A study of brain white matter plasticity in early blinds using tract-based spatial statistics and tract statistical analysis

Brain structural changes in blindness: a systematic review and an anatomical likelihood estimation (ALE) meta-analysis

In recent decades, numerous structural brain imaging studies investigated purported morphometric changes in early (EB) and late onset blindness (LB). The results of these studies have not yielded very consistent results, neither with respect to the type, nor to the anatomical locations of the brain morphometric alterations. To better characterize the effects of blindness on brain morphometry, we performed a systematic review and an Anatomical-Likelihood-Estimation (ALE) coordinate-based-meta-analysis of 65 eligible studies on brain structural changes in EB and LB, including 890 EB, 466 LB and 1257 sighted controls. Results revealed atrophic changes throughout the whole extent of the retino-geniculo-striate system in both EB and LB, whereas changes in areas beyond the occipital lobe occurred in EB only. We discuss the nature of some of the contradictory findings with respect to the used brain imaging methodologies and characteristics of the blind populations such as the onset, duration and cause of blindness. Future studies should aim for much larger sample sizes, eventually by merging data from different brain imaging centers using the same imaging sequences, opt for multimodal structural brain imaging, and go beyond a purely structural approach by combining functional with structural connectivity network analyses.

Reorganization of thalamocortical connections in congenitally blind humans

Cross-modal plasticity in blind individuals has been reported over the past decades showing that nonvisual information is carried and processed by "visual" brain structures. However, despite multiple efforts, the structural underpinnings of cross-modal plasticity in congenitally blind individuals remain unclear. We mapped thalamocortical connectivity and assessed the integrity of white matter of 10 congenitally blind individuals and 10 sighted controls. We hypothesized an aberrant thalamocortical pattern of connectivity taking place in the absence of visual stimuli from birth as a potential mechanism of cross-modal plasticity. In addition to the impaired microstructure of visual white matter bundles, we observed structural connectivity changes between the thalamus and occipital and temporal cortices. Specifically, the thalamic territory dedicated to connections with the occipital cortex was smaller and displayed weaker connectivity in congenitally blind individuals, whereas those connecting with the temporal cortex showed greater volume and increased connectivity. The abnormal pattern of thalamocortical connectivity included the lateral and medial geniculate nuclei and the pulvinar nucleus. For the first time in humans, a remapping of structural thalamocortical connections involving both unimodal and multimodal thalamic nuclei has been demonstrated, shedding light on the possible mechanisms of cross-modal plasticity in humans. The present findings may help understand the functional adaptations commonly observed in congenitally blind individuals.

White matter tract changes in pediatric posterior fossa brain tumor survivors after surgery and chemotherapy

BACKGROUND

Survivors of pediatric posterior fossa brain tumors are susceptible to the adverse effects of treatment as they grow into adulthood. While the exact neurobiological mechanisms of these outcomes are not yet understood, the effects of treatment on white matter (WM) tracts in the brain can be visualized using diffusion tensor (DT) imaging. We investigated these WM microstructural differences using the statistical method tract-specific analysis (TSA). We applied TSA to the DT images of 25 children with a history of posterior fossa tumor (15 treated with surgery, 10 treated with surgery and chemotherapy) along with 21 healthy controls. Between these 3 groups, we examined differences in the most used DTI metric, fractional anisotropy (FA), in 11 major brain WM tracts.

RESULTS

Lower FA was found in the splenium of the corpus callosum (CC), the bilateral corticospinal tract (CST), the right inferior frontal occipital fasciculus (IFOF) and the left uncinate fasciculus (UF) in children with brain tumors as compared to healthy controls. Lower FA, an indicator of microstructural damage to WM, was observed in 4 of the 11 WM tracts examined in both groups of children with a history of posterior fossa tumor, with an additional tract unique to children who received surgery and chemotherapy (left UF).

CONCLUSIONS

Our findings indicate that a history of tumor in the posterior fossa and surgical resection may have effects on the WM in other parts of the brain.

Tract-specific analysis and neurocognitive functioning in sickle cell patients without history of overt stroke

INTRODUCTION

Sickle cell disease (SCD) is a hereditary blood disorder in which the oxygen-carrying hemoglobin molecule in red blood cells is abnormal. SCD patients are at increased risks for strokes and neurocognitive deficit, even though neurovascular screening and treatments have lowered the rate of overt strokes. Tract-specific analysis (TSA) is a statistical method to evaluate microstructural WM damage in neurodegenerative disorders, using diffusion tensor imaging (DTI).

METHODS

We utilized TSA and compared 11 major brain WM tracts between SCD patients with no history of overt stroke, anemic controls, and healthy controls. We additionally examined the relationship between the most commonly used DTI metric of WM tracts and neurocognitive performance in the SCD patients and healthy controls.

RESULTS

Disruption of WM microstructure orientation-dependent metrics for the SCD patients was found in the genu of the corpus callosum (CC), cortico-spinal tract, inferior fronto-occipital fasciculus, right inferior longitudinal fasciculus, superior longitudinal fasciculus, and left uncinate fasciculus. Neurocognitive performance indicated slower processing speed and lower response inhibition skills in SCD patients compared to controls. TSA abnormalities in the CC were significantly associated with measures of processing speed, working memory, and executive functions.

CONCLUSION

Decreased DTI-derived metrics were observed on six tracts in chronically anemic patients, regardless of anemia subtype, while two tracks with decreased measures were unique to SCD patients. Patients with WMHs had more significant FA abnormalities. Decreased FA values in the CC significantly correlated with all nine neurocognitive tests, suggesting a critical importance for CC in core neurocognitive processes.

Overlapping Anatomical Networks Convey Cross-Modal Suppression in the Sighted and Coactivation of "Visual" and Auditory Cortex in the Blind

In the present combined DTI/fMRI study we investigated adaptive plasticity of neural networks involved in controlling spatial and nonspatial auditory working memory in the early blind (EB). In both EB and sighted controls (SC), fractional anisotropy (FA) within the right inferior longitudinal fasciculus correlated positively with accuracy in a one-back sound localization but not sound identification task. The neural tracts passing through the cluster of significant correlation connected auditory and "visual" areas in the right hemisphere. Activity in these areas during both sound localization and identification correlated with FA within the anterior corpus callosum, anterior thalamic radiation, and inferior fronto-occipital fasciculus. In EB, FA in these structures correlated positively with activity in both auditory and "visual" areas, whereas FA in SC correlated positively with activity in auditory and negatively with activity in visual areas. The results indicate that frontal white matter conveys cross-modal suppression of occipital areas in SC, while it mediates coactivation of auditory and reorganized "visual" cortex in EB.

Neuroplasticity in cerebral visual impairment (CVI): Assessing functional vision and the neurophysiological correlates of dorsal stream dysfunction

Cerebral visual impairment (CVI) results from perinatal injury to visual processing structures and pathways and is the most common individual cause of pediatric visual impairment and blindness in developed countries. While there is mounting evidence demonstrating extensive neuroplastic reorganization in early onset, profound ocular blindness, how the brain reorganizes in the setting of congenital damage to cerebral (i.e. retro-geniculate) visual pathways remains comparatively poorly understood. Individuals with CVI exhibit a wide range of visual deficits and, in particular, present with impairments of higher order visual spatial processing (referred to as "dorsal stream dysfunction") as well as object recognition (associated with processing along the ventral stream). In this review, we discuss the need for ongoing work to develop novel, neuroscience-inspired approaches to investigate functional visual deficits in this population. We also outline the role played by advanced structural and functional neuroimaging in helping to elucidate the underlying neurophysiology of CVI, and highlight key differences with regard to patterns of neural reorganization previously described in ocular blindness.

Changes of fractional anisotropy and RGMa in crossed cerebellar diaschisis induced by middle cerebral artery occlusion

Crossed cerebellar diaschisis (CCD) is the phenomenon of hypoperfusion and hypometabolism of the contralateral cerebellar hemisphere caused by dysfunction of the associated supratentorial region. The aim of the present study was to analyze the changes in fractional anisotropy (FA) in CCD induced by middle cerebral artery occlusion (MCAO) using magnetic resonance-diffusion tensor imaging (MR-DTI). Furthermore, the role of repulsive guidance molecule a (RGMa) in CCD was assessed by measuring RGMa expression using histochemical analysis. In the present study, the cerebellar hemisphere was serially scanned with T2-weighted, serial diffusion-weighted and diffusion tensor (DT) imaging using a 3.0T GE Signa HDxt Scanner to analyze the changes in FA over 72 h. Subsequently, immunohistochemistry analyses of the corresponding cerebellar hemisphere sections were performed to assess the expression of RGMa. Results indicated that FA of both sides of the cerebellar hemisphere, particularly that of the contralateral cerebellar hemisphere (right side) derived from DTI, was reduced during the 72-h time period following MCAO, and the decrease was maximal and statistically significant at 12 h (P<0.05). Immunohistochemistry analysis revealed a significant increase in the expression of RGMa protein in the affected region of the contralateral cerebellar hemisphere (right side) at 24 h following MCAO injury (P<0.05). Furthermore, the expression of RGMa and FA was negatively correlated in MCAO (P<0.05). The results suggest that MR-DTI is an important assessment to evaluate changes of FA in CCD induced by MCAO. Furthermore, the present results suggest that RGMa, which was negatively correlated with FA in MCAO rats, may serve an important role in CCD.

Structural alterations of the brain preceded functional alterations in major depressive disorder patients: Evidence from multimodal connectivity

BACKGROUND

Recent studies showed that major depressive disorder (MDD) has been involved in abnormal functional and structural connections in specific brain regions. However, comprehensive researches on MDD-related alterations in the topological organization of brain functional and structural networks are still limited.

METHODS

Functional network (FN) was constructed from resting-state functional MRI temporal series correlations and structural network (SN) was established by Diffusion tensor imaging (DTI) data in 58 MDD patients and 71 healthy controls (HC). The measurements of the network properties were calculated for two networks respectively. Correlations were conducted between altered network parameters and Hamilton depression scale (HAMD) score. Additionally, network resilient analysis were conducted on FN and SN.

RESULTS

The losses of small-worldness charateristics and the decline of nodal efficiency across FN and SN were found in MDD patients. Based on network-based statistic (NBS) approach, the decreased connections in MDD patients were mainly found in the superior occipital gyrus, superior temporal gyrus for FN and SN, while the increased connections were distributed in putamen, superior frontal gyrus only for SN. Compared with the FN, the SN showed less resilient to targeted or random node failure. Besides, altered edges in NBS and regions with decreased nodal efficiency were negatively associated with HAMD score in MDD patients.

LIMITATIONS

The samples size is small and most of the MDD patients take different antidepressant medications.

CONCLUSIONS

Alterations of SN in the brain of MDD patients preceded that of FN to some extent, and reorganization of the brain network was a mechanism which compensated for functional and structural alterations during disease progression.

White matter connectivity between occipital and temporal regions involved in face and voice processing in hearing and early deaf individuals

Neuroplasticity following sensory deprivation has long inspired neuroscience research in the quest of understanding how sensory experience and genetics interact in developing the brain functional and structural architecture. Many studies have shown that sensory deprivation can lead to cross-modal functional recruitment of sensory deprived cortices. Little is known however about how structural reorganization may support these functional changes. In this study, we examined early deaf, hearing signer and hearing non-signer individuals using diffusion MRI to evaluate the potential structural connectivity linked to the functional recruitment of the temporal voice area by face stimuli in deaf individuals. More specifically, we characterized the structural connectivity between occipital, fusiform and temporal regions typically supporting voice- and face-selective processing. Despite the extensive functional reorganization for face processing in the temporal cortex of the deaf, macroscopic properties of these connections did not differ across groups. However, both occipito- and fusiform-temporal connections showed significant microstructural changes between groups (fractional anisotropy reduction, radial diffusivity increase). We propose that the reorganization of temporal regions after early auditory deprivation builds on intrinsic and mainly preserved anatomical connectivity between functionally specific temporal and occipital regions.

Brain (re)organization following visual loss

The study of the neural consequences of sensory loss provides a unique window into the brain's functional and organizational principles. Although the blind visual cortex has been implicated in the cross-modal processing of nonvisual inputs for quite some time, recent research has shown that certain cortical organizational principles are preserved even in the case of complete sensory loss. Furthermore, a growing body of work has shown that markers of neuroplasticity extend to neuroanatomical metrics that include cortical thickness and myelinization. Although our understanding of the mechanisms that underlie sensory deprivation-driven cross-modal plasticity is improving, several critical questions remain unanswered. The specific pathways that underlie the rerouting of nonvisual information, for instance, have not been fully elucidated. The fact that important cross-modal recruitment occurs following transient deprivation in sighted individuals suggests that significant rewiring following blindness may not be required. Furthermore, there are marked individual differences regarding the magnitude and functional relevance of the cross-modal reorganization. It is also not clear to what extent precise environmental factors may play a role in establishing the degree of reorganization across individuals, as opposed to factors that might specifically relate to the cause or the nature of the visual loss. In sum, although many unresolved questions remain, sensory deprivation continues to be an excellent model for studying the plastic nature of the brain. This article is categorized under: Psychology > Brain Function and Dysfunction Psychology > Perception and Psychophysics Neuroscience > Plasticity.

Axo-myelinic neurotransmission: a novel mode of cell signalling in the central nervous system

It is widely recognized that myelination of axons greatly enhances the speed of signal transmission. An exciting new finding is the dynamic communication between axons and their myelin-forming oligodendrocytes, including activity-dependent signalling from axon to myelin. The oligodendrocyte-myelin complex may in turn respond by providing metabolic support or alter subtle myelin properties to modulate action potential propagation. In this Opinion, we discuss what is known regarding the molecular physiology of this novel, synapse-like communication and speculate on potential roles in disease states including multiple sclerosis, schizophrenia and Alzheimer disease. An emerging appreciation of the contribution of white-matter perturbations to neurological dysfunction identifies the axo-myelinic synapse as a potential novel therapeutic target.

Frontal EEG asymmetry and later behavior vulnerability in infants with congenital visual impairment

OBJECTIVE

Young children with congenital visual impairment (VI) are at increased risk of behavioral vulnerabilities. Studies on 'at risk' populations suggest that frontal EEG asymmetry may be associated with behavioral risk. We investigated frontal asymmetry at 1year (Time 1), behavior at 2years (Time 2) and their longitudinal associations within a sample of infants with VI. Frontal asymmetry in the VI sample at 1year was also compared cross-sectionally to an age-matched typically sighted (TS) group.

METHODS

At Time 1, 22 infants with VI and 10 TS infants underwent 128-channel EEG recording. Frontal asymmetry ratios were calculated from power spectral density values in the alpha frequency band. At Time 2, Achenbach Child Behavior Checklist data was obtained for the VI sample.

RESULTS

63.6% of the VI sample and 50% of the TS sample showed left frontal asymmetry; no significant difference in frontal asymmetry was found between the two groups. 22.7% of the VI sample had subclinical to clinical range 'internalizing' behavior difficulties. Greater left frontal asymmetry at one year was significantly associated with greater emotionally reactive scores at two years within the VI sample (r=0.50, p=0.02).

CONCLUSIONS

Left frontal asymmetry correlates with later behavior risk within this vulnerable population.

SIGNIFICANCE

These findings make an important first contribution regarding the utility of frontal EEG asymmetry as a method to investigate risk in infants with VI.

Multimodal MR-imaging reveals large-scale structural and functional connectivity changes in profound early blindness

In the setting of profound ocular blindness, numerous lines of evidence demonstrate the existence of dramatic anatomical and functional changes within the brain. However, previous studies based on a variety of distinct measures have often provided inconsistent findings. To help reconcile this issue, we used a multimodal magnetic resonance (MR)-based imaging approach to provide complementary structural and functional information regarding this neuroplastic reorganization. This included gray matter structural morphometry, high angular resolution diffusion imaging (HARDI) of white matter connectivity and integrity, and resting state functional connectivity MRI (rsfcMRI) analysis. When comparing the brains of early blind individuals to sighted controls, we found evidence of co-occurring decreases in cortical volume and cortical thickness within visual processing areas of the occipital and temporal cortices respectively. Increases in cortical volume in the early blind were evident within regions of parietal cortex. Investigating white matter connections using HARDI revealed patterns of increased and decreased connectivity when comparing both groups. In the blind, increased white matter connectivity (indexed by increased fiber number) was predominantly left-lateralized, including between frontal and temporal areas implicated with language processing. Decreases in structural connectivity were evident involving frontal and somatosensory regions as well as between occipital and cingulate cortices. Differences in white matter integrity (as indexed by quantitative anisotropy, or QA) were also in general agreement with observed pattern changes in the number of white matter fibers. Analysis of resting state sequences showed evidence of both increased and decreased functional connectivity in the blind compared to sighted controls. Specifically, increased connectivity was evident between temporal and inferior frontal areas. Decreases in functional connectivity were observed between occipital and frontal and somatosensory-motor areas and between temporal (mainly fusiform and parahippocampus) and parietal, frontal, and other temporal areas. Correlations in white matter connectivity and functional connectivity observed between early blind and sighted controls showed an overall high degree of association. However, comparing the relative changes in white matter and functional connectivity between early blind and sighted controls did not show a significant correlation. In summary, these findings provide complimentary evidence, as well as highlight potential contradictions, regarding the nature of regional and large scale neuroplastic reorganization resulting from early onset blindness.

Tract specific analysis in patients with sickle cell disease

Sickle cell disease (SCD) is a hereditary blood disorder in which the oxygen-carrying hemoglobin molecule in red blood cells is abnormal. It affects numerous people in the world and leads to a shorter life span, pain, anemia, serious infections and neurocognitive decline. Tract-Specific Analysis (TSA) is a statistical method to evaluate white matter alterations due to neurocognitive diseases, using diffusion tensor magnetic resonance images. Here, for the first time, TSA is used to compare 11 major brain white matter (WM) tracts between SCD patients and age-matched healthy subjects. Alterations are found in the corpus callosum (CC), the cortico-spinal tract (CST), inferior fronto-occipital fasciculus (IFO), inferior longitudinal fasciculus (ILF), superior longitudinal fasciculus (SLF), and uncinated fasciculus (UNC). Based on previous studies on the neurocognitive functions of these tracts, the significant areas found in this paper might be related to several cognitive impairments and depression, both of which are observed in SCD patients.