The association of macro- and microstructure of the corpus callosum and language lateralisation

Volumetric and diffusion MRI abnormalities associated with dysarthria in multiple sclerosis

Up to half of all people with multiple sclerosis experience communication difficulties due to dysarthria, a disorder that impacts the motor aspects of speech production. Dysarthria in multiple sclerosis is linked to cerebellar dysfunction, disease severity and lesion load, but the neuroanatomical substrates of these symptoms remain unclear. In this study, 52 participants with multiple sclerosis and 14 age- and sex-matched healthy controls underwent structural and diffusion MRI, clinical assessment of disease severity and cerebellar dysfunction and a battery of motor speech tasks. Assessments of regional brain volume and white matter integrity, and their relationships with clinical and speech measures, were undertaken. White matter tracts of interest included the interhemispheric sensorimotor tract, cerebello-thalamo-cortical tract and arcuate fasciculus, based on their roles in motor and speech behaviours. Volumetric analyses were targeted to Broca's area, Wernicke's area, the corpus callosum, thalamus and cerebellum. Our results indicated that multiple sclerosis participants scored worse on all motor speech tasks. Fixel-based diffusion MRI analyses showed significant evidence of white matter tract atrophy in each tract of interest. Correlational analyses further indicated that higher speech naturalness-a perceptual measure of dysarthria-and lower reading rate were associated with axonal damage in the interhemispheric sensorimotor tract and left arcuate fasciculus in people with multiple sclerosis. Axonal damage in all tracts of interest also correlated with clinical scales sensitive to cerebellar dysfunction. Participants with multiple sclerosis had lower volumes of the thalamus and corpus callosum compared with controls, although no brain volumetrics correlated with measures of dysarthria. These findings indicate that axonal damage, particularly when measured using diffusion metrics, underpin dysarthria in multiple sclerosis.

Tracing the development and lifespan change of population-level structural asymmetry in the cerebral cortex

Cortical asymmetry is a ubiquitous feature of brain organization that is subtly altered in some neurodevelopmental disorders, yet we lack knowledge of how its development proceeds across life in health. Achieving consensus on the precise cortical asymmetries in humans is necessary to uncover the developmental timing of asymmetry and the extent to which it arises through genetic and later influences in childhood. Here, we delineate population-level asymmetry in cortical thickness and surface area vertex-wise in seven datasets and chart asymmetry trajectories longitudinally across life (4-89 years; observations = 3937; 70% longitudinal). We find replicable asymmetry interrelationships, heritability maps, and test asymmetry associations in large-scale data. Cortical asymmetry was robust across datasets. Whereas areal asymmetry is predominantly stable across life, thickness asymmetry grows in childhood and peaks in early adulthood. Areal asymmetry is low-moderately heritable (max h2SNP ~19%) and correlates phenotypically and genetically in specific regions, indicating coordinated development of asymmetries partly through genes. In contrast, thickness asymmetry is globally interrelated across the cortex in a pattern suggesting highly left-lateralized individuals tend towards left-lateralization also in population-level right-asymmetric regions (and vice versa), and exhibits low or absent heritability. We find less areal asymmetry in the most consistently lateralized region in humans associates with subtly lower cognitive ability, and confirm small handedness and sex effects. Results suggest areal asymmetry is developmentally stable and arises early in life through genetic but mainly subject-specific stochastic effects, whereas childhood developmental growth shapes thickness asymmetry and may lead to directional variability of global thickness lateralization in the population.

Greater volumes of a callosal sub-region terminating in posterior language-related areas predict a stronger degree of language lateralization: A tractography study

Language lateralization is the most intriguing trait of functional asymmetry for cognitive functions. Nowadays, ontogenetic determinants of this trait are largely unknown, but there are efforts to find its anatomical correlates. In particular, a white matter interhemispheric connection-the corpus callosum-has been proposed as such. In the present study, we aimed to find the association between the degree of language lateralization and metrics of the callosal sub-regions. We applied a sentence completion fMRI task to measure the degree of language lateralization in a group of healthy participants balanced for handedness. We obtained the volumes and microstructural properties of callosal sub-regions with two tractography techniques, diffusion tensor imaging (DTI) and constrained spherical deconvolution (CSD). The analysis of DTI-based metrics did not reveal any significant associations with language lateralization. In contrast, CSD-based analysis revealed that the volumes of a callosal sub-region terminating in the core posterior language-related areas predict a stronger degree of language lateralization. This finding supports the specific inhibitory model implemented through the callosal fibers projecting into the core posterior language-related areas in the degree of language lateralization, with no relevant contribution of other callosal sub-regions.

Language lateralization in very preterm children: associating dichotic listening to interhemispheric connectivity and language performance

BACKGROUND

Language difficulties of very preterm (VPT) children might be related to weaker cerebral hemispheric lateralization of language. Language lateralization refers to the development of an expert region for language processing in the left hemisphere during the first years of life. Children born VPT might not develop such a dominant left hemisphere for language processing. A dichotic listening task may be a functional task to show the dominance of the left hemisphere during language processing. During this task, different acoustic events are simultaneously presented to both ears. Due to crossing fibers in the brain, right ear stimuli are transferred directly to the left hemisphere, and left ear stimuli are transferred first to the right hemisphere and then, through the corpus callosum (CC), to the left hemisphere. Dichotic listening typically shows a right ear advantage, assuming to reflect left hemispherical language dominance. The CC, in particular the splenium, is associated with auditory processing and is considered important for language lateralization. The objective of this work was to explore whether dichotic listening performance in school-aged VPT children are associated with language performance and interhemispheric connectivity.

METHODS

This is a cross-sectional study of 58 VPT children and 30 full term controls at age 10 years. Language performance and dichotic digit test (DDT) were assessed. In 44 VPT children, additionally diffusion weighted imaging (DWI) was performed using a 3 T MRI scanner. Fractional anisotropy (FA) and mean diffusivity (MD) values of the splenium of the CC were extracted.

RESULTS

Poorer right ear DDT scores were associated with poorer language performance in VPT children only (p = 0.015). Association between right ear DDT scores and MD of the splenium approached the level of significance (p = 0.051).

CONCLUSIONS

These results support the hypothesis that poor language performance in VPT children may be a consequence of weaker lateralized language organization, due to a poorly developed splenium of the CC. Dichotic listening may reflect the level of language lateralization in VPT children.

IMPACT

Poor language performance in VPT children may be a consequence of weaker lateralized language organization, due to a poorly developed splenium of the CC. Dichotic listening performance may reflect the level of language lateralization in VPT children and right ear scores of a dichotic listening task are associated with both the splenium of the corpus callosum and language performance. If our results could be validated in future research, it suggests that poor CC development may indicate VPT children at risk for long-term language problems.

Handedness and midsagittal corpus callosum morphology: a meta-analytic evaluation

Following a series of seminal studies in the 1980s, left or mixed hand preference is widely thought to be associated with a larger corpus callosum than right handedness, influencing the interpretation of findings and various theories related to interhemispheric processing, brain lateralisation, and hand preference. Recent reviews, however, find inconsistencies in the literature and cast doubt on the existence of such an association. The present study was conducted to clarify the relationship between hand preference and callosal morphology in a series of meta-analyses. For this purpose, articles were identified via a search in PubMed and Web Of Science databases. Studies reporting findings relating to handedness (assessed as hand preference) and corpus-callosum morphology in healthy participants were considered eligible. On the basis of a total of k = 24 identified studies and databases, random-effects meta-analyses were conducted considering four different group comparisons: (a) dominantly right- (dRH) and left-hand preference (dLH), (b) consistent right (cRH) and non-cRH preference, (c) cRH with mixed-hand preference (MH), and (d) cRH with consistent left-hand hand preference (cLH). For none of these meta-analyses did we find a significant effect of hand preference, and narrow confidence intervals suggest that the existence of population effects larger than 1% explained variance could be excluded. For example, considering the comparison of dRH and dLH (k = 14 studies; 1910 dRH and 646 dLH participants) the mean effect size was Hedge’s g = 0.016 (95% confidence interval: − 0.12 to 0.15; explained variance: < 0.001%). Thus, the common practice of assuming an increase in callosal connectivity based on mixed or left hand preference is likely invalid.

Frontal and parietal EEG alpha asymmetry: a large-scale investigation of short-term reliability on distinct EEG systems

EEG resting-state alpha asymmetry is one of the most widely investigated forms of functional hemispheric asymmetries in both basic and clinical neuroscience. However, studies yield inconsistent results. One crucial prerequisite to obtain reproducible results is the reliability of the index of interest. There is a body of research suggesting a moderate-to-good reliability of EEG resting-state alpha asymmetry, but unfortunately sample sizes in these studies are typically small. This study presents the first large-scale short-term reliability study of frontal and parietal EEG resting-state alpha asymmetry. We used the Dortmund Vital Study data set containing 370 participants. In each participant, EEG resting state was recorded eight times, twice with their eyes opened, twice with their eyes-closed, each on two different EEG systems. We found good reliability of EEG alpha power and alpha asymmetry on both systems for electrode pairs. We also found that alpha power asymmetry reliability is higher in the eyes-closed condition than in the eyes-open condition. The frontomedial electrode pair showed weaker reliability than the frontolateral and parietal electrode pairs. Interestingly, we found no population-level alpha asymmetry in frontal electrodes, one of the most investigated electrode sites in alpha asymmetry research. In conclusion, our results suggest that while EEG alpha asymmetry is an overall reliable measure, frontal alpha asymmetry should be assessed using multiple electrode pairs.

How does hemispheric specialization contribute to human-defining cognition?

Uniquely human cognitive faculties arise from flexible interplay between specific local neural modules, with hemispheric asymmetries in functional specialization. Here, we discuss how these computational design principles provide a scaffold that enables some of the most advanced cognitive operations, such as semantic understanding of world structure, logical reasoning, and communication via language. We draw parallels to dual-processing theories of cognition by placing a focus on Kahneman's System 1 and System 2. We propose integration of these ideas with the global workspace theory to explain dynamic relay of information products between both systems. Deepening the current understanding of how neurocognitive asymmetry makes humans special can ignite the next wave of neuroscience-inspired artificial intelligence.

Left-Handers Are Less Lateralized Than Right-Handers for Both Left and Right Hemispheric Functions

Many neuroscientific techniques have revealed that more left- than right-handers will have unusual cerebral asymmetries for language. After the original emphasis on frequency in the aphasia and epilepsy literatures, most neuropsychology, and neuroimaging efforts rely on estimates of central tendency to compare these two handedness groups on any given measure of asymmetry. The inevitable reduction in mean lateralization in the left-handed group is often postulated as being due to reversed asymmetry in a small subset of them, but it could also be due to a reduced asymmetry in many of the left-handers. These two possibilities have hugely different theoretical interpretations. Using functional magnetic resonance imaging localizer paradigms, we matched left- and right-handers for hemispheric dominance across four functions (verbal fluency, face perception, body perception, and scene perception). We then compared the degree of dominance between the two handedness groups for each of these four measures, conducting t-tests on the mean laterality indices. The results demonstrate that left-handers with typical cerebral asymmetries are less lateralized for language, faces, and bodies than their right-handed counterparts. These results are difficult to reconcile with current theories of language asymmetry or of handedness.

Investigating the effects of handedness on the consistency of lateralization for speech production and semantic processing tasks using functional transcranial Doppler sonography

The left hemisphere is dominant for language in most people, but lateralization strength varies between different tasks and individuals. A large body of literature has shown that handedness is associated with lateralization: left handers have weaker language lateralization on average, and a greater incidence of atypical (right hemisphere) lateralization; but typically, these studies have relied on a single measure of language lateralization. Here we consider the relationships between lateralization for two different language tasks. We investigated the influence of handedness on lateralization using functional transcranial Doppler sonography (fTCD), using an existing dataset (N = 151 adults, 21 left handed). We compared a speech production task (word generation) and a semantic association task. We demonstrated stronger left-lateralization for word generation than semantic association; and a moderate correlation between laterality indices for the two tasks (r = 0.59). Laterality indices were stronger for right than left handers, and left handers were more likely than right handers to have atypical (right hemisphere) lateralization or inconsistent lateralization between the two tasks. These results add to our knowledge of individual differences in lateralization and support the view that language lateralization is multifactorial rather than unitary.

An updated investigation of the multidimensional structure of language lateralization in left- and right-handed adults: a test-retest functional transcranial Doppler sonography study with six language tasks

A previous study we reported in this journal suggested that left and right-handers may differ in their patterns of lateralization for different language tasks (Woodhead et al. 2019 R. Soc. Open Sci. 6, 181801. (doi:10.1098/rsos.181801)). However, it had too few left-handers (N = 7) to reach firm conclusions. For this update paper, further participants were added to the sample to create separate groups of left- (N = 31) and right-handers (N = 43). Two hypotheses were tested: (1) that lateralization would be weaker at the group level in left-than right-handers; and (2) that left-handers would show weaker covariance in lateralization between tasks, supporting a two-factor model. All participants performed the same protocol as in our previous paper: lateralization was measured using functional transcranial Doppler sonography during six different language tasks, on two separate testing sessions. The results supported hypothesis 1, with significant differences in laterality between groups for four out of six tasks. For hypothesis 2, structural equation modelling showed that there was stronger evidence for a two-factor model in left than right-handers; furthermore, examination of the factor loadings suggested that the pattern of laterality across tasks may also differ between handedness groups. These results expand on what is known about the differences in laterality between left- and right-handers.

Mapping the principal gradient onto the corpus callosum

Gradients capture some of the variance of the resting-state functional magnetic resonance imaging (rsfMRI) signal. Amongst these, the principal gradient depicts a functional processing hierarchy that spans from sensory-motor cortices to regions of the default-mode network. While the cortex has been well characterised in terms of gradients little is known about its underlying white matter. For instance, comprehensive mapping of the principal gradient on the largest white matter tract, the corpus callosum, is still missing. Here, we mapped the principal gradient onto the midsection of the corpus callosum using the 7T human connectome project dataset. We further explored how quantitative measures and variability in callosal midsection connectivity relate to the principal gradient values. In so doing, we demonstrated that the extreme values of the principal gradient are located within the callosal genu and the posterior body, have lower connectivity variability but a larger spatial extent along the midsection of the corpus callosum than mid-range values. Our results shed light on the relationship between the brain's functional hierarchy and the corpus callosum. We further speculate about how these results may bridge the gap between functional hierarchy, brain asymmetries, and evolution.

The role of the corpus callosum in language network connectivity in children

The specific role of the corpus callosum (CC) in language network organization remains unclear, two contrasting models have been proposed: inhibition of homotopic areas allowing for independent functioning of the hemispheres versus integration of information from both hemispheres. This study aimed to add to this discussion with the first investigation of language network connectivity in combination with CC volume measures. In 38 healthy children aged 6–12, we performed task‐based functional magnetic resonance imaging to measure language network connectivity, used structural magnetic resonance imaging to quantify CC subsection volumes, and administered various language tests to examine language abilities. We found an increase in left intrahemispheric and bilateral language network connectivity and a decrease in right intrahemispheric connectivity associated with larger volumes of the posterior, mid‐posterior, and central subsections of the CC. Consistent with that, larger volumes of the posterior parts of the CC were significantly associated with better verbal fluency and vocabulary, the anterior CC volume was positively correlated with verbal span. Thus, children with larger volumes of CC subsections showed increased interhemispheric language network connectivity and were better in different language domains. This study presents the first evidence that the CC is directly linked to language network connectivity and underlines the excitatory role of the CC in the integration of information from both hemispheres.

Handedness and White Matter Networks

The development and persistence of laterality is a key feature of human motor behavior, with the asymmetry of hand use being the most prominent. The idea that asymmetrical functions of the hands reflect asymmetries in terms of structural and functional brain organization has been tested many times. However, despite advances in laterality research and increased understanding of this population-level bias, the neural basis of handedness remains elusive. Recent developments in diffusion magnetic resonance imaging enabled the exploration of lateralized motor behavior also in terms of white matter and connectional neuroanatomy. Despite incomplete and partly inconsistent evidence, structural connectivity of both intrahemispheric and interhemispheric white matter seems to differ between left and right-handers. Handedness was related to asymmetry of intrahemispheric pathways important for visuomotor and visuospatial processing (superior longitudinal fasciculus), but not to projection tracts supporting motor execution (corticospinal tract). Moreover, the interindividual variability of the main commissural pathway corpus callosum seems to be associated with handedness. The review highlights the importance of exploring new avenues for the study of handedness and presents the latest state of knowledge that can be used to guide future neuroscientific and genetic research.

Lifespan trajectories of relative corpus callosum thickness: Regional differences and cognitive relevance

The cerebral hemispheres are specialized for different cognitive functions and receive divergent information from the sensory organs, so that the interaction between the hemispheres is a crucial aspect of perception and cognition. At the same time, the major fiber tract responsible for this interaction, the corpus callosum, shows a structural development across the lifespan which is over-proportional. That is, compared to changes in overall forebrain volume, the corpus callosum shows an accentuated growth during childhood, adolescence, and early adulthood, as well as pronounced decline in older age. However, this over-proportionality of growth and decline along with potential consequences for cognition, have been largely overlooked in empirical research. In the present study we systematically address the proportionality of callosal development in a large mixed cross-sectional and longitudinal sample (1867 datasets from 1014 unique participants), covering the human lifespan (age range 4-93 years), and examine the cognitive consequences of the observed changes. Relative corpus callosum thickness was measured at 60 segments along the midsagittal surface, and lifespan trajectories were clustered to identify callosal subsections of comparable lifespan development. While confirming the expected inverted u-shaped lifespan trajectories, we also found substantial regional variation. Compared with anterior clusters, the most posterior sections exhibited an accentuated growth during development which extends well into the third decade of life, and a protracted decline in older age which is delayed by about 10 years (starting mid to late 50s). We further showed that the observed longitudinal changes in relative thickness of the mid splenium significantly mediates age-related changes in tests assessing verbal knowledge and non-verbal visual-spatial abilities across the lifespan. In summary, we demonstrate that analyzing the proportionality of callosal growth and decline offers valuable insight into lifespan development of structural connectivity between the hemispheres, and suggests consequences for the cognitive development of perception and cognition.

The Contribution of the Corpus Callosum to Language Lateralization

The development of hemispheric lateralization for language is poorly understood. In one hypothesis, early asymmetric gene expression assigns language to the left hemisphere. In an alternate view, language is represented a priori in both hemispheres and lateralization emerges via cross-hemispheric communication through the corpus callosum. To address this second hypothesis, we capitalized on the high temporal and spatial resolution of magnetoencephalographic imaging to measure cortical activity during language processing, speech preparation, and speech execution in 25 participants with agenesis of the corpus callosum (AgCC) and 21 matched neurotypical individuals. In contrast to strongly lateralized left hemisphere activations for language in neurotypical controls, participants with complete or partial AgCC exhibited bilateral hemispheric activations in both auditory or visually driven language tasks, with complete AgCC participants showing significantly more right hemisphere activations than controls or than individuals with partial AgCC. In AgCC individuals, language laterality positively correlated with verbal IQ. These findings suggest that the corpus callosum helps to drive language lateralization.

SIGNIFICANCE STATEMENT

The role that corpus callosum development has on the hemispheric specialization of language is poorly understood. Here, we used magnetoencephalographic imaging during linguistic tests (verb generation, picture naming) to test for hemispheric dominance in patients with agenesis of the corpus callosum (AgCC) and found reduced laterality (i.e., greater likelihood of bilaterality or right hemisphere dominance) in this cohort compared with controls, especially in patients with complete agenesis. Laterality was positively correlated with behavioral measures of verbal intelligence. These findings provide support for the hypothesis that the callosum aids in functional specialization throughout neural development and that the loss of this mechanism correlates with impairments in verbal performance.

Corpus Callosum Diffusion and Language Lateralization in Patients with Brain Tumors: A DTI and fMRI Study

BACKGROUND AND PURPOSE

Examining how left-hemisphere brain tumors might impact both the microstructure of the corpus callosum (CC) as measured by fractional anisotropy (FA) values in diffusion tensor imaging (DTI) as well as cortical language lateralization measured with functional MRI (fMRI).

METHODS

fMRI tasks (phonemic fluency and verb generation) were performed in order to detect activation in Broca's and Wernicke's area. Twenty patients with left-hemisphere brain tumors were investigated. fMRI results were divided into left dominant (LD), right dominant (RD), or codominant (CD) for language function. DTI was performed to generate FA maps in the anterior and posterior CC. FA values were correlated with the degree of language dominance.

RESULTS

Patients who were LD or RD for language in Broca's area had lower FA in the anterior CC than those who were CD for language (median for CD = .72, LD = .66, RD = .65, P < .09). Lateralized versus CD group level analysis also showed that CD patients had higher FA in the anterior CC than patients who displayed strong lateralization in either hemisphere (median for CD = .72, lateralized = .65, P < .05).

CONCLUSION

Our preliminary observations indicate that the greater FA in CD patients may reflect a more directional microstructure for the CC in this region, suggesting a greater need for interhemispheric transfer of information. Because brain tumors can cause compensatory codominance, our findings may suggest a mechanism by which interhemispheric transfer is facilitated during plasticity in the presence of a tumor.

Left-hemisphere lateralization for language and interhemispheric fiber tracking in patients with schizophrenia

BACKGROUND

It has been suggested that the degree of hemispheric specialization (HS) depends on the structural connectivity between the two hemispheres, that is to say the corpus callosum (CC). Studies, performed only on healthy participants, investigated this anatomo-functional relationship. Nevertheless, it has never been studied in schizophrenia. We therefore propose to study the anatomo-functional relationships between the integrity of interhemispheric connectivity and leftward functional lateralization for language in patients with schizophrenia compared with healthy participants, driven by a multimodal approach combining fMRI and DTI-based fiber tractography. We hypothesized that reduced leftward functional lateralization for language in patients with schizophrenia could be related to a callosal hypoconnectivity.

MATERIALS AND METHODS

Seventeen patients based on the DSM-IV, and 17 controls were included. The functional laterality index and interhemispheric diffusion values between homologue temporal regions, belonging to the language network, were individually extracted in order to study the anatomo-functional relationships.

RESULTS

In the patients, higher mean and radial diffusivity (RD) values (thicker myelin sheaths) were associated with less leftward lateralization. In contrast, the controls presented higher RD values and lower fractional anisotropy values (axonal loss) with more leftward lateralization.

CONCLUSIONS

Our study revealed a relationship between the CC and the HS for language, but did not provide evidence clarifying the direction of the relationship between callosal connectivity and functional lateralization for language. In particular, the present findings showed that the loss of integrity in interhemispheric callosal fibers was associated with reduced leftward cerebral dominance for language in patients with schizophrenia.

Quantifying cerebral asymmetries for language in dextrals and adextrals with random-effects meta analysis

Speech and language-related functions tend to depend on the left hemisphere more than the right in most right-handed (dextral) participants. This relationship is less clear in non-right handed (adextral) people, resulting in surprisingly polarized opinion on whether or not they are as lateralized as right handers. The present analysis investigates this issue by largely ignoring methodological differences between the different neuroscientific approaches to language lateralization, as well as discrepancies in how dextral and adextral participants were recruited or defined. Here we evaluate the tendency for dextrals to be more left hemisphere dominant than adextrals, using random effects meta analyses. In spite of several limitations, including sample size (in the adextrals in particular), missing details on proportions of groups who show directional effects in many experiments, and so on, the different paradigms all point to proportionally increased left hemispheric dominance in the dextrals. These results are analyzed in light of the theoretical importance of these subtle differences for understanding the cognitive neuroscience of language, as well as the unusual asymmetry in most adextrals.

Corpus callosal microstructure influences intermanual transfer in chimpanzees

Learning a new motor skill with one hand typically results in performance improvements in the alternate hand. The neural substrates involved with this skill acquisition are poorly understood. We combined behavioral testing and non-invasive brain imaging to study how the organization of the corpus callosum was related to intermanual transfer performance in chimpanzees. Fifty-three chimpanzees were tested for intermanual transfer of learning using a bent-wire task. Magnetic resonance and diffusion tensor images were collected from 39 of these subjects. The dominant hand showed greater performance benefits than the nondominant hand. Further, performance was associated with structural integrity of the motor and sensory regions of the CC. Subjects with better intermanual transfer of learning had lower fractional anisotropy values. The results are consistent with the callosal access model of motor programming.

FOXP2 variation modulates functional hemispheric asymmetries for speech perception

Left-hemispheric language dominance is a well-known characteristic of the human language system, but the molecular mechanisms underlying this crucial feature of vocal communication are still far from being understood. The forkhead box P2 gene FOXP2, which has been related to speech development, constitutes an interesting candidate gene in this regard. Therefore, the present study was aimed at investigating effects of variation in FOXP2 on individual language dominance. To this end, we used a dichotic listening and a visual half-field task in a sample of 456 healthy adults. The FOXP2 SNPs rs2396753 and rs12533005 were found to be significantly associated with the distribution of correct answers on the dichotic listening task. These results show that variation in FOXP2 may contribute to the inter-individual variability in hemispheric asymmetries for speech perception.

Structural white matter asymmetries in relation to functional asymmetries during speech perception and production

Functional hemispheric asymmetries of speech production and perception are a key feature of the human language system, but their neurophysiological basis is still poorly understood. Using a combined fMRI and tract-based spatial statistics approach, we investigated the relation of microstructural asymmetries in language-relevant white matter pathways and functional activation asymmetries during silent verb generation and passive listening to spoken words. Tract-based spatial statistics revealed several leftward asymmetric clusters in the arcuate fasciculus and uncinate fasciculus that were differentially related to activation asymmetries in the two functional tasks. Frontal and temporal activation asymmetries during silent verb generation were positively related to the strength of specific microstructural white matter asymmetries in the arcuate fasciculus. In contrast, microstructural uncinate fasciculus asymmetries were related to temporal activation asymmetries during passive listening. These findings suggest that white matter asymmetries may indeed be one of the factors underlying functional hemispheric asymmetries. Moreover, they also show that specific localized white matter asymmetries might be of greater relevance for functional activation asymmetries than microstructural features of whole pathways.

White matter lateralization and interhemispheric coherence to auditory modulations in normal reading and dyslexic adults

Neural activation of slow acoustic variations that are important for syllable identification is more lateralized to the right hemisphere than activation of fast acoustic changes that are important for phoneme identification. It has been suggested that this complementary function at different hemispheres is rooted in a different degree of white matter myelination in the left versus right hemisphere. The present study will investigate this structure-function relationship with Diffusion Tensor Imaging (DTI) and Auditory Steady-State Responses (ASSR), respectively. With DTI we examined white matter lateralization in the cortical auditory and language regions (i.e. posterior region of the superior temporal gyrus and the arcuate fasciculus) and white matter integrity in the splenium of the corpus callosum. With ASSR we examined interhemispheric coherence to slow, syllabic-rate (i.e. 4 Hz) and fast, phonemic-rate (i.e. 20 Hz) modulations. These structural and functional techniques were applied in a group of normal reading adults and a group of dyslexic adults for whom previously reduced functional interhemispheric connectivity at 20 Hz has been reported (Poelmans et al. (2012). Ear and Hearing, 33, 134-143). This sample was chosen since it is hypothesized that in dyslexic readers insufficient hemispheric asymmetry in myelination might relate to their auditory and phonological problems. Results demonstrate reduced white matter lateralization in the posterior superior temporal gyrus and the arcuate fasciculus in the dyslexic readers. Additionally, white matter lateralization in the posterior superior temporal gyrus and white matter integrity in the splenium of the corpus callosum related to interhemispheric coherence to phonemic-rate modulations (i.e. 20 Hz). Interestingly, this correlation pattern was opposite in normal versus dyslexic readers. These results might imply that less pronounced left white matter dominance in dyslexic adults might relate to their problems to process phonemic-rate acoustic information and to integrate them into the phonological system.

White-matter microstructure and language lateralization in left-handers: a whole-brain MRI analysis

Most people are left-hemisphere dominant for language. However the neuroanatomy of language lateralization is not fully understood. By combining functional magnetic resonance imaging (fMRI) and diffusion tensor imaging (DTI), we studied whether language lateralization is associated with cerebral white-matter (WM) microstructure. Sixteen healthy, left-handed women aged 20-25 were included in the study. Left-handers were targeted in order to increase the chances of involving subjects with atypical language lateralization. Language lateralization was determined by fMRI using a verbal fluency paradigm. Tract-based spatial statistics analysis of DTI data was applied to test for WM microstructural correlates of language lateralization across the whole brain. Fractional anisotropy and mean diffusivity were used as indicators of WM microstructural organization. Right-hemispheric language dominance was associated with reduced microstructural integrity of the left superior longitudinal fasciculus and left-sided parietal lobe WM. In left-handed women, reduced integrity of the left-sided language related tracts may be closely linked to the development of right hemispheric language dominance. Our results may offer new insights into language lateralization and structure-function relationships in human language system.

Speech dominance is a better predictor of functional brain asymmetry than handedness: a combined fMRI word generation and behavioral dichotic listening study

An unresolved issue in behavioral studies of hemispheric asymmetry is why both left-handers and right-handers show a right ear advantage at the group level. In the present study we screened left-handers for left- versus right-hemisphere speech dominance with fMRI by comparing right versus left hemisphere frontal lobe activity (in Broca's area) in a silent word generation task. A left hemisphere dominant right-handed control group was included as well. All participants took part in a dichotic listening task with consonant-vowel syllables. The results showed that left-handers and right-handers with left-hemisphere speech dominance showed a right ear advantage. However, the left-handers with right hemisphere speech dominance had a left ear advantage. Thus, at the group level the direction of the ear advantage in dichotic listening was predicted by language dominance but not by hand preference. At the individual level, the dichotic task we used showed more variability than the fMRI results. Further research will have to indicate whether this is a feature inherent to dichotic listening, or whether the variability is due to alternative explanations such as a more bilateral representation of speech perception compared to speech production.

Diffusion tensor quantification and cognitive correlates of the macrostructure and microstructure of the corpus callosum in typically developing and dyslexic children

Noninvasive quantitative MRI methods, such as diffusion tensor imaging (DTI), can offer insights into the structure-function relationships in human developmental brain disorders. In this article, we quantified the macrostructural and microstructural attributes of the corpus callosum (CC) in children with dyslexia and in typically developing readers of comparable age and gender. Diffusion anisotropy, and mean, radial and axial diffusivities of cross-sectional CC subregions were computed using a validated DTI methodology. The normalized posterior CC area was enlarged in children with dyslexia relative to that in typically developing children. Moreover, the callosal microstructural attributes, such as the mean diffusivity of the posterior middle sector of the CC, correlated significantly with measures of word reading and reading comprehension. Reading group differences in fractional anisotropy, mean diffusivity and radial diffusivity were observed in the posterior CC (CC5). This study demonstrates the utility of regional DTI measurements of the CC in understanding the neurobiology of reading disorders.

The Corpus Callosum in Monozygotic Twins Concordant and Discordant for Handedness and Language Dominance

We used diffusion tensor imaging to assess callosal morphology in 35 pairs of monozygotic twins, of which 17 pairs were concordant for handedness and 18 pairs were discordant for handedness. Functional hemispheric language dominance was established for each twin member using fMRI, resulting in 26 twin pairs concordant and 9 twin pairs discordant for language dominance. On the basis of genetic models of handedness and language dominance, which assume one “right shift” (RS) gene with two alleles, an RS+ allele biasing toward right-handedness and left cerebral language dominance and an RS− allele leaving both asymmetries to chance, all twins were classified according to their putative genotypes, and the possible effects of the gene on callosal morphology was assessed. Whereas callosal size was under a high genetic control that was independent of handedness and language dominance, twin pairs with a high probability of carrying the putative RS+ allele showed a connectivity pattern characterized by a genetically controlled, low anisotropic diffusion over the whole corpus callosum. In contrast, the high connectivity pattern exhibited by twin pairs more likely to lack the RS+ allele was under significantly less genetic influence. The data suggest that handedness and hemispheric dominance for speech production might be at least partly dependent on genetically controlled processes of axonal pruning in the corpus callosum.

Sex differences in language asymmetry are age-dependent and small: a large-scale, consonant-vowel dichotic listening study with behavioral and fMRI data

Men are often believed to have a functionally more asymmetrical brain organization than women, but the empirical evidence for sex differences in lateralization is unclear to date. Over the years we have collected data from a vast number of participants using the same consonant-vowel dichotic listening task, a reliable marker for language lateralization. One dataset comprised behavioral data from 1782 participants (885 females, 125 non-right-handers), who were divided in four age groups (children <10 yrs, adolescents = 10-15 yrs, younger adults = 16-49 yrs, and older adults >50 yrs). In addition, we had behavioral and functional imaging (fMRI) data from another 104 younger adults (49 females, aged 18-45 yrs), who completed the same dichotic listening task in a 3T scanner. This database allowed us to comprehensively test whether there is a sex difference in functional language lateralization. Across all participants and in both datasets a right ear advantage (REA) emerged, reflecting left-hemispheric language lateralization. Accordingly, the fMRI data revealed a leftward asymmetry in superior temporal lobe language processing areas. In the N = 1782 dataset no main effect of sex but a significant sex by age interaction emerged: the REA increased with age in both sexes but as a result of an earlier onset in females the REA was stronger in female than male adolescents. In turn, male younger adults showed greater asymmetry than female younger adults (accounting for <1% of variance). There were no sex differences in children and older adults. The males in the fMRI dataset (N = 104) also had a greater REA than females (accounting for 4% of variance), but no sex difference emerged in the neuroimaging data. Handedness did not affect these findings. Taken together, our findings suggest that sex differences in language lateralization as assessed with dichotic listening exist, but they are (a) not necessarily reflected in fMRI data, (b) age-dependent and (c) relatively small.

Changes in white matter integrity follow excitatory rTMS treatment of post-stroke aphasia

PURPOSE

In this study, we examine whether an excitatory repetitive transcranial magnetic stimulation (rTMS) protocol called intermittent theta burst stimulation (iTBS) applied to the affected left hemisphere leads to changes in white matter fractional anisotropy (FA).

METHODS

Diffusion tensor imaging (DTI) data were collected in 8 aphasic stroke patients before and after 10 daily iTBS treatments. Alignment of structural and DTI data and derivation of diffusion index maps were performed using Analysis of Functional NeuroImages software followed by Tract-Based Spatial Statistics using FMRIB Software Library. Paired t-tests were performed to compare pre- to post-rTMS changes in FA.

RESULTS

There were significant (p < 0.001) left-hemispheric FA increases near the inferior and superior frontal gyri and anterior corpus callosum. FA also increased in the right midbrain and bilaterally near temporal, parietal and posterior cingulate regions. FA decreased bilaterally near the fusiform gyrus and in left cerebellum.

CONCLUSIONS

Overall, left-hemispheric regions that showed increased FA corresponded to areas previously shown to have increases in fMRI language activation after iTBS. The increased white matter integrity near the stimulation sites may reflect improvements in cortical function mediated by excitatory rTMS through its ability to facilitate synaptic connections.

A qualitative and quantitative review of diffusion tensor imaging studies in reading and dyslexia

In this review paper we address whether deficits in reading (i.e. developmental dyslexia) are rooted in neurobiological anomalies in white matter tracts. Diffusion tensor imaging (DTI) offers an index of the connections between brain regions (via tractography) and of the white matter properties of these connections (via fractional anisotropy, FA). The reported studies generally show that lower FA values in left temporoparietal and frontal areas are indicative of poorer reading ability or dyslexia. Second, most studies have indicated that these regions coincide with the left arcuate fasciculus and corona radiata, with fewer studies suggesting a role for the posterior part of the corpus callosum or for more ventral tracts such as the inferior longitudinal fasciculus or the inferior fronto-occipital fasciculus. Finally, a quantitative activation likelihood estimation (ALE) meta-analysis on all reported studies that used a voxel-based approach reveals a cluster located close to the left temporoparietal region (x=-29, y=-17, z=26). Fibertracking through this cluster demonstrates that this region hosts both the left arcuate fasciculus and the left corona radiata.

Diminished language lateralization in schizophrenia corresponds to impaired inter-hemispheric functional connectivity

INTRODUCTION

A consistent brain imaging finding in schizophrenia is decreased language-asymmetry, already evident in first episode patients, thus arguing for a biomarker of the disorder. Nonetheless, its specificity to schizophrenia is questionable. Furthermore, while previous studies suggested that enhanced right hemisphere activation underlies this diminished asymmetry, the mechanism for this anomaly is yet unknown. This study aimed to examine the role of inter-hemispheric relations in such abnormality through functional connectivity analysis driven by left inferior frontal gyrus (IFG) activation. To test for disorder specificity we compared schizophrenia patients not only to healthy controls but also to patients with obsessive compulsive disorder (OCD).

METHODS

Functional magnetic resonance imaging (fMRI) was applied during an auditory verb generation task in the 3 groups. Language-related activation in BA44/45 located in the IFG was used for regional estimation of brain asymmetry and for assessment of inter-hemispheric functional connectivity.

RESULTS

Schizophrenia, but not OCD patients showed reduced language asymmetry in the IFG relative to healthy controls and diminished functional connectivity between the left and right IFG. Importantly, decreased inter-hemispheric functional connectivity in the IFG was related to more negative symptoms among the schizophrenia patients.

CONCLUSIONS

Diminished language-related asymmetry in the IFG seems to be an early disorder specific neural marker of schizophrenia, supporting its pathogenic role. The relation of this regional abnormality to reduced inter-hemispheric functional connectivity and symptom severity supports the role of large-scale brain disorganization in schizophrenia. This may relate to the known structural abnormalities of the corpus callosum leading to functional hemispheric dysconnection.

Individual differences in reading skill and language lateralisation: a cluster analysis

Individual differences in reading and cerebral lateralisation were investigated in 200 college students who completed reading assessments and divided visual field word recognition tasks, and received a structural MRI scan. Prior studies on this data set indicated that little variance in brain-behaviour correlations could be attributed to the effects of sex and handedness variables (Chiarello, Welcome, Halderman, & Leonard, 2009; Chiarello, Welcome, Halderman, Towler, et al., 2009; Welcome et al., 2009). Here a more bottom-up approach to behavioural classification (cluster analysis) was used to explore individual differences that need not depend on a priori decisions about relevant subgroups. The cluster solution identified four subgroups of college age readers with differing reading skill and visual field lateralisation profiles. These findings generalised to measures that were not included in the cluster analysis. Poorer reading skill was associated with somewhat reduced VF asymmetry, while average readers demonstrated exaggerated RVF/left hemisphere advantages. Skilled readers had either reduced asymmetries, or asymmetries that varied by task. The clusters did not differ by sex or handedness, suggesting that there are identifiable sources of variance among individuals that are not captured by these standard participant variables. All clusters had typical leftward asymmetry of the planum temporale. However, the size of areas in the posterior corpus callosum distinguished the two subgroups with high reading skill. A total of 17 participants, identified as multivariate outliers, had unusual behavioural profiles and differed from the remainder of the sample in not having significant leftward asymmetry of the planum temporale. A less buffered type of neurodevelopment that is more open to the effects of random genetic and environmental influences may characterise such individuals.

Attention and cognitive control networks assessed in a dichotic listening fMRI study

A meaningful interaction with our environment relies on the ability to focus on relevant sensory input and to ignore irrelevant information, i.e. top-down control and attention processes are employed to select from competing stimuli following internal goals. In this, the demands for the recruitment of top-down control processes depend on the relative perceptual salience of the competing stimuli. In the present functional magnetic resonance imaging (fMRI) study, we investigated the recruitment of top-down control processes in response to varying degrees of control demands in the auditory modality. For this purpose, we tested 20 male and 20 female subjects with a dichotic listening paradigm, in which the relative perceptual salience of two simultaneously presented stimuli was systematically manipulated by varying the inter-aural intensity difference (IID) and asking the subjects to selectively attend to either ear. The analysis showed that the interaction between IID and attentional direction involves two networks in the brain. A fronto-parietal network, including the pre-supplementary motor area, anterior cingulate cortex, inferior frontal junction, insula and inferior parietal lobe, was recruited during cognitively demanding conditions and can thus be seen as a top-down cognitive control network. In contrast, a second network including the superior temporal and the post-central gyri was engaged under conditions with low cognitive control demands. These findings demonstrate how cognitive control is achieved through the interplay of distinct brain networks, with their differential engagement determined as a function of the level of competition between the sensory stimuli.

Hemispheric asymmetry: contributions from brain imaging

A series of studies using functional and structural magnetic resonance imaging, including diffusion tensor imaging measures also, to elucidate the aspects of hemispheric asymmetry are reviewed. It is suggested that laterality evolved as a response to the demands of language and the need for air-based communication which may have necessitated a division of labor between the hemispheres in order to avoid having duplicate copies in both the hemispheres that would increase processing redundancy. This would have put pressure on brain structures related to the evolution of language and speech, such as the left peri-Sylvian region. MRI data are provided showing structural and functional asymmetry in this region of the brain and how fibers connecting the right and left peri-Sylvian regions pass through the corpus callosum. It is further suggested that the so-called Yakelovian-torque, i.e., the twisting of the brain along the longitudinal axis, with the right frontal and left occipital poles protruding beyond the corresponding left and right sides, was necessary for the expansion of the left peri-Sylvian region and the right occipito-parietal regions subserving the processing of spatial relations. Functional magnetic resonance imaging data related to sex differences for visuo-spatial processing are presented showing enhanced right-sided activation in posterior parts of the brain in both sexes, and frontal activation including Broca's area in the female group only, suggesting that males and females use different strategies when solving a cognitive task. The paper ends with a discussion of the role of the corpus callosum in laterality and the role played by structural asymmetry in understanding corresponding functional asymmetry. WIREs Cogni Sci 2011 2 461-478 DOI: 10.1002/wcs.122 For further resources related to this article, please visit the WIREs website.

Callosal tracts and patterns of hemispheric dominance: a combined fMRI and DTI study

Left-hemispheric dominance for language and right-hemispheric dominance for spatial processing are distinctive characteristics of the human brain. However, variations of these hemispheric asymmetries have been observed, with a minority showing crowding of both functions to the same hemisphere or even a mirror reversal of the typical lateralization pattern. Here, we used diffusion tensor imaging and functional magnetic imaging to investigate the role of the corpus callosum in participants with atypical hemispheric dominance. The corpus callosum was segmented according to the projection site of the underlying fibre tracts. Analyses of the microstructure of the identified callosal segments revealed that atypical hemispheric dominance for language was associated with high anisotropic diffusion through the corpus callosum as a whole. This effect was most evident in participants with crowding of both functions to the right. The enhanced anisotropic diffusion in atypical hemispheric dominance implies that in these individuals the two hemispheres are more heavily interconnected.

Corpus callosum function in verbal dichotic listening: inferences from a longitudinal follow-up of Relapsing-Remitting Multiple Sclerosis patients

This study conducted a follow-up of 13 early-onset slightly disabled Relapsing-Remitting Multiple Sclerosis (RRMS) patients within an year, evaluating both CC area measurements in a midsagittal Magnetic Resonance (MR) image, and Dichotic Listening (DL) testing with stop consonant vowel (C-V) syllables. Patients showed a significant progressive loss of posterior CC areas (isthmus and splenium) related to increasing EDSS scores and an enhancing right ear advantage (REA) over time. A significant correlation between posterior CC areas and DL scores emerged in both evaluations, being negative for the right and positive for the left ear. The pattern of correlations suggests that the CC can serve an inhibitory and also excitatory influence on the contralateral hemisphere when studying the phonological processing of language. STATEMENT OF SIGNIFICANCE TO THE NEUROSCIENCE OF LANGUAGE: The scope of the manuscript is language lateralization. The task used in the experiment is a verbal dichotic listening task, tapping the most basic phonological aspects of language. Finally, the available research is scarce when focusing on the interhemispheric excitation or inhibition of the corpus callosum in linguistic functioning.

The effects of background noise on dichotic listening to consonant-vowel syllables: An fMRI study

The present fMRI study attempts to identify brain areas that may underlie the effect of different background noises on functional brain asymmetry in a dichotic listening task. Previous studies have shown that the prominent right ear advantage in dichotic listening to consonant-vowel syllables is affected by background noise. To explore the underlying neuronal processes, haemodynamic brain responses using fMRI were recorded while participants performed the dichotic listening task in two different noisy backgrounds (conversational "babble" and traffic noise). The behavioural results showed a reduction of the right ear advantage in the background noise conditions, especially in the traffic noise condition. The behavioural results are discussed in terms of alertness-attentional mechanisms. The effects of background noise on brain activation involved significant activations in a speech-processing network. Specifically the changes in activations in the peri-Sylvian region of the superior temporal gyrus and in the temporo-parietal junction part in the left hemisphere, as well as in the superior temporal gyrus/sulcus area in the right hemisphere may mirror the effects of noise on behavioural performance. The effects of noise on brain activation are discussed with regard to pre-activation mechanisms.

Neurofunctional view of psychiatry: clinical brain imaging revisited

PURPOSE OF REVIEW

Despite an exponential increase in the use of brain imaging in neuroscience, it has as yet hardly been integrated into clinical psychiatry. Our aim is to examine the potentials and perspectives of functional brain imaging in the diagnosis and treatment of mental disorders.

RECENT FINDINGS

This review focuses on functional MRI in probing neural activation and on diffusion tensor imaging in delineating functionally related fibre-track organization. As a case study, it examines the state-of-the-art in applying these methods in schizophrenia by referring to several hurdles in the common clinical practice of psychiatry. First, we evaluate the ability of functional brain imaging to target various neuropathological mechanisms such as neurodegeneration, disrupted development and dysconnection. Then we discuss the use of brain-imaging genomics in identifying disease-specific genetic-based neuroendophenotypes. Lastly, we describe the current effort in using brain imaging to designate the most effective and least adverse treatment for patients with schizophrenia.

SUMMARY

By examining the use of advanced MRI techniques in schizophrenia, we show both the remarkable variety of functional inferences, as well as their possible implications in clinical psychiatry. We advocate the need for extending the view on neuropathology from region-based to network-based, for integrating neurogenomic features as biological markers for illness definition and for relying on neural parameters to guide effective individual treatment.

Behavioral correlates of corpus callosum size: anatomical/behavioral relationships vary across sex/handedness groups

There are substantial individual differences in the size and shape of the corpus callosum and such differences are thought to relate to behavioral lateralization. We report findings from a large scale investigation of relationships between brain anatomy and behavioral asymmetry on a battery of visual word recognition tasks. A sample of 200 individuals was divided into groups on the basis of sex and consistency of handedness. We investigated differences between sex/handedness groups in callosal area and relationships between callosal area and behavioral predictors. Sex/handedness groups did not show systematic differences in callosal area or behavioral asymmetry. However, the groups differed in the relationships between area of the corpus callosum and behavioral asymmetry. Among consistent-handed males, callosal area was negatively related to behavioral laterality. Among mixed-handed males and consistent-handed females, behavioral laterality was not predictive of callosal area. The most robust relationship was observed in mixed-handed females, in whom behavioral asymmetry was positively related to callosal area. Our study demonstrates the importance of considering brain/behavior relationships within sub-populations, as relationships between behavioral asymmetry and callosal anatomy varied across subject groups.

What Is Left Is Right

The present paper is based on a talk on hemispheric asymmetry given by Kenneth Hugdahl at the Xth European Congress of Psychology, Praha July 2007. Here, we propose that hemispheric asymmetry evolved because of a left hemisphere speech processing specialization. The evolution of speech and the need for air-based communication necessitated division of labor between the hemispheres in order to avoid having duplicate copies in both hemispheres that would increase processing redundancy. It is argued that the neuronal basis of this labor division is the structural asymmetry observed in the peri-Sylvian region in the posterior part of the temporal lobe, with a left larger than right planum temporale area. This is the only example where a structural, or anatomical, asymmetry matches a corresponding functional asymmetry. The increase in gray matter volume in the left planum temporale area corresponds to a functional asymmetry of speech processing, as indexed from both behavioral, dichotic listening, and functional neuroimaging studies. The functional anatomy of the corpus callosum also supports such a view, with regional specificity of information transfer between the hemispheres.

Splenium microstructure is related to two dimensions of reading skill

Inconsistent differences in the corpus callosum (CC) structure between dyslexic readers (DRs) and typical readers (TRs) have been reported. We examine differences in CC splenium microstructure and the association of splenium microstructure with reading-related skills. Nine DRs and 18 TRs completed a reading skills battery and diffusion tensor imaging. DRs had higher splenium fractional anisotropy (FA) and axial diffusivity (LA) as compared with TRs. Retrieval of orthographic information from the language lexicon was negatively associated with FA and LA within both reading groups. Phonological awareness was positively associated with splenium FA and LA in TRs but not DRs. This study suggests two white matter pathways that may be differentially associated with reading skills in the CC splenium.

Diffusion tensor quantification of the macrostructure and microstructure of human midsagittal corpus callosum across the lifespan

The midsagittal cross-sectional area of the human corpus callosum (CC) has been used by many researchers as a marker of development, natural aging, and neurodegenerative and acquired pathologies. The availability of non-invasive MRI methods for quantifying the macrostructural and microstructural organization of the CC would help to clarify the CC contribution to behavior and cognition in both health and disease. In this report, we extended and validated the ability of a recently described semi-automated diffusion tensor imaging tissue segmentation method to utilize the high orientation contrast of the CC on diffusion tensor imaging. Using a cohort of healthy right-handed children and adults aged 7-59 years, we show gender-independent non-linear (quadratic) and strongly correlated growth trends in the CC area and the corresponding diffusion tensor fractional anisotropy (r = 0.67; P < 1 x 10(-10)). Our results provide preliminary evidence that diffusion tensor anisotropy in the living CC may be related to the number of small myelinated fibers.

Neural mechanisms underlying the processing of Chinese and English words in a word generation task: an event-related potential study

Event-related brain potentials (ERPs) were used to examine the spatiotemporal cortical activation patterns underlying Chinese and English word generation (forming a new word by adding a stroke or a letter to an old Chinese or English word) for low- and high-proficiency Chinese-English bilinguals. The results revealed that early visual perceptual processing and word identification were similar between the two languages for the N120 and P220 waveforms. However, a greater negative potential (N250-350) was associated with Chinese words than with English words between 250 and 350 ms. Subsequently, for fluent Chinese bilinguals, Chinese words elicited a more positive ERP deflection (LPC) than did English words between 350 and 800 ms. The differences in ERP components between Chinese and English words indicates that there might be a real difference in the processing demands between these languages, and that the processing of English might be affected by the proficiency of the second language.

Grey and white matter loss along cerebral midline structures in myotonic dystrophy type 2

Myotonic dystrophy type 2 (DM2) is an autosomal dominantly inherited multisystemic disorder and a common cause of muscular dystrophy in adults. Although neuromuscular symptoms predominate, there is clinical and imaging evidence of cerebral involvement. We used voxel-based morphometry (VBM) based on T1-weighted magnetic resonance images to investigate brain morphology in 13 DM2 patients in comparison to 13 sex- and age-matched controls. Further, we employed novel computational surface-based methods that specifically assess callosal thickness. We found grey and white matter loss along cerebral midline structures in our patient group. Grey matter reductions were present in brainstem and adjacent hypothalamic and thalamic regions, while white matter was mainly reduced in corpus callosum. The reduced callosal size was highly significant and independently confirmed by different methods. Our data provide first evidence for grey and white matter loss along brain midline structures in DM2 patients. The reduced size of the corpus callosum further extends the spectrum of white matter changes in DM2 and may represent the morphological substrate of neuropsychological abnormalities previously described in this disorder.

Auditory interhemispheric transfer in relation to patterns of partial agenesis and hypoplasia of the corpus callosum in spina bifida meningomyelocele

Spina bifida meningomyelocele with hydrocephalus (SBM) is commonly associated with anomalies of the corpus callosum (CC). We describe MRI patterns of regional CC agenesis and relate CC anomalies to functional laterality based on a dichotic listening test in 90 children with SBM and 27 typically developing controls. Many children with SBM (n = 40) showed regional CC anomalies in the form of agenesis of the rostrum and/or splenium, and a smaller number (n = 20) showed hypoplasia (thinning) of all CC regions (rostrum, genu, body, and splenium). The expected right ear advantage (REA) was exhibited by normal controls and children with SBM having a normal or hypoplastic splenium. It was not shown by children with SBM who were left handed, missing a splenium, or had a higher level spinal cord lesion. Perhaps the right hemisphere of these children is more involved in processing some aspects of linguistic stimuli.