Alterations of the visual pathways in congenital blindness

Reorganization of integration and segregation networks in brain-based visual impairment

Growing evidence suggests that cerebral connectivity changes its network organization by altering modular topology in response to developmental and environmental experience. However, changes in cerebral connectivity associated with visual impairment due to early neurological injury are still not fully understood. Cerebral visual impairment (CVI) is a brain-based visual disorder associated with damage and maldevelopment of retrochiasmal pathways and areas implicated in visual processing. In this study, we used a multimodal imaging approach and connectomic analyses based on structural (voxel-based morphometry; VBM) and resting state functional connectivity (rsfc) to investigate differences in weighted degree and link-level connectivity in individuals with CVI compared to controls with neurotypical development. We found that participants with CVI showed significantly reduced grey matter volume within the primary visual cortex and intraparietal sulcus (IPS) compared to controls. Participants with CVI also exhibited marked reorganization characterized by increased integration of visual connectivity to somatosensory and multimodal integration areas (dorsal and ventral attention regions) and lower connectivity from visual to limbic and default mode networks. Link-level functional changes in CVI were also associated with key clinical outcomes related to visual function and development. These findings provide early insight into how visual impairment related to early brain injury distinctly reorganizes the functional network architecture of the human brain.

Gray matter volume of the feline cerebral cortex and structural plasticity following perinatal deafness

In response to sensory deprivation, the brain adapts according to contemporary demands to efficiently navigate a modified perceptual environment. This reorganization may result in improved processing of the remaining senses-a phenomenon referred to as compensatory crossmodal plasticity. One approach to explore this neuroplasticity is to consider the macrostructural changes in neural tissue that mirror this functional optimization. The current study is the first of its kind to measure MRI-derived gray matter (GM) volumes of control felines (n=30), while additionally identifying volumetric differences in response to perinatal deafness (30 ototoxically-deafened cats). To accomplish this purpose, regional and morphometric methods were performed in parallel. The regional analysis evaluated volumetric alterations of global GM, as well as the volumes of 146 regions of interest (ROIs) and 12 functional subgroupings of these ROIs. Results revealed whole-brain GM preservation; however, somatosensory and visual cortices exhibited an overall increase in volume. On a smaller scale, this analysis uncovered two auditory ROIs (second auditory cortex, A2, and ventral auditory field, VAF) that decreased in volume alongside two visual regions (anteromedial lateral suprasylvian area, AMLS and splenial visual area, SVA) that increased-all localized within the right hemisphere. Comparatively, the findings of tensor-based morphometry (TBM) generally aligned with those of the ROI-based method, as this voxel-wise approach demonstrated clusters of expansion coincident with visual- and somatosensory-related loci; although, it failed to detect any GM reductions following deafness. As distinct differences were identified in each analysis, the current study highlights the importance of employing multiple methods when exploring MRI volumetry. Overall, this study proposes that volumetric alterations within sensory loci allude to a redistribution of cortical space arising from modified perceptual demands following auditory deprivation.

Plastic reorganization of the topological asymmetry of hemispheric white matter networks induced by congenital visual experience deprivation

Congenital blindness offers a unique opportunity to investigate human brain plasticity. The influence of congenital visual loss on the asymmetry of the structural network remains poorly understood. To address this question, we recruited 21 participants with congenital blindness (CB) and 21 age-matched sighted controls (SCs). Employing diffusion and structural magnetic resonance imaging, we constructed hemispheric white matter (WM) networks using deterministic fiber tractography and applied graph theory methodologies to assess topological efficiency (i.e., network global efficiency, network local efficiency, and nodal local efficiency) within these networks. Statistical analyses revealed a consistent leftward asymmetry in global efficiency across both groups. However, a different pattern emerged in network local efficiency, with the CB group exhibiting a symmetric state, while the SC group showed a leftward asymmetry. Specifically, compared to the SC group, the CB group exhibited a decrease in local efficiency in the left hemisphere, which was caused by a reduction in the nodal properties of some key regions mainly distributed in the left occipital lobe. Furthermore, interhemispheric tracts connecting these key regions exhibited significant structural changes primarily in the splenium of the corpus callosum. This result confirms the initial observation that the reorganization in asymmetry of the WM network following congenital visual loss is associated with structural changes in the corpus callosum. These findings provide novel insights into the neuroplasticity and adaptability of the brain, particularly at the network level.

Reduced lateralization of the language network in the blind and its relationship with white matter tract neuroanatomy

Several previous studies reported reduced leftward lateralization in blind participants' samples compared to the sighted population. The origins of this difference remain unknown. Here, we tested whether functional lateralization is connected with the structural characteristics of white matter tracts [corpus callosum (CC), uncinate fasciculus (UF), and superior longitudinal fasciculus (SLF)], as suggested by previous studies conducted in the typical sighted population. Twenty-three blind and 21 sighted adult participants were tested during fMRI with a semantic decision paradigm presented both auditorily and in the modality appropriate for reading (tactually for the blind and visually for the sighted). Lateralization indices (LI) were calculated based on the activations. The fractional anisotropy (FA) measure was extracted from the white matter tracts of interest. Correlation analyses testing the relationship between FA and LI were conducted. The reduced leftward lateralization of both speech processing and reading-related activations was replicated. Nevertheless, the relationship between the structural integrity of the CC and LI and between the asymmetry of the intrahemispheric tracts and LI was not confirmed, possibly due to the lack of power. The sources of the reduced lateralization of the language network in the sensory-deprived population remain unknown. Further studies should account for environmental variables (e.g., the frequency of contact with written language) and the complexity of the factors that may influence the functional lateralization of the human brain.

Blind individuals' enhanced ability to sense their own heartbeat is related to the thickness of their occipital cortex

Blindness is associated with heightened sensory abilities, such as improved hearing and tactile acuity. Moreover, recent evidence suggests that blind individuals are better than sighted individuals at perceiving their own heartbeat, suggesting enhanced interoceptive accuracy. Structural changes in the occipital cortex have been hypothesized as the basis of these behavioral enhancements. Indeed, several studies have shown that congenitally blind individuals have increased cortical thickness within occipital areas compared to sighted individuals, but how these structural differences relate to behavioral enhancements is unclear. This study investigated the relationship between cardiac interoceptive accuracy and cortical thickness in 23 congenitally blind individuals and 23 matched sighted controls. Our results show a significant positive correlation between performance in a heartbeat counting task and cortical thickness only in the blind group, indicating a connection between structural changes in occipital areas and blind individuals' enhanced ability to perceive heartbeats.

Changes in primary visual and auditory cortex of blind and sighted adults following 10 weeks of click-based echolocation training

Recent work suggests that the adult human brain is very adaptable when it comes to sensory processing. In this context, it has also been suggested that structural "blueprints" may fundamentally constrain neuroplastic change, e.g. in response to sensory deprivation. Here, we trained 12 blind participants and 14 sighted participants in echolocation over a 10-week period, and used MRI in a pre-post design to measure functional and structural brain changes. We found that blind participants and sighted participants together showed a training-induced increase in activation in left and right V1 in response to echoes, a finding difficult to reconcile with the view that sensory cortex is strictly organized by modality. Further, blind participants and sighted participants showed a training induced increase in activation in right A1 in response to sounds per se (i.e. not echo-specific), and this was accompanied by an increase in gray matter density in right A1 in blind participants and in adjacent acoustic areas in sighted participants. The similarity in functional results between sighted participants and blind participants is consistent with the idea that reorganization may be governed by similar principles in the two groups, yet our structural analyses also showed differences between the groups suggesting that a more nuanced view may be required.

The impact of early and late blindness on language and verbal working memory: A brain-constrained neural model

Neural circuits related to language exhibit a remarkable ability to reorganize and adapt in response to visual deprivation. Particularly, early and late blindness induce distinct neuroplastic changes in the visual cortex, repurposing it for language and semantic processing. Interestingly, these functional changes provoke a unique cognitive advantage - enhanced verbal working memory, particularly in early blindness. Yet, the underlying neuromechanisms and the impact on language and memory-related circuits remain not fully understood. Here, we applied a brain-constrained neural network mimicking the structural and functional features of the frontotemporal-occipital cortices, to model conceptual acquisition in early and late blindness. The results revealed differential expansion of conceptual-related neural circuits into deprived visual areas depending on the timing of visual loss, which is most prominent in early blindness. This neural recruitment is fundamentally governed by the biological principles of neural circuit expansion and the absence of uncorrelated sensory input. Critically, the degree of these changes is constrained by the availability of neural matter previously allocated to visual experiences, as in the case of late blindness. Moreover, we shed light on the implication of visual deprivation on the neural underpinnings of verbal working memory, revealing longer reverberatory neural activity in 'blind models' as compared to the sighted ones. These findings provide a better understanding of the interplay between visual deprivations, neuroplasticity, language processing and verbal working memory.

Accelerated Brain Atrophy, Microstructural Decline and Connectopathy in Age-Related Macular Degeneration

Age-related macular degeneration (AMD) has recently been linked to cognitive impairment. We hypothesized that AMD modifies the brain aging trajectory, and we conducted a longitudinal diffusion MRI study on 40 participants (20 with AMD and 20 controls) to reveal the location, extent, and dynamics of AMD-related brain changes. Voxel-based analyses at the first visit identified reduced volume in AMD participants in the cuneate gyrus, associated with vision, and the temporal and bilateral cingulate gyrus, linked to higher cognition and memory. The second visit occurred 2 years after the first and revealed that AMD participants had reduced cingulate and superior frontal gyrus volumes, as well as lower fractional anisotropy (FA) for the bilateral occipital lobe, including the visual and the superior frontal cortex. We detected faster rates of volume and FA reduction in AMD participants in the left temporal cortex. We identified inter-lingual and lingual-cerebellar connections as important differentiators in AMD participants. Bundle analyses revealed that the lingual gyrus had a lower streamline length in the AMD participants at the first visit, indicating a connection between retinal and brain health. FA differences in select inter-lingual and lingual cerebellar bundles at the second visit showed downstream effects of vision loss. Our analyses revealed widespread changes in AMD participants, beyond brain networks directly involved in vision processing.

Lifelong olfactory deprivation-dependent cortical reorganization restricted to orbitofrontal cortex

Prolonged sensory deprivation has repeatedly been linked to cortical reorganization. We recently demonstrated that individuals with congenital anosmia (CA, complete olfactory deprivation since birth) have seemingly normal morphology in piriform (olfactory) cortex despite profound morphological deviations in the orbitofrontal cortex (OFC), a finding contradictory to both the known effects of blindness on visual cortex and to the sparse literature on brain morphology in anosmia. To establish whether these unexpected findings reflect the true brain morphology in CA, we first performed a direct replication of our previous study to determine if lack of results was due to a deviant control group, a confound in cross sectional studies. Individuals with CA (n = 30) were compared to age and sex matched controls (n = 30) using voxel- and surface-based morphometry. The replication results were near identical to the original study: bilateral clusters of group differences in the OFC, including CA atrophy around the olfactory sulci and volume increases in the medial orbital gyri. Importantly, no group differences in piriform cortex were detected. Subsequently, to assess any subtle patterns of group differences not detectable by our mass-univariate analysis, we explored the data from a multivariate perspective. Combining the newly collected data with data from the replicated study (CA = 49, control = 49), we performed support vector machine classification based on gray matter volume. In line with the mass-univariate analyses, the multivariate analysis could accurately differentiate between the groups in bilateral OFC, whereas the classification accuracy in piriform cortex was at chance level. Our results suggest that despite lifelong olfactory deprivation, piriform (olfactory) cortex is morphologically unaltered and the morphological deviations in CA are confined to the OFC.

Developmental plasticity of the structural network of the occipital cortex in congenital blindness

The occipital cortex is the visual processing center in the mammalian brain. An unanswered scientific question pertains to the impact of congenital visual deprivation on the development of various profiles within the occipital network. To address this issue, we recruited 30 congenitally blind participants (8 children and 22 adults) as well as 31 sighted participants (10 children and 21 adults). Our investigation focused on identifying the gray matter regions and white matter connections within the occipital cortex, alongside behavioral measures, that demonstrated different developmental patterns between blind and sighted individuals. We discovered significant developmental changes in the gray matter regions and white matter connections of the occipital cortex among blind individuals from childhood to adulthood, in comparison with sighted individuals. Moreover, some of these structures exhibited cognitive functional reorganization. Specifically, in blind adults, the posterior occipital regions (left calcarine fissure and right middle occipital gyrus) showed reorganization of tactile perception, and the forceps major tracts were reorganized for braille reading. These plastic changes in blind individuals may be attributed to experience-dependent neuronal apoptosis, pruning, and myelination. These findings provide valuable insights into the longitudinal neuroanatomical and cognitive functional plasticity of the occipital network following long-term visual deprivation.

Achromatopsia-Visual Cortex Stability and Plasticity in the Absence of Functional Cones

Purpose

Achromatopsia is a rare inherited disorder rendering retinal cone photoreceptors nonfunctional. As a consequence, the sizable foveal representation in the visual cortex is congenitally deprived of visual input, which prompts a fundamental question: is the cortical representation of the central visual field in patients with achromatopsia remapped to take up processing of paracentral inputs? Such remapping might interfere with gene therapeutic treatments aimed at restoring cone function.

Methods

We conducted a multicenter study to explore the nature and plasticity of vision in the absence of functional cones in a cohort of 17 individuals affected by autosomal recessive achromatopsia and confirmed biallelic disease-causing CNGA3 or CNGB3 mutations. Specifically, we tested the hypothesis of foveal remapping in human achromatopsia. For this purpose, we applied two independent functional magnetic resonance imaging (fMRI)-based mapping approaches, i.e. conventional phase-encoded eccentricity and population receptive field mapping, to separate data sets.

Results

Both fMRI approaches produced the same result in the group comparison of achromatopsia versus healthy controls: sizable remapping of the representation of the central visual field in the primary visual cortex was not apparent.

Conclusions

Remapping of the cortical representation of the central visual field is not a general feature in achromatopsia. It is concluded that plasticity of the human primary visual cortex is less pronounced than previously assumed. A pretherapeutic imaging workup is proposed to optimize interventions.

Visuo-spatial imagery in dreams of congenitally and early blind: a systematic review

Background

The presence of visual imagery in dreams of congenitally blind people has long been a matter of substantial controversy. We set to systematically review body of published work on the presence and nature of oneiric visuo-spatial impressions in congenitally and early blind subjects across different areas of research, from experimental psychology, functional neuroimaging, sensory substitution, and sleep research.

Methods

Relevant studies were identified using the following databases: EMBASE, MEDLINE and PsychINFO.

Results

Studies using diverse imaging techniques and sensory substitution devices broadly suggest that the "blind" occipital cortex may be able to integrate non-visual sensory inputs, and thus possibly also generate visuo-spatial impressions. Visual impressions have also been reported by blind subjects who had near-death or out-of-body experiences.

Conclusion

Deciphering the mechanistic nature of these visual impression could open new possibility in utilization of neuroplasticity and its potential role for treatment of neurodisability.

Electromagnetic-field theories of qualia: can they improve upon standard neuroscience?

How do brains create all our different colors, pains, and other conscious qualities? These various qualia are the most essential aspects of consciousness. Yet standard neuroscience (primarily based on synaptic information processing) has not found the synaptic-firing codes, sometimes described as the "spike code," to account for how these qualia arise and how they unite to form complex perceptions, emotions, et cetera. Nor is it clear how to get from these abstract codes to the qualia we experience. But electromagnetic field (versus synaptic) approaches to how qualia arise have been offered in recent years by Pockett, McFadden, Jones, Bond, Ward and Guevera, Keppler and Shani, Hunt and Schooler, et cetera. These EM-field approaches show promise in offering more viable accounts of qualia. Yet, until now, they have not been evaluated together. We review various EM field theories of qualia, highlight their strengths and weaknesses, and contrast these theories with standard neuroscience approaches.

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.

The influence of visual deprivation on the development of the thalamocortical network: Evidence from congenitally blind children and adults

The thalamus is heavily involved in relaying sensory signals to the cerebral cortex. A relevant issue is how the deprivation of congenital visual sensory information modulates the development of the thalamocortical network. The answer is unclear because previous studies on this topic did not investigate network development, structure-function combinations, and cognition-related behaviors in the same study. To overcome these limitations, we recruited 30 congenitally blind subjects (8 children, 22 adults) and 31 sighted subjects (10 children, 21 adults), and conducted multiple analyses [i.e., gray matter volume (GMV) analysis using the voxel-based morphometry (VBM) method, resting-state functional connectivity (FC), and brain-behavior correlation]. We found that congenital blindness elicited significant changes in the development of GMV in visual and somatosensory thalamic regions. Blindness also resulted in significant changes in the development of FC between somatosensory thalamic regions and visual cortical regions as well as advanced information processing regions. Moreover, the somatosensory thalamic regions and their FCs with visual cortical regions were reorganized to process high-level tactile language information in blind individuals. These findings provide a refined understanding of the neuroanatomical and functional plasticity of the thalamocortical network.

Gyrification in relation to cortical thickness in the congenitally blind

Greater cortical gyrification (GY) is linked with enhanced cognitive abilities and is also negatively related to cortical thickness (CT). Individuals who are congenitally blind (CB) exhibits remarkable functional brain plasticity which enables them to perform certain non-visual and cognitive tasks with supranormal abilities. For instance, extensive training using touch and audition enables CB people to develop impressive skills and there is evidence linking these skills to cross-modal activations of primary visual areas. There is a cascade of anatomical, morphometric and functional-connectivity changes in non-visual structures, volumetric reductions in several components of the visual system, and CT is also increased in CB. No study to date has explored GY changes in this population, and no study has explored how variations in CT are related to GY changes in CB. T1-weighted 3D structural magnetic resonance imaging scans were acquired to examine the effects of congenital visual deprivation in cortical structures in a healthy sample of 11 CB individuals (6 male) and 16 age-matched sighted controls (SC) (10 male). In this report, we show for the first time an increase in GY in several brain areas of CB individuals compared to SC, and a negative relationship between GY and CT in the CB brain in several different cortical areas. We discuss the implications of our findings and the contributions of developmental factors and synaptogenesis to the relationship between CT and GY in CB individuals compared to SC. F.

Neural substrates of spatial processing and navigation in blindness: An activation likelihood estimation meta-analysis

Even though vision is considered the best suited sensory modality to acquire spatial information, blind individuals can form spatial representations to navigate and orient themselves efficiently in space. Consequently, many studies support the amodality hypothesis of spatial representations since sensory modalities other than vision contribute to the formation of spatial representations, independently of visual experience and imagery. However, given the high variability in abilities and deficits observed in blind populations, a clear consensus about the neural representations of space has yet to be established. To this end, we performed a meta-analysis of the literature on the neural correlates of spatial processing and navigation via sensory modalities other than vision, like touch and audition, in individuals with early and late onset blindness. An activation likelihood estimation (ALE) analysis of the neuroimaging literature revealed that early blind individuals and sighted controls activate the same neural networks in the processing of non-visual spatial information and navigation, including the posterior parietal cortex, frontal eye fields, insula, and the hippocampal complex. Furthermore, blind individuals also recruit primary and associative occipital areas involved in visuo-spatial processing via cross-modal plasticity mechanisms. The scarcity of studies involving late blind individuals did not allow us to establish a clear consensus about the neural substrates of spatial representations in this specific population. In conclusion, the results of our analysis on neuroimaging studies involving early blind individuals support the amodality hypothesis of spatial representations.

Alteration of Degree Centrality in Adolescents With Early Blindness

Congenital nystagmus in infants and young children can lead to early blindness (EB). Previous neuroimaging studies have demonstrated that EB is accompanied by alterations in brain structure and function. However, the effects of visual impairment and critical developmental periods on brain functional connectivity at rest have been unclear. Here, we used the voxel-wise degree centrality (DC) method to explore the underlying functional network brain activity in adolescents with EB. Twenty-one patients with EBs and 21 sighted controls (SCs) underwent magnetic resonance imaging. Differences between the two groups were assessed using the DC method. Moreover, the support vector machine (SVM) method was used to differentiate patients with EB patients from the SCs according to DC values. Compared with the SCs, the patients with EB had increased DC values in the bilateral cerebellum_6, cerebellum vermis_4_5, bilateral supplementary motor areas (SMA), and left fusiform gyrus; the patients with EB had decreased DC values in the bilateral rectal gyrus and left medial orbital frontal gyrus. The SVM classification of the DC values achieved an overall accuracy of 70.45% and an area under the curve of 0.86 in distinguishing between the patients with EB and the SCs. Our study may reveal the neuromechanism of neuroplasticity in EB; the findings provide an imaging basis for future development of restorative visual therapies and sensory substitution devices, and future assessments of visual rehabilitation efficacy.

Brain Gray Matter Atrophy and Functional Connectivity Remodeling in Patients With Chronic LHON

Purpose

The aim of this study was to investigate the brain gray matter volume (GMV) and spontaneous functional connectivity (FC) changes in patients with chronic Leber's hereditary optic neuropathy (LHON), and their relations with clinical measures.

Methods

A total of 32 patients with chronic LHON and matched sighted healthy controls (HC) underwent neuro-ophthalmologic examinations and multimodel magnetic resonance imaging (MRI) scans. Voxel-based morphometry (VBM) was used to detect the GMV differences between the LHON and HC. Furthermore, resting-state FC analysis using the VBM-identified clusters as seeds was carried out to detect potential functional reorganization in the LHON. Finally, the associations between the neuroimaging and clinical measures were performed.

Results

The average peripapillary retinal nerve fiber layer (RNFL) thickness of the chronic LHON was significantly thinner (T = −16.421, p < 0.001), and the mean defect of the visual field was significantly higher (T = 11.28, p < 0.001) than the HC. VBM analysis demonstrated a significantly lower GMV of bilateral calcarine gyri (CGs) in the LHON than in the HC (p < 0.05). Moreover, in comparison with the HC, the LHON had significantly lower FC between the centroid of the identified left CG and ipsilateral superior occipital gyrus (SOG) and higher FC between this cluster and the ipsilateral posterior cingulate gyrus (p < 0.05, corrected). Finally, the GMV of the left CG was negatively correlated with the LHON duration (r = −0.535, p = 0.002), and the FC between the left CG and the ipsilateral posterior cingulate gyrus of the LHON was negatively correlated with the average peripapillary RNFL thickness (r = −0.522, p = 0.003).

Conclusion

The atrophied primary visual cortex of the chronic LHON may be caused by transneuronal degeneration following the retinal damage. Moreover, our findings suggest that the functional organization of the atrophied primary visual cortex has been reshaped in the chronic LHON.

Grey Matter Hypertrophy and Atrophy in Early-Blind Adolescents: A Surface-Based Morphometric Study

Objective

This study is aimed at exploring the regional changes in brain cortical morphology (thickness, volume, and surface area) in the early-blind adolescents (EBAs) by using the surface-based morphometric (SBM) method.

Methods

High-resolution structural T1-weighted images (T1WI) of 23 early-blind adolescents (EBAs) and 21 age- and gender-matched normal-sighted controls (NSCs) were acquired. Structural indices, including cortical thickness (CT), cortical volume (CV), and surface area (SA), were analyzed by using FreeSurfer software, and the correlations between structural indices and the blindness duration were computed by Pearson correlation analysis.

Results

Compared to controls, EBAs had significantly reduced CV and SA mainly in the primary visual cortex (V1) and decreased CV in the left vision-related cortices (r-MFC). There were no regions that EBAs had a significantly larger CV or SA than NSCs. EBAs had significantly increased CT in the V1 and strongly involved the visual cortex (right lateral occipital gyrus, LOG.R) and the left superior temporal gyrus (STG.L), while it had decreased CT in the left superior parietal lobule (SPL.L) and the right lingual gyrus (LING.R). Additionally, no correlation was found between cortical morphometric measures and clinical variables in the EBA group.

Conclusions

SBM is a useful method for detecting human brain structural abnormalities in blindness. The results showed that these structural abnormalities in the visual cortex and visual-related areas outside the occipital cortex in the EBAs not only may be influenced by neurodevelopment, degeneration, plasticity, and so on but also involved the interaction of these factors after the early visual deprivation.

Brain Morphological Modifications in Congenital and Acquired Auditory Deprivation: A Systematic Review and Coordinate-Based Meta-Analysis

Neuroplasticity following deafness has been widely demonstrated in both humans and animals, but the anatomical substrate of these changes is not yet clear in human brain. However, it is of high importance since hearing loss is a growing problem due to aging population. Moreover, knowing these brain changes could help to understand some disappointing results with cochlear implant, and therefore could improve hearing rehabilitation. A systematic review and a coordinate-based meta-analysis were realized about the morphological brain changes highlighted by MRI in severe to profound hearing loss, congenital and acquired before or after language onset. 25 papers were included in our review, concerning more than 400 deaf subjects, most of them presenting prelingual deafness. The most consistent finding is a volumetric decrease in gray matter around bilateral auditory cortex. This change was confirmed by the coordinate-based meta-analysis which shows three converging clusters in this region. The visual areas of deaf children is also significantly impacted, with a decrease of the volume of both gray and white matters. Finally, deafness is responsible of a gray matter increase within the cerebellum, especially at the right side. These results are largely discussed and compared with those from deaf animal models and blind humans, which demonstrate for example a much more consistent gray matter decrease along their respective primary sensory pathway. In human deafness, a lot of other factors than deafness could interact on the brain plasticity. One of the most important is the use of sign language and its age of acquisition, which induce among others changes within the hand motor region and the visual cortex. But other confounding factors exist which have been too little considered in the current literature, such as the etiology of the hearing impairment, the speech-reading ability, the hearing aid use, the frequent associated vestibular dysfunction or neurocognitive impairment. Another important weakness highlighted by this review concern the lack of papers about postlingual deafness, whereas it represents most of the deaf population. Further studies are needed to better understand these issues, and finally try to improve deafness rehabilitation.

Structural changes to primary visual cortex in the congenital absence of cone input in achromatopsia

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Anatomy of primary visual cortex (V1) assessed with surface-based morphmetry in those with congenital achromatopsia (ACHM).

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Reduction in cortical surface area in foveal, parafoveal and paracentral representations of V1 in those with ACHM.

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In ACHM a localized thickening in the area of V1 that represents the region of retina occupied solely by cones.

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V1 changes in ACHM may limit its ability to take on normal properties if retinal function were to be restored.

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Early intervention, before the development plastic period is over, may offer better restoration of vision in ACHM.

Autosomal recessive Achromatopsia (ACHM) is a rare inherited disorder associated with dysfunctional cone photoreceptors resulting in a congenital absence of cone input to visual cortex. This might lead to distinct changes in cortical architecture with a negative impact on the success of gene augmentation therapies. To investigate the status of the visual cortex in these patients, we performed a multi-centre study focusing on the cortical structure of regions that normally receive predominantly cone input. Using high-resolution T1-weighted MRI scans and surface-based morphometry, we compared cortical thickness, surface area and grey matter volume in foveal, parafoveal and paracentral representations of primary visual cortex in 15 individuals with ACHM and 42 normally sighted, healthy controls (HC). In ACHM, surface area was reduced in all tested representations, while thickening of the cortex was found highly localized to the most central representation. These results were comparable to more widespread changes in brain structure reported in congenitally blind individuals, suggesting similar developmental processes, i.e., irrespective of the underlying cause and extent of vision loss. The cortical differences we report here could limit the success of treatment of ACHM in adulthood. Interventions earlier in life when cortical structure is not different from normal would likely offer better visual outcomes for those with ACHM.

Defining neuroplasticity

Neuroplasticity, i.e., the modifiability of the brain, is different in development and adulthood. The first includes changes in: (i) neurogenesis and control of neuron number; (ii) neuronal migration; (iii) differentiation of the somato-dendritic and axonal phenotypes; (iv) formation of connections; (v) cytoarchitectonic differentiation. These changes are often interrelated and can lead to: (vi) system-wide modifications of brain structure as well as to (vii) acquisition of specific functions such as ocular dominance or language. Myelination appears to be plastic both in development and adulthood, at least, in rodents. Adult neuroplasticity is limited, and is mainly expressed as changes in the strength of excitatory and inhibitory synapses while the attempts to regenerate connections have met with limited success. The outcomes of neuroplasticity are not necessarily adaptive, but can also be the cause of neurological and psychiatric pathologies.

The functional characterization of callosal connections

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The functional characterization of callosal connections is informed by anatomical data.

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Callosal connections play a conditional driving role depending on the brain state and behavioral demands.

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Callosal connections play a modulatory function, in addition to a driving role.

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The corpus callosum participates in learning and interhemispheric transfer of sensorimotor habits.

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The corpus callosum contributes to language processing and cognitive functions.

The brain operates through the synaptic interaction of distant neurons within flexible, often heterogeneous, distributed systems. Histological studies have detailed the connections between distant neurons, but their functional characterization deserves further exploration. Studies performed on the corpus callosum in animals and humans are unique in that they capitalize on results obtained from several neuroscience disciplines. Such data inspire a new interpretation of the function of callosal connections and delineate a novel road map, thus paving the way toward a general theory of cortico-cortical connectivity. Here we suggest that callosal axons can drive their post-synaptic targets preferentially when coupled to other inputs endowing the cortical network with a high degree of conditionality. This might depend on several factors, such as their pattern of convergence-divergence, the excitatory and inhibitory operation mode, the range of conduction velocities, the variety of homotopic and heterotopic projections and, finally, the state-dependency of their firing. We propose that, in addition to direct stimulation of post-synaptic targets, callosal axons often play a conditional driving or modulatory role, which depends on task contingencies, as documented by several recent studies.

Structural Differences Across Multiple Visual Cortical Regions in the Absence of Cone Function in Congenital Achromatopsia

Most individuals with congenital achromatopsia (ACHM) carry mutations that affect the retinal phototransduction pathway of cone photoreceptors, fundamental to both high acuity vision and colour perception. As the central fovea is occupied solely by cones, achromats have an absence of retinal input to the visual cortex and a small central area of blindness. Additionally, those with complete ACHM have no colour perception, and colour processing regions of the ventral cortex also lack typical chromatic signals from the cones. This study examined the cortical morphology (grey matter volume, cortical thickness, and cortical surface area) of multiple visual cortical regions in ACHM (n = 15) compared to normally sighted controls (n = 42) to determine the cortical changes that are associated with the retinal characteristics of ACHM. Surface-based morphometry was applied to T1-weighted MRI in atlas-defined early, ventral and dorsal visual regions of interest. Reduced grey matter volume in V1, V2, V3, and V4 was found in ACHM compared to controls, driven by a reduction in cortical surface area as there was no significant reduction in cortical thickness. Cortical surface area (but not thickness) was reduced in a wide range of areas (V1, V2, V3, TO1, V4, and LO1). Reduction in early visual areas with large foveal representations (V1, V2, and V3) suggests that the lack of foveal input to the visual cortex was a major driving factor in morphological changes in ACHM. However, the significant reduction in ventral area V4 coupled with the lack of difference in dorsal areas V3a and V3b suggest that deprivation of chromatic signals to visual cortex in ACHM may also contribute to changes in cortical morphology. This research shows that the congenital lack of cone input to the visual cortex can lead to widespread structural changes across multiple visual areas.

Cortical Visual Impairments and Learning Disabilities

Medical advances in neonatology have improved the survival rate of premature infants, as well as children who are born under difficult neurological conditions. As a result, the prevalence of cerebral dysfunctions, whether minimal or more severe, is increasing in all industrialized countries and in some developing nations. Whereas in the past, ophthalmological diseases were considered principally responsible for severe visual impairment, today, all recent epidemiological studies show that the primary cause of blindness and severe visual impairment in children in industrialized countries is now neurological, with lesions acquired around the time of birth currently comprising the commonest contributor. The resulting cortical or cerebral visual impairments (CVIs) have long been ignored, or have been confused either with other ophthalmological disorders causing low vision, or with a range of learning disabilities. We present here the deleterious consequences that CVI can have upon learning and social interaction, and how these can be given behavioral labels without the underlying visual causes being considered. We discuss the need to train and inform clinicians in the identification and diagnosis of CVI, and how to distinguish the diagnosis of CVI from amongst other visual disorders, including the specific learning disorders. This is important because the range of approaches needed to enhance the development of children with CVI is specific to each child's unique visual needs, making incorrect labeling or diagnosis potentially detrimental to affected children because these needs are not met.

A quantitative analysis of the retinofugal projections in congenital and late-onset blindness

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Congenital (CB) and late blind (LB) affects the integrity brain visual structures.

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We measured the integrity of the retino-fugal system using structural MRI images.

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Optic nerve, optic tract, optic chiasm and LGN were reduced by 50 to 60% in CB and LB.

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There were no differences between CB and LB.

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In LB, optic nerve volume correlated negatively with blindness duration.

Vision loss early in life has dramatic consequences on the organization of the visual system and hence on structural plasticity of its remnant components. Most of the studies on the anatomical changes in the brain following visual deprivation have focused on the re-organization of the visual cortex and its afferent and efferent projections. In this study, we performed a quantitative analysis of the volume and size of the optic chiasm, optic nerve, optic tract and the lateral geniculate nucleus (LGN), the retino recipient thalamic nucleus. Analysis was carried out on structural T1-weighted MRIs from 22 congenitally blind (CB), 14 late blind (LB) and 29 age -and sex-matched sighted control (SC) subjects. We manually segmented the optic nerve, optic chiasm and optic tract, while LGN volumes were extracted using in-house software. We also measured voxel intensity of optic nerve, optic chiasm and optic tract. Mean volumes of the optic nerve, optic tract and optic chiasm were reduced by 50 to 60% in both CB and LB participants. No significant differences were found between the congenitally and late-onset blind participants for any of the measures. Our data further revealed reduced white matter voxel intensities in optic nerve, optic chiasm and optic tract in blind compared to sighted participants, suggesting decreased myelin content in the atrophied white matter. The LGN was reduced by 50% and 44% in CB and LB, respectively. In LB, optic nerve volume correlated negatively with the blindness duration index; no such correlation was found for optic chiasm, optic tract and LGN. The observation that despite the absence of visual input about half of the subcortical retinofugal projections are structurally preserved raises the question of their functional role. One possibility is that the surviving fibers play a role in the maintenance of circadian rhythms in the blind through the intrinsically photosensitive melanopsin-containing retinal ganglion cells.

Data-Driven Classification of Spectral Profiles Reveals Brain Region-Specific Plasticity in Blindness

Congenital blindness has been shown to result in behavioral adaptation and neuronal reorganization, but the underlying neuronal mechanisms are largely unknown. Brain rhythms are characteristic for anatomically defined brain regions and provide a putative mechanistic link to cognitive processes. In a novel approach, using magnetoencephalography resting state data of congenitally blind and sighted humans, deprivation-related changes in spectral profiles were mapped to the cortex using clustering and classification procedures. Altered spectral profiles in visual areas suggest changes in visual alpha-gamma band inhibitory-excitatory circuits. Remarkably, spectral profiles were also altered in auditory and right frontal areas showing increased power in theta-to-beta frequency bands in blind compared with sighted individuals, possibly related to adaptive auditory and higher cognitive processing. Moreover, occipital alpha correlated with microstructural white matter properties extending bilaterally across posterior parts of the brain. We provide evidence that visual deprivation selectively modulates spectral profiles, possibly reflecting structural and functional adaptation.

Mice with mutations in Trpm1, a gene in the locus of 15q13.3 microdeletion syndrome, display pronounced hyperactivity and decreased anxiety-like behavior

The 15q13.3 microdeletion syndrome is a genetic disorder characterized by a wide spectrum of psychiatric disorders that is caused by the deletion of a region containing 7 genes on chromosome 15 (MTMR10, FAN1, TRPM1, MIR211, KLF13, OTUD7A, and CHRNA7). The contribution of each gene in this syndrome has been studied using mutant mouse models, but no single mouse model recapitulates the whole spectrum of human 15q13.3 microdeletion syndrome. The behavior of Trpm1-/- mice has not been investigated in relation to 15q13.3 microdeletion syndrome due to the visual impairment in these mice, which may confound the results of behavioral tests involving vision. We were able to perform a comprehensive behavioral test battery using Trpm1 null mutant mice to investigate the role of Trpm1, which is thought to be expressed solely in the retina, in the central nervous system and to examine the relationship between TRPM1 and 15q13.3 microdeletion syndrome. Our data demonstrate that Trpm1-/- mice exhibit abnormal behaviors that may explain some phenotypes of 15q13.3 microdeletion syndrome, including reduced anxiety-like behavior, abnormal social interaction, attenuated fear memory, and the most prominent phenotype of Trpm1 mutant mice, hyperactivity. While the ON visual transduction pathway is impaired in Trpm1-/- mice, we did not detect compensatory high sensitivities for other sensory modalities. The pathway for visual impairment is the same between Trpm1-/- mice and mGluR6-/- mice, but hyperlocomotor activity has not been reported in mGluR6-/- mice. These data suggest that the phenotype of Trpm1-/- mice extends beyond that expected from visual impairment alone. Here, we provide the first evidence associating TRPM1 with impairment of cognitive function similar to that observed in phenotypes of 15q13.3 microdeletion syndrome.

The Effect of Congenital and Acquired Bilateral Anophthalmia on Brain Structure

The aim of this study was to compare the pattern of changes in brain structure resulting from congenital and acquired bilateral anophthalmia. Brain structure was investigated using 3T magnetic resonance imaging (MRI) in Oxford (congenital) or Manchester (acquired). T1-weighted structural and diffusion-weighted scans were acquired from people with anophthalmia and sighted control participants. Differences in grey matter between the groups were quantified using voxel-based morphometry and differences in white matter microstructure using tract-based spatial statistics. Quantification of optic nerve volume and cortical thickness in visual regions was also performed in all groups. The optic nerve was reduced in volume in both anophthalmic populations, but to a greater extent in the congenital group and anophthalmia acquired at an early age. A similar pattern was found for the white matter microstructure throughout the occipitotemporal regions of the brain, suggesting a greater reduction of integrity with increasing duration of anophthalmia. In contrast, grey matter volume changes differed between the two groups, with the acquired anophthalmia group showing a decrease in the calcarine sulcus, corresponding to the area that would have been peripheral primary visual cortex. In contrast, the acquired anophthalmia group showed a decrease in grey matter volume in the calcarine sulcus corresponding to the area that would have been peripheral primary visual cortex. There are both qualitative and quantitative differences in structural brain changes in congenital and acquired anophthalmia, indicating differential effects of development and degeneration.

Visual Cortex Alterations in Early and Late Blind Subjects During Tactile Perception

Involvement of visual cortex varies during tactile perception tasks in early blind (EB) and late blind (LB) human subjects. This study explored differences in sensory motor networks associated with tactile task in EB and LB subjects and between children and adolescents. A total of 40 EB subjects, 40 LB subjects, and 30 sighted controls were recruited in two subgroups: children (6-12 years) and adolescents (13-19 years). Data were acquired using a 3T MR scanner. Analyses of blood oxygen level dependent (BOLD), functional connectivity (FC), correlation, and post hoc test for multiple comparisons were carried out. Difference in BOLD activity was observed in EB and LB groups in visual cortex during tactile perception, with increased FC of visual with dorsal attention and sensory motor networks in EB. EB adolescents exhibited increased connectivity with default mode and salience networks when compared with LB. Functional results correlated with duration of training, suggestive of better performance in EB. Alteration in sensory and visual networks in EB and LB correlated with duration of tactile training. Age of onset of blindness has an effect in cross-modal reorganization of visual cortex in EB and multimodal in LB in children and adolescents.

Brain-Machine Interfaces to Assist the Blind

The loss or absence of vision is probably one of the most incapacitating events that can befall a human being. The importance of vision for humans is also reflected in brain anatomy as approximately one third of the human brain is devoted to vision. It is therefore unsurprising that throughout history many attempts have been undertaken to develop devices aiming at substituting for a missing visual capacity. In this review, we present two concepts that have been prevalent over the last two decades. The first concept is sensory substitution, which refers to the use of another sensory modality to perform a task that is normally primarily sub-served by the lost sense. The second concept is cross-modal plasticity, which occurs when loss of input in one sensory modality leads to reorganization in brain representation of other sensory modalities. Both phenomena are training-dependent. We also briefly describe the history of blindness from ancient times to modernity, and then proceed to address the means that have been used to help blind individuals, with an emphasis on modern technologies, invasive (various type of surgical implants) and non-invasive devices. With the advent of brain imaging, it has become possible to peer into the neural substrates of sensory substitution and highlight the magnitude of the plastic processes that lead to a rewired brain. Finally, we will address the important question of the value and practicality of the available technologies and future directions.

Structural brain plasticity induced by early blindness

It is well established that early blindness results in behavioural adaptations. While the functional effects of visual deprivation have been well researched, anatomical studies are scarce. The aim of this study was to investigate whole brain structural plasticity in a mouse model of congenital blindness. Volumetric analyses were conducted on high-resolution MRI images and histological sections from the same brains. These morphometric measurements were compared between anophthalmic and sighted ZRDBA mice obtained by breeding ZRDCT and DBA mice. Results from MRI analyses using the Multiple Automatically Generated Templates (MAGeT) method showed smaller volume for the primary visual cortex and superior colliculi in anophthalmic compared with sighted mice. Deformation-based morphometry revealed smaller volumes within the dorsal lateral geniculate nuclei and the lateral secondary visual cortex and larger volumes within olfactory areas, piriform cortex, orbital areas and the amygdala, in anophthalmic compared with sighted mice. Histological analyses revealed a larger volume for the amygdala and smaller volume for the superior colliculi, primary visual cortex and medial secondary visual cortex, in anophthalmic compared with sighted mice. The absence of superficial visual layers of the superior colliculus and the thinner cortical layer IV of the primary and secondary visual cortices may explain the smaller volume of these areas, although this was observed in a limited sample. The present study shows large-scale brain plasticity in a mouse model of congenital blindness. In addition, the congruence of MRI and histological findings support the use of MRI to investigate structural brain plasticity in the mouse.

Morphometric evaluation of cerebellar structures in late monocular blindness

BACKGROUND

Only a few studies have investigated structural and functional changes in monocular blind individuals. Our aim in this study was to segmentally investigate the cerebellar structures of subjects with late-onset monocular blindness (LMB) using a Voxel-based volumetric analysis system.

METHODS

The segmental volumetric values, cortical thickness, gray matter volumes and percentage ratios of the cerebellar lobules of individuals with LMB due to trauma and in healthy individuals with bilateral sight as the control group were calculated at the volBrain CERES 1.0 website ( https://volbrain.upv.es/ ) by using brain 3D fast spoiled gradient recall acquisition in steady-state (3D T1 FSPGR) MRI sequence images in our prospective study.

RESULTS

We studied 11 subjects with LMB (8 males/3 females) and 11 healthy control subjects (8 males/3 females). The mean age was 41.45 ± 14.15 and 40 ± 11.11 years, respectively (p > 0.05). The mean duration of the LMB status was 20.8 ± 11.2 years. Cerebellar lobule crus II volume and cerebellar lobule VIIB/VIIIA volume/percentage were higher in the LMB group, and mean cerebellar cortical thickness, cerebellar lobule VI-cerebellar lobule crus I-II cortical thickness, and cerebellar lobule VI gray matter volume values were lower in the LMB group (p < 0.05).

CONCLUSION

In this study, cerebellar lobule VIIB/VIIIA volume/total percent ratio, cerebellar cortical thickness and cerebellar gray matter volume in the LMB group were found to be different from the control group. To our knowledge, this is the first study to report cerebellar anatomical changes in patients with LMB.

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.

The Effect of Onset Age of Visual Deprivation on Visual Cortex Surface Area Across-Species

Blindness early in life induces permanent alterations in brain anatomy, including reduced surface area of primary visual cortex (V1). Bilateral enucleation early in development causes greater reductions in primary visual cortex surface area than at later times. However, the time at which cortical surface area expansion is no longer sensitive to enucleation is not clearly established, despite being an important milestone for cortical development. Using histological and MRI techniques, we investigated how reductions in the surface area of V1 depends on the timing of blindness onset in rats, ferrets and humans. To compare data across species, we translated ages of all species to a common neuro-developmental event-time (ET) scale. Consistently, blindness during early cortical expansion induced large (~40%) reductions in V1 surface area, in rats and ferrets, while blindness occurring later had diminishing effects. Longitudinal measurements on ferrets confirmed that early enucleation disrupted cortical expansion, rather than inducing enhanced pruning. We modeled the ET associated with the conclusion of the effect of blindness on surface area at maturity (ETc), relative to the normal conclusion of visual cortex surface area expansion, (ETdev). A final analysis combining our data with extant published data confirmed that ETc occurred well before ETdev.

Available Sensory Input Determines Motor Performance and Strategy in Early Blind and Sighted Short-Tailed Opossums

The early loss of vision results in a reorganized neocortex, affecting areas of the brain that process both the spared and lost senses, and leads to heightened abilities on discrimination tasks involving the spared senses. Here, we used performance measures and machine learning algorithms that quantify behavioral strategy to determine if and how early vision loss alters adaptive sensorimotor behavior. We tested opossums on a motor task involving somatosensation and found that early blind animals had increased limb placement accuracy compared with sighted controls, while showing similarities in crossing strategy. However, increased reliance on tactile inputs in early blind animals resulted in greater deficits in limb placement and behavioral flexibility when the whiskers were trimmed. These data show that compensatory cross-modal plasticity extends beyond sensory discrimination tasks to motor tasks involving the spared senses and highlights the importance of whiskers in guiding forelimb control.

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Early blind opossums outperform sighted controls during ladder rung walking

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Whisker trimming causes forelimb accuracy deficits in blind and sighted opossums

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Whisker trimming, but not the loss of vision, impacts stereotypical movements

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Both groups adopt conservative approaches to ladder crossing after whisker trimming

Normal Olfactory Functional Connectivity Despite Lifelong Absence of Olfactory Experiences

Congenital blindness is associated with atypical morphology and functional connectivity within and from visual cortical regions; changes that are hypothesized to originate from a lifelong absence of visual input and could be regarded as a general (re) organization principle of sensory cortices. Challenging this is the fact that individuals with congenital anosmia (lifelong olfactory sensory loss) display little to no morphological changes in the primary olfactory cortex. To determine whether olfactory input from birth is essential to establish and maintain normal functional connectivity in olfactory processing regions, akin to the visual system, we assessed differences in functional connectivity within the olfactory cortex between individuals with congenital anosmia (n = 33) and matched controls (n = 33). Specifically, we assessed differences in connectivity between core olfactory processing regions as well as differences in regional homogeneity and homotopic connectivity within the primary olfactory cortex. In contrast to congenital blindness, none of the analyses indicated atypical connectivity in individuals with congenital anosmia. In fact, post-hoc Bayesian analysis provided support for an absence of group differences. These results suggest that a lifelong absence of olfactory experience has a limited impact on the functional connectivity in the olfactory cortex, a finding that indicates a clear difference between sensory modalities in how sensory cortical regions develop.

Spatial Competence and Brain Plasticity in Congenital Blindness via Sensory Substitution Devices

In congenital blindness (CB), tactile, and auditory information can be reinterpreted by the brain to compensate for visual information through mechanisms of brain plasticity triggered by training. Visual deprivation does not cause a cognitive spatial deficit since blind people are able to acquire spatial knowledge about the environment. However, this spatial competence takes longer to achieve but is eventually reached through training-induced plasticity. Congenitally blind individuals can further improve their spatial skills with the extensive use of sensory substitution devices (SSDs), either visual-to-tactile or visual-to-auditory. Using a combination of functional and anatomical neuroimaging techniques, our recent work has demonstrated the impact of spatial training with both visual to tactile and visual to auditory SSDs on brain plasticity, cortical processing, and the achievement of certain forms of spatial competence. The comparison of performances between CB and sighted people using several different sensory substitution devices in perceptual and sensory-motor tasks uncovered the striking ability of the brain to rewire itself during perceptual learning and to interpret novel sensory information even during adulthood. We discuss here the implications of these findings for helping blind people in navigation tasks and to increase their accessibility to both real and virtual environments.

Altered Temporal Dynamic Intrinsic Brain Activity in Late Blindness

Previous neuroimaging studies demonstrated that visual deprivation triggers significant crossmodal plasticity in the functional and structural architecture of the brain. However, prior neuroimaging studies focused on the static brain activity in blindness. It remains unknown whether alterations of dynamic intrinsic brain activity occur in late blindness (LB). This study investigated dynamic intrinsic brain activity changes in individuals with late blindness by assessing the dynamic amplitude of low-frequency fluctuations (dALFFs) using sliding-window analyses. Forty-one cases of late blindness (LB) (29 males and 12 females, mean age: 39.70 ± 12.66 years) and 48 sighted controls (SCs) (17 males and 31 females, mean age: 43.23 ± 13.40 years) closely matched in age, sex, and education level were enrolled in this study. The dALFF with sliding-window analyses was used to compare the difference in dynamic intrinsic brain activity between the two groups. Compared with SCs, individuals with LB exhibited significantly lower dALFF values in the bilateral lingual gyrus (LING)/calcarine (CAL) and left thalamus (THA). LB cases also showed considerably decreased dFC values between the bilateral LING/CAL and the left middle frontal gyrus (MFG) and between the left THA and the right LING/cerebelum_6 (CER) (two-tailed, voxel-level P < 0.01, Gaussian random field (GRF) correction, cluster-level P < 0.05). Our study demonstrated that LB individuals showed lower-temporal variability of dALFF in the visual cortices and thalamus, suggesting lower flexibility of visual thalamocortical activity, which might reflect impaired visual processing in LB individuals. These findings indicate that abnormal dynamic intrinsic brain activity might be involved in the neurophysiological mechanisms of LB.

Efficiency of Sensory Substitution Devices Alone and in Combination With Self-Motion for Spatial Navigation in Sighted and Visually Impaired

Human adults can optimally combine vision with self-motion to facilitate navigation. In the absence of visual input (e.g., dark environments and visual impairments), sensory substitution devices (SSDs), such as The vOICe or BrainPort, which translate visual information into auditory or tactile information, could be used to increase navigation precision when integrated together or with self-motion. In Experiment 1, we compared and assessed together The vOICe and BrainPort in aerial maps task performed by a group of sighted participants. In Experiment 2, we examined whether sighted individuals and a group of visually impaired (VI) individuals could benefit from using The vOICe, with and without self-motion, to accurately navigate a three-dimensional (3D) environment. In both studies, 3D motion tracking data were used to determine the level of precision with which participants performed two different tasks (an egocentric and an allocentric task) and three different conditions (two unisensory conditions and one multisensory condition). In Experiment 1, we found no benefit of using the devices together. In Experiment 2, the sighted performance during The vOICe was almost as good as that for self-motion despite a short training period, although we found no benefit (reduction in variability) of using The vOICe and self-motion in combination compared to the two in isolation. In contrast, the group of VI participants did benefit from combining The vOICe and self-motion despite the low number of trials. Finally, while both groups became more accurate in their use of The vOICe with increased trials, only the VI group showed an increased level of accuracy in the combined condition. Our findings highlight how exploiting non-visual multisensory integration to develop new assistive technologies could be key to help blind and VI persons, especially due to their difficulty in attaining allocentric information.

Compensatory Cross-Modal Plasticity Persists After Sight Restoration

Sensory deprivation prompts extensive structural and functional reorganizations of the cortex resulting in the occupation of space for the lost sense by the intact sensory systems. This process, known as cross-modal plasticity, has been widely studied in individuals with vision or hearing loss. However, little is known on the neuroplastic changes in restoring the deprived sense. Some reports consider the cross-modal functionality maladaptive to the return of the original sense, and others view this as a critical process in maintaining the neurons of the deprived sense active and operational. These controversial views have been challenged in both auditory and vision restoration reports for decades. Recently with the approval of Luxturna as the first retinal gene therapy (GT) drug to reverse blindness, there is a renewed interest for the crucial role of cross-modal plasticity on sight restoration. Employing a battery of task and resting state functional magnetic resonance imaging (rsfMRI), in comparison to a group of sighted controls, we tracked the functional changes in response to auditory and visual stimuli and at rest, in a group of patients with biallelic mutations in the RPE65 gene (“RPE65 patients”) before and 3 years after GT. While the sighted controls did not present any evidence for auditory cross-modal plasticity, robust responses to the auditory stimuli were found in occipital cortex of the RPE65 patients overlapping visual responses and significantly elevated 3 years after GT. The rsfMRI results showed significant connectivity between the auditory and visual areas for both groups albeit attenuated in patients at baseline but enhanced 3 years after GT. Taken together, these findings demonstrate that (1) RPE65 patients present with an auditory cross-modal component; (2) visual and non-visual responses of the visual cortex are considerably enhanced after vision restoration; and (3) auditory cross-modal functions did not adversely affect the success of vision restitution. We hypothesize that following GT, to meet the demand for the newly established retinal signals, remaining or dormant visual neurons are revived or unmasked for greater participation. These neurons or a subset of these neurons respond to both the visual and non-visual demands and further strengthen connectivity between the auditory and visual cortices.

Impaired interhemispheric synchrony in late blindness

Background

Several neuroimaging studies demonstrated that visual deprivation led to significant cross-modal plasticity in the brain’s functional and anatomical architecture.

Purpose

To investigate the pattern of the interhemispheric functional connectivity in individuals with late blindness using the voxel-mirrored homotopic connectivity (VMHC) and seed-based functional connectivity (FC) methods.

Material and Methods

Forty-four individuals with late blindness (22 men, 22 women; mean age = 39.88 ± 12.84 years) and 55 sighted control individuals (35 men, 20 women; mean age = 43.13 ± 13.98 years)—closely matched for age, sex, and education—underwent resting-state magnetic resonance imaging scans. The VMHC and seed-based FC methods were applied to assess interhemispheric coordination in a voxel-wise manner.

Results

Compared with the sighted control groups, the late blindness groups showed decreased VMHC values in the bilateral cuneus/calcarine/lingual gyrus (CUN/CAL/LING) (BA 17/18/19) (voxel level: P < 0.001, Gaussian random field [GRF] correction, cluster level: P < 0.005). Meanwhile, for seed-based FC analysis, compared with the sighted control group, the late blindness group showed a decreased FC between the right lower VMHC regions and the bilateral CUN/LING/CAL/precuneus (PreCUN)/left middle occipital gyrus (MOG) (BA 18/19/30/31) and left precentral gyrus (PreCG) and postcentral gyrus (PostCG) (BA 2/3/4/6). The late blindness group showed a decreased FC between the left lower VMHC regions and the bilateral CUN/LING/CAL/PreCUN (BA 18/19/31) and left PreCG and PostCG (BA 2/3/4/6) relative to the sighted control group (voxel level: P < 0.001, GRF correction, cluster level: P < 0.005). Moreover, a negative correlation was observed between the duration of blindness and VMHC values in the bilateral CUN/CAL/LING (r = −0.393, P = 0.008) in individuals with late blindness.

Conclusion

Our results indicated that late blindness induced substantial impairment of interhemispheric coordination in the visual cortex. This might reflect impaired visual fusion, visual recognition function, and top-down modulations in individuals with late blindness.

Neural Networks Mediating Perceptual Learning in Congenital Blindness

Despite the fact that complete visual deprivation leads to volumetric reductions in brain structures associated with spatial learning, blind individuals are still able to navigate. The neural structures involved in this function are not fully understood. Our study aims to correlate the performance of congenitally blind individuals (CB) and blindfolded sighted controls (SC) in a life-size obstacle-course using a visual-to-tactile sensory substitution device, with the size of brain structures (voxel based morphometry-VBM-) measured through structural magnetic resonance Imaging (MRI). VBM was used to extract grey matter volumes within several a-priori defined brain regions in all participants. Principal component analysis was utilized to group brain regions in factors and orthogonalize brain volumes. Regression analyses were then performed to link learning abilities to these factors. We found that (1) both CB and SC were able to learn to detect and avoid obstacles; (2) their learning rates for obstacle detection and avoidance correlated significantly with the volume of brain structures known to be involved in spatial skills. There is a similar relation between regions of the dorsal stream network and avoidance for both SC and CB whereas for detection, SC rely more on medial temporal lobe structures and CB on sensorimotor areas.

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.

Differences in the major fiber-tracts of people with congenital and acquired blindness

In order to better understand how our visual system processes information, we must understand the underlying brain connectivity architecture, and how it can get reorganized under visual deprivation. The full extent to which visual development and visual loss affect connectivity is not well known. To investigate the effect of the onset of blindness on structural connectivity both at the whole-brain voxel-wise level and at the level of all major white-matter tracts, we applied two complementary Diffusion-Tension Imaging (DTI) methods, TBSS and AFQ. Diffusion-weighted brain images were collected from three groups of participants: congenitally blind (CB), acquired blind (AB), and fully sighted controls. The differences between these groups were evaluated on a voxel-wise scale with Tract-Based Spatial Statistics (TBSS) method, and on larger-scale with Automated Fiber Quantification (AFQ), a method that allows for between-group comparisons at the level of the major fiber tracts. TBSS revealed that both blind groups tended to have higher FA than sighted controls in the central structures of the brain. AFQ revealed that, where the three groups differed, congenitally blind participants tended to be more similar to sighted controls than to those participants who had acquired blindness later in life. These differences were specifically manifested in the left uncinated fasciculus, the right corticospinal fasciculus, and the left superior longitudinal fasciculus, areas broadly associated with a range of higher-level cognitive systems.