences in right-handers and universal synphase shifts in left-handers are particularly interesting. These facts, together with the data on the origin of different ranges of EEG rhythms, indicate different involvement of the cortical and subcortical cerebral structures and their specific interactions in the regulation and performance of voluntary movements.
Differences between right- and left-handers in the EEG spatial and time organization under conditions of activity were confirmed in an experimental situation of an opposite type-during falling asleep. The inter-hemispheric asymmetry of the brain, characteristic of conscious state, decreased in both right- and left-handers at the expense of more pronounced reduction in the mean coherence levels in the dominant hemisphere. During experiment the volunteers were suggested to simultaneously press the buttons by both hands in response to accidentally presented acoustic stimuli. It was found that as the right-handers were falling asleep and EEG coherence in the dominant hemisphere reduced, motor responses by the right hand ceased first, while the responses of the left hand were retained, and ceased only after complete falling asleep; an opposite picture was observed during awakening. In left-handers the same regularity was the opposite or manifested itself less clearly.
However, a more important fact is that even in the above situation the right-handers exhibited opposite changes in the coherence of alpha-beta- and theta-delta-rhythm range fluctuations during alteration of functional conditions (consciousness/sleeping), while in lefthanders universal, synphasic changes in the coherence of all above ranges of the same rhythms was observed in all cortical regions.
In a word, it can be assumed that different behavior of EEG rhythms in the right- and left-handers reflects different specialization of the cerebral regulatory systems in the formation of inter-hemispheric asymmetry. Thus, in right-handers the maintenance of left-hemispheric asymmetry at rest, supported mainly by the alpha range, can be provided by intracortical interactions. In contrast to left-hemispheric, the right-hemispheric asymmetry, observed during falling asleep, when the coherence of theta-delta ranges increases, indicates the presence of functionally
* Hippocampus is a hemispheric gyrus at the base of the temporal lobe; it is a part of the limbic system, is involved in emotional reactions and memory mechanisms. - Ed.
Scheme of interaction of the hemispheric cortex and subcortical regulatory systems of the brain in right- and left-handers.
significant relationships of the right hemisphere to the diencephalic and, presumably, limbic* structures (including the hippocampus). The left-hemispheric asymmetry is maintained in them also with participation of the beta range, which indicates functional relationships of the left hemisphere to the stem structures. On the one hand, lefthanders, as well as right-handers, exhibit signs of functional bondage of the right hemisphere to the diencephalic and, presumably, limbic structures, while on the other, the specificity of the hemispheres is less and these bonds are less shaped as regards the stem structures. Opposite behavior of EEG fast and slow rhythms, detected in right-handers, reflects the presence of elements of competition between both the hemispheres and cortical and subcortical structures, functioning as the negative feedback mechanism. In left-handers the synphasic pattern of EEG rhythm behavior seems to indicate the formation of predominant interactions between the hemispheres and between the cortical and subcortical structures as the positive feedback mechanism.
This concept of brain organization in right- and lefthanders promotes better understanding of various clinical manifestations detected in brain diseases. For example, our data indicate that left-handers participating in the Chernobyl power plant accident aftermath developed after 5 - 7 years more severe psychoneurological disorders in comparison with right-handers (up to epileptic seizures). This is in line with clinical data indicating that the risk of epilepsy is higher for left-handers vs. righthanders and indicates a higher sensitivity of the former to radiation exposure.
Along with this, in collaboration with scientists from N. N. Burdenko Institute of Neurosurgery, Russian Academy of Medical Sciences, we compared the specific features of the recovery of impaired functions in right- and left-handers with the same type, severity, and side of brain injuries. This process was more rapid and complete in left-handers, which is explained not only by lesser functional specialization of the cortical centers, but also the basic organization of their brain.
In conclusion, we should like to emphasize that the above specific features of the brain in left-handers most likely form the physiological base for their extraordinary traits, adoption of non-standard decisions, including the creative activity (when a bright idea suddenly downs on somebody) described in literature not once and characteristic of them. It seems that these characteristics are responsible for the fact that a genius is more often seen among left-handers than in the population of right-handers. We should like to remind once more about these facts, in contrast to not a rare opinion claiming that the formation of brain asymmetry in left-handers is pathological. We believe that the principle of brain organization other than in the right-handers, preferable in certain activities, including operator activity, is also highly effective for other activities, including creative work.
Illustrations supplied by the author
Photos from the web-site smistar.ru
* Limbic system is a sum of cerebral structures involved in the regulation of visceral functions, olfaction, instinctive behavior, emotions, memory, sleeping, consciousness, etc. - Ed.
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