Interhemispheric Integration Research Articles

Putting the brain back together: Mechanisms of interhemispheric integration in face perception

Putting the brain back together: Mechanisms of interhemispheric integration in face perception

Abnormal moral reasoning in complete and partial callosotomy patients

Recent neuroimaging studies suggest lateralized cerebral mechanisms in the right temporal parietal junction are involved in complex social and moral reasoning, such as ascribing beliefs to others. Based on this evidence, we tested 3 anterior-resected and 3 complete callosotomy patients along with 22 normal subjects on a reasoning task that required verbal moral judgments. All 6 patients based their judgments primarily on the outcome of the actions, disregarding the beliefs of the agents. The similarity in performance between complete and partial callosotomy patients suggests that normal judgments of morality require full interhemispheric integration of information critically supported by the right temporal parietal junction and right frontal processes.

Cross‐sectional analysis of the association between age and corpus callosum size in chimpanzees (Pan troglodytes)

The CC is the major white matter tract connecting the cerebral hemispheres and provides for interhemispheric integration of sensory, motor and higher-order cognitive information. The midsagittal area of the CC has been frequently used as a marker of brain development in humans. We report the first investigation into the development of the corpus callosum and its regional subdivisions in chimpanzees (Pan troglodytes). Magnetic resonance images were collected from 104 chimpanzees (female n = 63, male n = 41) ranging in age from 6 years (pre-pubescent period) to 54 years (old age). Sustained linear growth was observed in the area of the CC subdivision of the genu; areas of the posterior midbody and anterior midbody displayed nonlinear growth during development. After adjusting for total brain size, we observed linear growth trajectories of the total CC and CC subdivisions of the genu, posterior midbody, isthmus and splenium, and nonlinear growth trajectories of the rostral body and anterior midbody. These developmental patterns are similar to the development of the CC in humans. As the growth curves of the CC mirrors growth seen in the percentage of white matter in humans, our results suggest chimpanzees show continued white matter development in regions related to cognitive development.

HOW DO BILINGUALS HANDLE INTERHEMISPHERIC INTEGRATION? EVIDENCE FROM A CROSS-LANGUAGE STUDY

The focus on interhemispheric interaction and integration has become a prominent aspect of laterality research. The aim of the present behavioral study was to determine whether hemisphere advantage differs between language groups. This was done by comparing how hemisphere advantage affects interhemispheric integration in monolingual and in bilingual individuals. Sixty university students (20 English monolinguals, 20 Hebrew bilinguals, and 20 balanced Arabic bilinguals) participated in two experiments, in which a lexical decision task was performed in the left and/or right visual field. Stimuli were presented unilaterally and bilaterally, whereby participants were cued to respond to the stimuli. In Experiment 1, all three groups showed an effect of lexicality, that is, participants responded to word stimuli faster than to non-word stimuli, with the Hebrew and Arabic groups showing a word advantage in spotting errors. In addition, all groups except the Hebrew group showed the expected right visual field advantage in accuracy, and the English group demonstrated this advantage in reaction time as well. In Experiment 2, responses to non-word stimuli were equally accurate in the left and right visual fields, but reaction time were faster for stimuli presented in the left visual field. The performance of balanced bilingual Arabic and unbalanced bilingual Hebrew reading groups was significantly better in the bilateral condition than in the unilateral condition. The results supported the notion that bilingual individuals show more effective interhemispheric communication and that they enjoy relative superiority in their interhemispheric processing in response to task demands.

Transcallosal transfer of information and functional asymmetry of the human brain

The corpus callosum is the largest commissure in the brain and acts as a “bridge” of nerve fibres connecting the two cerebral hemispheres. It plays a crucial role in interhemispheric integration and is responsible for normal communication and cooperation between the two hemispheres. Evolutionary pressures guiding brain size are accompanied by reduced interhemispheric and enhanced intrahemispheric connectivity. Some lines of evidence suggest that the speed of transcallosal conduction is limited in large brains (e.g., in humans), thus favouring intrahemispheric processing and brain lateralisation. Patterns of directional symmetry/asymmetry of transcallosal transfer time may be related to the degree of brain lateralisation. Neural network modelling and electrophysiological studies on interhemispheric transmission provide data supporting this supposition.

Ontogenesis of Laterality in 3- to 10-yr.-Old Children: Increased Unimanual Independence Grounded on Improved Bimanual Motor Function

Underlying sensorimotor factors, such as intermanual coupling, contributing to optimal laterality, long remain immature. Using the handmotor laterality blackboard, developmental change in symmetric bimanual motor function for 413 children in groups of 3 to 10 years of age shows synchronicity increased between groups 3 and 5 years of age; but between groups 5 and 7 years of age, given immature coupling, one hand may still disturb the movements of the other one. Between groups 7 and 9 years of age, the hands gradually stop disturbing each other and move independently and fluently. Changes in intermanual coupling with increasing unimanual independence represent expression of changing interhemispheric integration across groups. This promotes optimal laterality and task distribution between the hands. Maturation of the corpus callosum is inferred to be a factor in these ontogenetic changes which ultimately lead to optimal left hemisphere specialization for actions such as fluent writing.

Brain regional lesion burden and impaired mobility in the elderly

This study investigated the relationship of brain white matter (WM) lesions affecting specific neural networks with decreased mobility in ninety-nine healthy community-dwelling subjects ≥75 years old prospectively enrolled by age and mobility status. We assessed lesion burden in the genu, body and splenium of corpus callosum; anterior, superior and posterior corona radiata; anterior and posterior limbs of internal capsule; corticospinal tract; and superior longitudinal fasciculus. Burden in the splenium of corpus callosum (SCC) demonstrated the highest correlation particularly with walking speed (r=0.4, p<10(-4)), and in logistic regression it was the best regional predictor of low mobility performance. We also found that independent of mobility, corona radiata has the largest lesion burden with anterior (ACR) and posterior (PCR) aspects being the most frequently affected. The results suggest that compromised inter-hemispheric integration of visuospatial information through the SCC plays an important role in mobility impairment in the elderly. The relatively high lesion susceptibility of ACR and PCR in all subjects may obscure the importance of these lesions in mobility impairment.

Language status and hemispheric involvement in reading: Evidence from trilingual Arabic speakers tested in Arabic, Hebrew, and English.

This study explores the effects of language status on hemispheric involvement in lexical decision. The authors looked at the responses of native Arabic speakers in Arabic (L1 for reading) and in two second languages (L2): Hebrew, which is similar to L1 in morphological structure, and English, which is very different from L1. Two groups of Arabic speakers performed lateralized lexical decision tasks in the three languages, using unilateral presentations and bilateral presentations. These paradigms allowed us to infer both hemispheric specialization and interhemispheric communication in the three languages, and the effects of language status (native vs. nonnative) and similarity on hemispheric patterns of responses. In general the authors show an effect of language status in the right visual field (RVF), reflecting the greater facility of the left hemisphere (LH) in recognizing words in the participant's native Arabic than in their other languages. The participants revealed similar patterns of interhemispheric integration across the languages, with more integration occurring for words than for nonwords. Both hemispheres revealed sensitivity to morphological complexity, a pattern similar to that of native Hebrew readers and different from that of native English readers.

Effects of sex hormone therapy on interhemispheric crosstalk in postmenopausal women.

Evidence exists that fluctuating levels of sex hormones affect interhemispheric interaction in women during the menstrual cycle. The present study investigated whether interhemispheric interaction is susceptible to direct hormonal manipulations via hormone therapy (HT). Sixty-eight postmenopausal women who received HT either with estrogen alone (n = 15), an estrogen-gestagen combination (n = 22) or without HT (n = 31) were investigated. Participants were asked to match letters according to their physical or name identity. Matches were presented either within or across visual half-fields. Additionally, a simple reaction time task, assumed to estimate interhemispheric transfer time (IHTT), was used. Overall, postmenopausal women showed an across-field advantage in the more demanding name-identity task but not in the less demanding physical-identity task. However, across both tasks, the groups differed in responses to within- and across-field trials: the control group performed better on across- than within-field trials, whereas both HT groups showed faster responses on within- than across-field trials. IHTT did not differ between groups. The findings suggest that postmenopausal estrogen-therapy affects the relative efficiency of interhemispheric integration by modulating within-hemisphere functioning.

Interhemispheric EEG differences in olfactory bulbectomized rats with different cognitive abilities and brain beta-amyloid levels

Alterations in electroencephalogram (EEG) asymmetry and deficits in interhemispheric integration of information have been shown in patients with Alzheimer's disease (AD). However, no direct evidence of an association between EEG asymmetry, morphological markers in the brain, and cognition was found either in AD patients or in AD models. In this study we used rats with bilateral olfactory bulbectomy (OBX) as one of the AD models and measured their learning/memory abilities, brain beta-amyloid levels and EEG spectra in symmetrical frontal and occipital cortices. One year after OBX or sham-surgery, the rats were tested with the Morris water paradigm and assigned to three groups: sham-operated rats, SO, and OBX rats with virtually normal, OBX(+), or abnormal, OBX(−), learning (memory) abilities. In OBX vs. SO, the theta EEG activity was enhanced to a higher extent in the right frontal cortex and in the left occipital cortex. This produced significant interhemispheric differences in the frontal cortex of the OBX(−) rats and in the occipital cortex of both OBX groups. The beta 1 EEG asymmetry in SO was attenuated in OBX(+) and completely eliminated in OBX(−). OBX produced highly significant beta 2 EEG decline in the right frontal cortex, with OBX(−) > OBX(+) rank order of strength. The beta-amyloid level, examined by post-mortem immunological DOT-analysis in the cortex–hippocampus samples, was about six-fold higher in OBX(−) than in SO, but significantly less (enhanced by 82% vs. SO) in OBX(+) than in OBX(−). The involvement of the brain mediatory systems in the observed EEG asymmetry differences is discussed.

Development of interhemispheric visual integration in children: Does myelination affect the function? A multimodal brain imaging study

Development of interhemispheric visual integration in children: Does myelination affect the function? A multimodal brain imaging study

Enhanced redundancy gain in schizophrenics: A correlate of callosal dysfunction?

An abnormal pattern of hemispheric asymmetry, possibly as a result of disturbed interhemispheric communication, is widely, albeit by no means unanimously, held as a major cause of schizophrenia. To behaviourally test interhemispheric communication in schizophrenia we used a task that has been shown to be a reliable indicator of callosal functioning, namely, the redundant signals effect (RSE). It consists of the speeding of simple reaction time when responding to double as opposed to single visual stimuli. When the stimuli in a pair are presented to different hemispheres patients who underwent total commissurotomy or suffer from callosal agenesis show a paradoxically enhanced RSE with respect to healthy controls. Therefore, if schizophrenia patients have a callosal abnormality they ought to show a similar effect. In three experiments we tested a total of 55 patients with a diagnosis of schizophrenia and 51 healthy controls. In Experiment 1 we presented unilateral single stimuli and bilateral simultaneous double stimuli. The RSE was reliably larger in schizophrenics than in controls. In Experiment 2 the temporal interval between the two stimuli in a pair was varied. We found that while in controls the RSE disappeared with interstimulus intervals longer than 17 ms, in schizophrenia patients there was a RSE only for simultaneous double stimuli. Finally, in Experiment 3 we found that there was no enhanced redundancy gain in schizophrenics when the double stimuli were presented to one and the same hemisphere, and therefore, with no need for callosal transmission. All in all, the present results provide evidence of a callosal dysfunction in schizophrenia that impairs interhemispheric integration.

Meta-analysis of magnetic resonance imaging studies of the corpus callosum in bipolar disorder

Objective: The corpus callosum (CC) plays a pivotal role in inter-hemispheric transfer and integration of information and is a relatively understudied structure in bipolar disorder (BD). Magnetic resonance imaging (MRI) studies have reported callosal abnormalities in this condition but findings have been inconsistent. Structural changes affecting the CC may underlie functional abnormalities in BD and could contribute to, or explain the pathophysiology of, the condition. Method: A systematic review was carried out to identify, appraise and summarize MRI studies which compared callosal areas in BD with an unrelated control group. The findings were then synthesized using random effects meta-analysis. Consideration was given to a number of variables to explain heterogeneity. Results: Five case–control studies were identified. Bipolar patients showed reduced callosal areas in comparison with healthy volunteers with no evidence of heterogeneity or publication bias. Conclusion: Findings from this study indicate that callosal areas are reduced in BD and suggest that a failure to integrate information across the hemispheres may contribute to the pathophysiology of the disorder. Further research is necessary to clarify the underlying cellular changes leading to these morphometric differences.

Interhemispheric interaction during the menstrual cycle

Fluctuating levels of sex hormones and high levels of progesterone (P), in particular, have been suggested to reduce interhemispheric inhibition. The present study focuses on hormone-dependent modulation of interhemispheric integration. In two versions of the Banich–Belger task, participants were asked to match letters according to their physical (e.g., A vs. A) and semantic identity (e.g., A vs. a). Matches were presented either within or across visual half-fields. Moreover, a simple reaction-time task (Poffenberger task) that is assumed to estimate interhemispheric transfer time (IHTT) was used. Seventeen normally cycling women were tested during low P menses and high P midluteal phase. Saliva levels of P were analysed using chemiluminescence assays. Fifteen postmenopausal women performed the same tasks in corresponding time intervals. Additionally, 28 younger male controls were tested once. In agreement with previous results, the more demanding (semantic) interhemispheric-integration task revealed a typical across-field advantage (AFA) for all three groups. However, in normally cycling women, the AFA was significantly reduced during menses. IHTT did not change across the cycle phases. The results indicate that interhemispheric integration fluctuates across the menstrual cycle and is reduced during menses. During the luteal phase, however, the AFA is increased, suggesting that accompanying hormonal conditions favour an efficient interhemispheric integration. We conclude that transcallosal mechanisms involved in interhemispheric integration are profoundly altered when sex hormones are permanently reduced as in men and postmenopausal women. This difference enables an efficient interhemispheric integration without modulatory effects of P.

Interhemispheric transmission time in persons with Down syndrome

The study of cerebral specialization in persons with Down syndrome (DS) has revealed an anomalous pattern of organization. Specifically, persons with DS elicit a right cerebral hemisphere lateralization for receptive language and a left cerebral hemisphere lateralization for the production of simple and complex movements: a pattern quite different from the left hemisphere lateralization typically characterizing the aforementioned processes in the non-DS population. It is thought that the putative separation between speech perception and movement planning systems as well as the cost of interhemispheric integration impedes verbal-motor behaviours in persons with DS. Moreover, morphological anomalies of callosal structure may further amplify between-hemisphere communication difficulties in the DS population. In the present investigation, we employed a behavioural technique (i.e. the Poffenberger paradigm; Poffenberger) to determine whether global anomalies of callosal structure further amplify deficits in interhemispheric communication. Fourteen individuals with DS and 25 chronological age-matched and gender-equated participants without intellectual disability performed a visuomotor reaction time (RT) test with their left or right hand to visual stimuli appearing left or right of visual fixation. Typically it is reported that responses to visual stimuli appearing ipsilateral to the responding hand (i.e. the uncrossed condition) are faster than responses wherein visual stimuli and responding hand are contralaterally mapped (i.e. the crossed condition). The increased RT associated with the crossed condition is reported on the order of 4 ms and has been interpreted to reflect the physiological result of interhemispheric transmission. Not surprisingly persons with DS exhibited slower and more variable RTs relative to control counterparts. In addition, a reliable RT advantage favouring the uncrossed conditions was observed among control participants but not persons with DS. In keeping with the extant literature, RT performance of the DS group was slower and more variable than control counterparts. This finding has been interpreted to reflect an 'adaptive reaction' wherein the perceptual-motor abilities of persons with DS are not optimized to respond to externally paced stimuli. In terms of evaluating interhemispheric transmission via the Poffenberger paradigm, our results show the finite measures of explicit brain-behaviour relations characterizing so-called healthy controls are not always tenable in the DS population. Indeed, we believe such a finding underpins the aforementioned 'adaptive reaction' exemplifying preferred movement control in persons with DS.

Neuropsychologie, plasticité et épilepsie infantile

Epilepsy is one of the most frequent childhood disorders. While most cases are well controlled, approximately 30-50% are resistant to medical treatment. In these cases, neurosurgery may be an option. Since 1979, our team at the Sainte-Justine Hospital in Montreal has studied the impact of epilepsy on the psycho-motor and cognitive development of the affected children. The aim of a first series of studies was to explore the extent and limits of cerebral plasticity by investigating the neuropsychological sequelae of early versus late callosotomy and hemispherectomy. In keeping with the plasticity hypothesis, the results revealed that the children who were operated before puberty showed fewer deficits than those operated during adolescence or adulthood. However, the compensatory mechanisms available to them appeared to be limited with respect to the nature and complexity of the information they can process. For instance, young children having undergone section of the corpus callosum resembled individuals born without a corpus callosum (callosal agenesis) in that there did not show the typical disconnection deficits seen in adult ''split-brain'' patients. However, they exhibited deficits on tasks requiring interhemispheric integration of motor and visuo-motor information. By the same token, hemispherectomy patients were still able to make visual judgements in their ''blind'' visual field but they were found to be impaired on a variety of visual and auditory tasks (localization of a sound sources in space) requiring the participation of both hemispheres. In a second series of studies, carried out in collaboration with a Parisian team, we intended to describe the neuropsychological profile of focal epilepsies, specifically frontal and temporal epilepsy. Contrary to the common belief that childhood epilepsy would result in diffuse impairments, we were able to demonstrate that children manifest the same localized deficits as adult patients. In studies presently underway in our laboratories, we make use of functional neuroimaging techniques such as high density electrophysiology, optical imaging and magnetoencephalography to investigate pre-and postoperative language and memory functions and to study the impact of epilepsy on brain maturation.

Understanding Intervention for (C)APD: As Easy as A-B-C

Understanding Intervention for (C)APD: As Easy as A-B-C

Interhemispheric cooperation and non-cooperation during word recognition: Evidence for callosal transfer dysfunction in dyslexic adults

Participants report briefly-presented words more accurately when two copies are presented, one in the left visual field (LVF) and another in the right visual field (RVF), than when only a single copy is presented. This effect is known as the ‘redundant bilateral advantage’ and has been interpreted as evidence for interhemispheric cooperation. We investigated the redundant bilateral advantage in dyslexic adults and matched controls as a means of assessing communication between the hemispheres in dyslexia. Consistent with previous research, normal adult readers in Experiment 1 showed significantly higher accuracy on a word report task when identical word stimuli were presented bilaterally, compared to unilateral RVF or LVF presentation. Dyslexics, however, did not show the bilateral advantage. In Experiment 2, words were presented above fixation, below fixation or in both positions. In this experiment both dyslexics and controls benefited from the redundant presentation. Experiment 3 combined whole words in one visual field with word fragments in the other visual field (the initial and final letters separated by spaces). Controls showed a bilateral advantage but dyslexics did not. In Experiments 1 and 3, the dyslexics showed significantly lower accuracy for LVF trials than controls, but the groups did not differ for RVF trials. The findings suggest that dyslexics have a problem of interhemispheric integration and not a general problem of processing two lexical inputs simultaneously.

Interhemispheric Integration of Visual Processing during Task-Driven Lateralization

The mechanisms underlying interhemispheric integration (IHI) remain poorly understood, particularly for lateralized cognitive processes. To test competing theories of IHI, we constructed and fitted dynamic causal models to functional magnetic resonance data from two visual tasks that operated on identical stimuli but showed opposite hemispheric dominance. Using a systematic Bayesian model selection procedure, we found that, in the ventral visual stream, which was activated by letter judgments, interhemispheric connections mediated asymmetric information transfer from the nonspecialized right to the specialized left hemisphere when the latter did not have direct access to stimulus information. Notably, this form of IHI did not engage all areas activated by the task but was specific for areas in the lingual and fusiform gyri. In the dorsal stream, activated by spatial judgments, it did not matter which hemisphere received the stimulus: interhemispheric coupling increased bidirectionally, reflecting recruitment of the nonspecialized left hemisphere. Again, not all areas activated by the task were involved in this form of IHI; instead, it was restricted to interactions between areas in the superior parietal gyrus. Overall, our results provide direct neurophysiological evidence, in terms of effective connectivity, for the existence of context-dependent mechanisms of IHI that are implemented by specific visual areas during task-driven lateralization.

Interhemispheric integration in psychopathic offenders.

The goal of the present study was to determine whether a reduced capacity for interhemispheric integration can explain the attention deficits seen in psychopathic individuals under conditions that place substantial demands on left-hemisphere-specific resources. The present study examined the performance of 54 incarcerated psychopathic and nonpsychopathic male offenders on a same-different global-local paradigm that permits manipulation of both the magnitude of processing demands and the demand for interhemispheric coordination. Prior studies with similar paradigms have demonstrated that the cerebral hemispheres can function more efficiently as relatively independent processors on simple tasks, whereas communication between the hemispheres improves performance when processing demands are heavy. Analyses indicated that psychopathic offenders are not deficient in interhemispheric integration but provided additional evidence consistent with the left hemisphere activation hypothesis of psychopathy.