The significance of interhemispheric asymmetry in the development of compensatory processes in the human brain

For more precise definition of the role of hemispheric interconnections in mechanisms of human CNS compensation the intercentral relations of the electrical activity of the left and right cerebral hemispheres were studied on physiological model of focal interhemispheric asymmetry. Spectral-coherent EEG characteristics of 36 patients with tumoral damage of one hemisphere were studied in condition of chronic (prior to operation) and acute (early terms of postoperative period) brain decompensation. In was shown that the reorganization of the structure of the EEG intercentral relations correlated with definite stages of CNS compensatory processes and that the character of hemispheric interconnections depended on the lateralization of the damage focus. The primary role was revealed of the degree of the left (dominant) hemisphere preservation in restoration of normal pattern of the interhemispheric asymmetry of the coherence of human brain electrical processes.     

Effect of the unilateral activation of brain hemispheres on visceral pain sensitivity in BALB/c mice

A hemispheric asymmetry of the visceral pain sensitivity control was revealed in the BALB/c mice: animals with the left hemisphere inactivation did not differ from sham-operated control mice in respect to the pain response parameters. A right hemisphere inactivation reduced or suppressed the pain response. This suggests that the right hemisphere domineers in the visceral pain control.     

The hemispheric lateralization of the recognition of noisy visual stimuli in man

At tachistoscopic unilateral presentation of noisy visual stimuli and application of "yes-no" method in man predominance was found of the right hemisphere by the number and "yes" reaction time and of the left hemisphere by the number of responses "no". At verbal mnemic load preceding the presentation of visual patterns the left hemisphere asymmetry was observed by the number of "yes" responses and reactions time of both types. FMA was more clearly expressed in men in the first case and in women--in the second one. In more difficult conditions of recognition of several types of patterns, FMA was noticed mainly in women: initial left hemisphere advantage during the increase of the disturbance was changed to the right hemispheric one and appeared again. Preferential participation of the right hemisphere in singling out of the visual signal from noise is supposed. Possibility of the left hemispheric asymmetry manifestation was determined by the specificity and complexity of the visual task, by the level of the disturbance, presentation of competitive task and sexual composition of the group.     

The effect of sex steroid hormones on interhemispheric asymmetry in rats]

The effect of gonadectomy and sex-steroid hormones treatment on functional interhemispheric asymmetry to the reaction of pain cry avoidance of another species (emotional reactions) and motor and exploratory activity of open-field behavior in Wistar rats of 3 months old has been investigated. A spreading depression technique for hemisphere inactivation has been used. The hemispheric asymmetry of the reactions in intact rats was characterized by sex dimorphism; the left hemisphere dominated to a great extent in males than in females under the control of emotional reactions; in motor and exploratory activity in open-field behavior of rats the left hemisphere dominated in males and the right one--in female. In both sexes the neonatal gonadectomy levelled the interhemispheric differences in reactions under investigation. The following treatment of females with estradiol and males with testosterone didn't restore the asymmetry. After the castration at the age of 3 months the correlation between the size and direction of interhemispheric differences became reverse. The treatment of females with testosterone and males with estradiol both castrated in adulthood restored the interhemispheric asymmetry in males and had no effect in females. The treatment of intact rats with hormones of opposite sex led to the enhancement of left hemisphere dominance in motor and exploratory activity in males and levelled the asymmetry in females. It has been shown that in adult rats sex-steroids effect predominantly the right hemisphere.     

Changes in the EEG power in male and female subjects during visual creative thinking

Gender-related differences in EEG patterns during creative visual thinking were investigated in 10 men and 10 women. The spectral power density was analyzed in the range of 4-30 Hz. Gender differences in the hemispheric asymmetry of theta1-rhythm desynchronization were found: females demonstrated greater desynchronization in the right hemisphere than in the left hemisphere, whereas no asymmetry was revealed in males. Only in women numerous negative correlations between creativity indices and task-induced theta1 desynchronization were found.     

Spontaneous asymmetric circling behavior in hemi-parkinsonism; a human equivalent of the lesioned-circling rodent behavior

When induced experimentally in rodents, hemispheric asymmetry in basal ganglia dopamine results in spontaneous asymmetric circling toward the hemisphere with the lower level of dopamine. A similar asymmetry has long been thought to exist in the brains of hemi-Parkinsonian patients. Using an electronic turn counter, we demonstrated that, like unilaterally lesioned rats, and without being aware of it, five ambulating outpatients with hemi-Parkinson's disease exhibit spontaneous rotation toward the hemisphere containing less striatal dopaminergic activity.     

Interhemispheric EEG asymmetries during unilateral bright-light exposure and subsequent sleep in humans

We tested whether evening exposure to unilateral photic stimulation has repercussions on interhemispheric EEG asymmetries during wakefulness and later sleep. Because light exerts an alerting response in humans, which correlates with a decrease in waking EEG theta/alpha-activity and a reduction in sleep EEG delta activity, we hypothesized that EEG activity in these frequency bands show interhemispheric asymmetries after unilateral bright light (1,500 lux) exposure. A 2-h hemi-field light exposure acutely suppressed occipital EEG alpha activity in the ipsilateral hemisphere activated by light. Subjects felt more alert during bright light than dim light, an effect that was significantly more pronounced during activation of the right than the left visual cortex. During subsequent sleep, occipital EEG activity in the delta and theta range was significantly reduced after activation of the right visual cortex but not after stimulation of the left visual cortex. Furthermore, hemivisual field light exposure was able to shift the left predominance in occipital spindle EEG activity toward the stimulated hemisphere. Time course analysis revealed that this spindle shift remained significant during the first two sleep cycles. Our results reflect rather a hemispheric asymmetry in the alerting action of light than a use-dependent recovery function of sleep in response to the visual stimulation during prior waking. However, the observed shift in the spindle hemispheric dominance in the occipital cortex may still represent subtle local use-dependent recovery functions during sleep in a frequency range different from the delta range.     

Hemispheric asymmetry of the nigrostriatal system of the brain in rats genetically predisposed to catalepsy]

Hemispheric asymmetry of nigro-striate system in a strain of rats GC bred from Wistar for a predisposition to cataleptic reaction was studied by means of biochemical and morphological methods. Hemispheric asymmetry was found in GC and Wistar rats with respect to aminopeptidase activity in neurons of caudate nucleus, with a more pronounced left-side increase in GC rats, the asymmetry index being 13.7%. Acetylcholine esterase activity in subcellular particles of caudate nucleus showed an inversion of asymmetry with higher activity in the left hemisphere of Wistar and right hemisphere of GC rats, and asymmetry index of 15.5%. With respect to the number of astroglia cells in S. nigra, and astroglia and oligodendroglia in N. accumbens there was also an inversion of asymmetry in GC rats who had more cells in the structures of the left hemisphere, whereas Wistar rats had more in the right hemisphere. The asymmetry index was high and equal to 29.8% for astroglia in S. nigra, and 17% for astroglia and 21.4% for oligodendroglia in N. accumbens. However, in S. nigra the number of neurons and oligodendroglia cells was equally increased in the right hemisphere in GC and Wistar rats. The data suggest that the mechanism of hereditary pathology of brain nigro-striate system involves both enhancement and inversion of the hemispheric asymmetry.     

Unilateral Left-Hand Contractions Produce Widespread Depression of Cortical Activity after Their Execution

The execution of unilateral hand contractions before performance has been reported to produce behavioral aftereffects in various tasks. These effects have been regularly attributed to an induced shift in activation asymmetry to the contralateral hemisphere produced by the contractions. An alternative explanation proposes a generalized state of reduced bilateral cortical activity following unilateral hand contractions. The current experiment contrasted the above explanation models and tested the state of cortical activity after the termination of unilateral hand contractions. Twenty right-handed participants performed hand contractions in two blocks, one for each hand. Using electroencephalogram (EEG), the broad alpha band and its asymmetry between hemispheres before, during, and after hand contractions were analyzed. During contractions, significant bilateral decrease in alpha amplitudes (indicating cortical activation) emerged for both hands around sensory-motor regions. After contractions, alpha amplitudes increased significantly over the whole scalp when compared to baseline, but only for the left hand. No modulation of hemispheric asymmetry was observed at any phase. The results suggest that unilateral hand contractions produce a state of reduced cortical activity after their termination, which is more pronounced if the left hand was used. Consequently, we propose that the reduced cortical activity (and not the persistent activation asymmetry) may facilitate engagement in subsequent behavior, probably due to preventing interference from other, nonessential cortical regions.     

Hemispheric asymmetries in cortical electrical activity during sleep. I

The hypothesis of a predominance of the right hemisphere in stage REM as compared to NREM has been tested through a spectral analysis of the EEG recorded from left (T3) and right (T4) temporal sites in 5 young healthy right-handed male subjects. Variations in the asymmetry coefficient R - L/R + L in different sleep stages have been analyzed by one way ANOVAs and Sheffé's tests. The hypothesis of a progressive increase in left hemisphere activity throughout different REM cycles as one approaches final awakenings have been investigated by comparing variations in the asymmetry coefficient for epochs of REM and stage 2 NREM sampled in different phases of the REM cycle. EEG results do not support either the hypothesized stage dependent or cycle dependent variation in EEG activity during sleep. We question whether variations in EEG amplitude and synchronization can be used as indices of hemispheric asymmetries during sleep.     

The effect of gonadectomy on interhemispheric asymmetry in rats

The influence was studied of the gonadectomy in the newborn and mature male and female rats on functional interhemispheric asymmetry of the reaction of avoidance of pain scream of another rat ("emotional resonance"), and motor and investigatory activity in the open field. Consecutive inactivation of the hemispheres was realized by K+ spreading depression. It has been shown that neonatally gonadectomized rats have no interhemispheric asymmetry of the studied reactions. In male rats gonadectomized in mature state, interhemispheric asymmetry of "the emotional resonance" reaction is not significant and in the motor and investigatory activity in the open field, in contrast to intact animals, the right hemisphere is dominant and not the left one. Ovariectomy of mature female rats led to the increase of the dominance of the left hemisphere in the control of "the emotional resonance" and change of the right hemispheric dominance in the control of the motor and investigatory activity in the open field for the left hemispheric one. Gonadectomy of male and female mature rats had an opposite effect on the functioning of the right hemisphere: facilitating in male rats and inhibitory in female ones.     

The topography of functional interhemispheric asymmetry in the visual cortex]

Functional interhemispheric asymmetry was investigated by evoked potentials method in experiments on ten cats under ethaminal anaesthesia at 200 points of the visual cortex during the action of binocular and monocular photic flashes of submaximal intensity. Topographic maps have been plotted of the functional interhemispheric asymmetry. In most of the animals a hemisphere dominant and non-dominant at the given moment can be singled out. Section of the callosal body leads to reduction of the functional interhemispheric asymmetry due to a decrease of the focus of maximum activity in the dominant hemisphere and its increase in the non-dominant one. A mozaic pattern of functional interhemispheric asymmetry has been demonstrated, as expressed in the existence of zones of inverse dominance along with prevailing zones of direct dominance. Section of the callosal body produced a decrease in the area of direct dominance and an increase in that of inverse dominance. Absolute interhemispheric asymmetry was most pronounced in the central part of the visual cortex (field 18 and its medial boundary) and the relative one, on the periphery of the visual area (fields 17 and 19).     

Application of transcranial magnetic stimulation for monitoring of the motor cortex condition in dynamics in healthy subjects

Transcranial magnetic stimulation (TMS) is used already for sixteen years for studying human central nervous system. The main objective of this work was to study motor thresholds and their hemispheric asymmetry in healthy subjects during TMS. We examined 31 righthanded healthy students. Their motor thresholds were measured in May (before vacations), September (immediately after vacations), and November (two months after vacations). Magnetic stimulator Neurosoft-MS (Ivanovo, Russia) was used for TNS of the motor cortex. It was shown that in the absence of regular active functional loads on the right hand, the motor thresholds in healthy righthanders significantly increased under the TMS of the left hemisphere, and hemispheric asymmetry disappeared under conditions both of muscle relaxation and voluntary contraction. Motor thresholds under the left-side TMS decreased and hemispheric asymmetry recovered with the restart of the regular active functional loads on the right hand.     

Relationship of focal epileptogenesis to hemispheric motor prevalence in human ontogenesis

The authors describe the clinical and electroencephalographic findings in 220 children with focal and secondary generalized epilepsies without gross structural aetiology, with the aim of evaluating the significance of hemispheric motor prevalence for the formation, activity and possibility of compensation of the focus. Focal epileptogenesis was observed significantly more often: a) in partial seizures with elementary symptomatology in the motor non-dominant hemisphere, b) in the motor non-dominant hemisphere in boys, c) in the whole series, irrespective of the form of epilepsy and sex, in the motor nondominant hemisphere in children under the age of six and in the motor dominant hemisphere in children over the age of six. The findings in partial seizures with an elementary motor symptomatology probably arise from the very close relationship to motor areas of the cerebral cortex, in boys from evidently greater hemispheric functional asymmetry than in girls and the age correlation from unequal maturation of functionally identical parts of the brain hemispheres during ontogenesis.     

Interhemisphere asymmetry of the CSF dynamics and biomechanical properties of the skull

This investigation shows that both intracranial liquor circulation and skull biomechanical properties evaluated by its pliability (compliance) to intracranial pressure are characterised by marked interhemisphere asymmetry. The interhemisphere differences of cerebrospinal fluid mobility were evaluated by means of asymmetry coefficient (right/left ratio of liquor mobility) which was found to be 1.25-1.45 in healthy middle-age persons. For the skull pliability (compliance) the coefficient of hemispheric asymmetry was 0.75-0.95. These two hemisphere asymmetry coefficients are characterized by reciprocal relationships. These coefficients demonstrated no dominancy related to right/left hemisphere as well as no correlation with neurophysiological parameter. Functional tests (apnoea, hyperventilation, Stookey test) gave rise to significant changes of these coefficient values. At ageing, the magnitudes of these coefficients decreased. The spectral analysis of pulse waves of dopplerogram and rheoencephalogram reveals hemisphere asymmetry, too. It should be suggested that the interhemisphere asymmetry of the CSF dynamics and skull biomechanical properties is a special mechanism which contributes in the process of circulatory-metabolic support of brain activity.     

Sex differences in the relationship between creativity and hemispheric information selection at the global and local levels

Sex-related differences in creativity-related hemispheric selective processes at the global and local levels were studied using hierarchically presented letters under the conditions of their identification and comparison. The results showed that, irrespective of the sex and the type of creative thinking, its originality was associated with the acceleration of right-hemispheric processes of information selection at the global level and deceleration of interhemispheric communication. Relationships between the originality of ideas and hemispheric attentional characteristics were stronger in men during the solution of verbal creative problems and in women during figurative original thinking. The originality of verbal activity in men was more closely associated with the success of selective processes in the left hemisphere; in women, with the selective functions of both hemispheres. Figurative thinking in men was less related to hemispheric characteristics of attention compared to women. An increase in figurative originality in women was accompanied by the acceleration of information selection in the right hemisphere and a higher efficiency of local attention and speeds of global processing in the left hemisphere.     

Differences in vulnerability between the hemispheres in early childhood and adulthood

There are more left hemisphere damaged (LHD) than right hemisphere damaged (RHD) children and adults, if one relies on studies of congenital hemiparesis, as well as on those done on groups with radiologically demonstrated perinatal and postnatal unilateral stroke, hemispherectomy or unilateral epilepsy in childhood or adulthood. The main pathogenetic factor seems to be a hemodynamic one, responsible for insufficient blood supply to the left hemisphere (LH). Since adults show a difference in the same direction as children, the blood supply factor to the left internal carotid artery would seem to be the crucial factor. Around birth, an open ductus arteriosus may play an additional role. The hemodynamic asymmetry does not exclude an intrinsic maturational hemispheric tissue factor, making the left hemisphere more vulnerable than the right to detrimental influences around birth and during the early postnatal period. Immature white matter is especially vulnerable to asphyxia.     

Hemispheric specialization for global and local processing: the effect of stimulus category.

Neuropsychological evidence indicates that the global aspect of complex visual scenes is preferentially processed by the right hemisphere, and local aspects are preferentially processed by the left hemisphere. Using letter-based hierarchical stimuli (Navon figures), we recently demonstrated, in a directed-attention task, lateralized neural activity (assessed by positron emission tomography) in the left prestriate cortex during local processing, and in the right prestriate cortex during global processing. Furthermore, temporal-parietal cortex was critically activated bilaterally in a divided-attention task that involved varying the number of target switches between local and global levels of letter-based hierarchical stimuli. Little is known about whether such stimulus categories influence such hemispheric lateralization. We now present data on brain activity, derived from positron emission tomography, in normal subjects scanned during either local or global processing of object-based hierarchical stimuli. We again observe attentional modulation of neural activity in prestriate cortex. There is now greater right-sided activation for local processing and greater left-sided activation for global processing, which is the opposite of that seen with letter-based stimuli. The results suggest that the relative differential hemispheric activations in the prestriate areas during global and local processing are modified by stimulus category.     

On Disturbances of Interhemispheric Interaction in Psychopathologic States (Development of Ideas of L.Ya. Balonov and V.L. Deglin)

The current state of the problem of changes of brain functional asymmetry in psychopathology is reviewed from the point of view of disorders of interhemispheric interaction as the main chain in genesis of affective disorders and schizophrenia. Based on studies of individual activation of the left and right hemispheres as well as on evaluation of cognitive activity, it is shown that psychopathology is accompanied by a disorder of hemispheric interaction with shift of the interhemispheric activation balance towards the right hemisphere in depression and towards the left hemisphere in maniacal states and paranoid schizophrenia. Changes of the activation balance lead to disorder of cognitive activity and human adaptation in the environment. A conclusion is made that disinhibition of subcortical and brainstem structures in psychoses occurs not only vertically, but also horizontally, when the complementary and superpositional interaction of hemispheres shifts to the evolutionary earlier reciprocal type of interaction with domination of cognitive functions of the right hemisphere in depression and of the left hemisphere in the maniacal state and schizophrenia.     

Differences in vulnerability between the hemispheres in early childhood and adulthood

There are more left hemisphere damaged than right hemisphere damaged children and adults if one relies on studies of congenital hemiparesis as well as on those done on groups with radiologically demonstrated perinatal and postnatal unilateral stroke, hemispherectomy, or unilateral epilepsy in childhood or adulthood. The main pathogenetic factor seems to be a hemodynamic one, responsible for insufficient blood supply to the left hemisphere. Since adults show a difference in the same direction as children, the blood supply to the left internal carotid artery would seem to be the crucial factor. Around birth, an open ductus arteriosus may play an additional role. The hemodynamic asymmetry does not exclude an intrinsic maturational hemispheric tissue factor, making the left hemisphere more vulnerable than the right to detrimental influences around birth and during the early postnatal period. Immature white matter is especially vulnerable to asphyxia. The text was submitted by the author in English.