Early blindness results in abnormal corticocortical and thalamocortical connections

Sound suppresses earliest visual cortical processing after sight recovery in congenitally blind humans

Neuroscientific research has consistently shown more extensive non-visual activity in the visual cortex of congenitally blind humans compared to sighted controls; a phenomenon known as crossmodal plasticity. Whether or not crossmodal activation of the visual cortex retracts if sight can be restored is still unknown. The present study, involving a rare group of sight-recovery individuals who were born pattern vision blind, employed visual event-related potentials to investigate persisting crossmodal modulation of the initial visual cortical processing stages. Here we report that the earliest, stimulus-driven retinotopic visual cortical activity (<100 ms) was suppressed in a spatially specific manner in sight-recovery individuals when concomitant sounds accompanied visual stimulation. In contrast, sounds did not modulate the earliest visual cortical response in two groups of typically sighted controls, nor in a third control group of sight-recovery individuals who had suffered a transient phase of later (rather than congenital) visual impairment. These results provide strong evidence for persisting crossmodal activity in the visual cortex after sight recovery following a period of congenital visual deprivation. Based on the time course of this modulation, we speculate on a role of exuberant crossmodal thalamic input which may arise during a sensitive phase of brain development.

Spoken language processing activates the primary visual cortex

Primary visual cortex (V1) is generally thought of as a low-level sensory area that primarily processes basic visual features. Although there is evidence for multisensory effects on its activity, these are typically found for the processing of simple sounds and their properties, for example spatially or temporally-congruent simple sounds. However, in congenitally blind individuals, V1 is involved in language processing, with no evidence of major changes in anatomical connectivity that could explain this seemingly drastic functional change. This is at odds with current accounts of neural plasticity, which emphasize the role of connectivity and conserved function in determining a neural tissue's role even after atypical early experiences. To reconcile what appears to be unprecedented functional reorganization with known accounts of plasticity limitations, we tested whether V1's multisensory roles include responses to spoken language in sighted individuals. Using fMRI, we found that V1 in normally sighted individuals was indeed activated by comprehensible spoken sentences as compared to an incomprehensible reversed speech control condition, and more strongly so in the left compared to the right hemisphere. Activation in V1 for language was also significant and comparable for abstract and concrete words, suggesting it was not driven by visual imagery. Last, this activation did not stem from increased attention to the auditory onset of words, nor was it correlated with attentional arousal ratings, making general attention accounts an unlikely explanation. Together these findings suggest that V1 responds to spoken language even in sighted individuals, reflecting the binding of multisensory high-level signals, potentially to predict visual input. This capability might be the basis for the strong V1 language activation observed in people born blind, re-affirming the notion that plasticity is guided by pre-existing connectivity and abilities in the typically developed brain.

Early retinal deprivation crossmodally alters nascent subplate circuits and activity in the auditory cortex during the precritical period

Sensory perturbation in one modality results in the adaptive reorganization of neural pathways within the spared modalities, a phenomenon known as "crossmodal plasticity," which has been examined during or after the classic "critical period." Because peripheral perturbations can alter the auditory cortex (ACX) activity and functional connectivity of the ACX subplate neurons (SPNs) even before the critical period, called the precritical period, we investigated if retinal deprivation at birth crossmodally alters the ACX activity and SPN circuits during the precritical period. We deprived newborn mice of visual inputs after birth by performing bilateral enucleation. We performed in vivo widefield imaging in the ACX of awake pups during the first two postnatal weeks to investigate cortical activity. We found that enucleation alters spontaneous and sound-evoked activities in the ACX in an age-dependent manner. Next, we performed whole-cell patch clamp recording combined with laser scanning photostimulation in ACX slices to investigate circuit changes in SPNs. We found that enucleation alters the intracortical inhibitory circuits impinging on SPNs, shifting the excitation-inhibition balance toward excitation and this shift persists after ear opening. Together, our results indicate that crossmodal functional changes exist in the developing sensory cortices at early ages before the onset of the classic critical period.

Microglia enable cross-modal plasticity by removing inhibitory synapses

Cross-modal plasticity is the repurposing of brain regions associated with deprived sensory inputs to improve the capacity of other sensory modalities. The functional mechanisms of cross-modal plasticity can indicate how the brain recovers from various forms of injury and how different sensory modalities are integrated. Here, we demonstrate that rewiring of the microglia-mediated local circuit synapse is crucial for cross-modal plasticity induced by visual deprivation (monocular deprivation [MD]). MD relieves the usual inhibition of functional connectivity between the somatosensory cortex and secondary lateral visual cortex (V2L). This results in enhanced excitatory responses in V2L neurons during whisker stimulation and a greater capacity for vibrissae sensory discrimination. The enhanced cross-modal response is mediated by selective removal of inhibitory synapse terminals on pyramidal neurons by the microglia in the V2L via matrix metalloproteinase 9 signaling. Our results provide insights into how cortical circuits integrate different inputs to functionally compensate for neuronal damage.

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.

Early retinal deprivation crossmodally alters nascent subplate circuits and activity in the auditory cortex during the precritical period

Sensory perturbation in one modality results in adaptive reorganization of neural pathways within the spared modalities, a phenomenon known as "crossmodal plasticity", which has been examined during or after the classic 'critical period'. Because peripheral perturbations can alter auditory cortex (ACX) activity and functional connectivity of the ACX subplate neurons (SPNs) even before the classic critical period, called the precritical period, we investigated if retinal deprivation at birth crossmodally alters ACX activity and SPN circuits during the precritical period. We deprived newborn mice of visual inputs after birth by performing bilateral enucleation. We performed in vivo imaging in the ACX of awake pups during the first two postnatal weeks to investigate cortical activity. We found that enucleation alters spontaneous and sound-evoked activity in the ACX in an age-dependent manner. Next, we performed whole-cell patch clamp recording combined with laser scanning photostimulation in ACX slices to investigate circuit changes in SPNs. We found that enucleation alters the intracortical inhibitory circuits impinging on SPNs shifting the excitation-inhibition balance towards excitation and this shift persists after ear opening. Together, our results indicate that crossmodal functional changes exist in the developing sensory cortices at early ages before the onset of the classic critical period.

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.

Altered time-varying local spontaneous brain activity pattern in patients with high myopia: a dynamic amplitude of low-frequency fluctuations study

PURPOSE

To investigate the abnormal time-varying local spontaneous brain activity in patients with high myopia (HM) on the basis of the dynamic amplitude of low-frequency fluctuations (dALFF) approach.

METHODS

Age and gender matching were performed based on resting-state functional magnetic resonance imaging data from 86 HM patients and 87 healthy controls (HCs). Local spontaneous brain activities were evaluated using the time-varying dALFF method. Support vector machine combined with the radial basis function kernel was used for pattern classification analysis.

RESULTS

Inter-group comparison between HCs and HM patients has demonstrated that dALFF variability in the left inferior frontal gyrus (orbital part), left lingual gyrus, right anterior cingulate and paracingulate gyri, and right calcarine fissure and surrounding cortex was decreased in HM patients, while increased in the left thalamus, left paracentral lobule, and left inferior parietal (except supramarginal and angular gyri). Pattern classification between HM patients and HCs displayed a classification accuracy of 85.5%.

CONCLUSION

In this study, the findings mentioned above have suggested the association between local brain activities of HM patients and abnormal variability in brain regions performing visual sensorimotor and attentional control functions. Several useful information has been provided to elucidate the mechanism-related alterations of the myopic nervous system. In addition, the significant role of abnormal dALFF variability has been highlighted to achieve an in-depth comprehension of the pathological alterations and neuroimaging mechanisms in the field of HM.

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.

Altered functional connectivity of the thalamus in primary angle-closure glaucoma patients: A resting-state fMRI study

Background and objectives

Glaucoma is one of the leading irreversible causes of blindness worldwide, and previous studies have shown that there is abnormal functional connectivity (FC) in the visual cortex of glaucoma patients. The thalamus is a relay nucleus for visual signals; however, it is not yet clear how the FC of the thalamus is altered in glaucoma. This study investigated the alterations in thalamic FC in patients with primary angle-closure glaucoma (PACG) by using resting-state functional MRI (rs-fMRI). We hypothesized that PACG patients have abnormal FC between the thalamus and visual as well as extravisual brain regions.

Methods

Clinically confirmed PACG patients and age- and gender-matched healthy controls (HCs) were evaluated by T1 anatomical and functional MRI on a 3 T scanner. Thirty-four PACG patients and 33 HCs were included in the rs-fMRI analysis. All PACG patients underwent complete ophthalmological examinations; included retinal nerve fiber layer thickness (RNFLT), intraocular pressure (IOP), average cup-to-disc ratio (A-C/D), and vertical cup-to-disc ratio (V-C/D). After the MRI data were preprocessed, the bilateral thalamus was chosen as the seed point; and the differences in resting-state FC between groups were evaluated. The brain regions that significantly differed between PACG patients and HCs were identified, and the correlations were then evaluated between the FC coefficients of these regions and clinical variables.

Results

Compared with the HCs, the PACG patients showed decreased FC between the bilateral thalamus and right transverse temporal gyrus, between the bilateral thalamus and left anterior cingulate cortex, and between the left thalamus and left insula. Concurrently, increased FC was found between the bilateral thalamus and left superior frontal gyrus in PACG patients. The FC between the bilateral thalamus and left superior frontal gyrus was positively correlated with RNFLT and negatively correlated with the A-C/D and V-C/D. The FC between the left thalamus and left insula was negatively correlated with IOP.

Conclusion

Extensive abnormal resting-state functional connections between the thalamus and visual and extravisual brain areas were found in PACG patients, and there were certain correlations with clinical variables, suggesting that abnormal thalamic FC plays an important role in the progression of PACG.

Cross-Modal Plasticity in Brains Deprived of Visual Input Before Vision

Unimodal sensory loss leads to structural and functional changes in both deprived and nondeprived brain circuits. This process is broadly known as cross-modal plasticity. The evidence available indicates that cross-modal changes underlie the enhanced performances of the spared sensory modalities in deprived subjects. Sensory experience is a fundamental driver of cross-modal plasticity, yet there is evidence from early-visually deprived models supporting an additional role for experience-independent factors. These experience-independent factors are expected to act early in development and constrain neuronal plasticity at later stages. Here we review the cross-modal adaptations elicited by congenital or induced visual deprivation prior to vision. In most of these studies, cross-modal adaptations have been addressed at the structural and functional levels. Here, we also appraise recent data regarding behavioral performance in early-visually deprived models. However, further research is needed to explore how circuit reorganization affects their function and what brings about enhanced behavioral performance.

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.

Magnetic resonance imaging and diffusion tensor imaging reconstruction of connectomes in a macropod, the quokka (Setonix brachyurus)

The diversity of the diprotodontids provides an excellent opportunity to study how a basic marsupial cortical plan has been modified for the needs of the mammals living in different habitats. Very little is known about the connections of the cerebral cortex with the deep brain structures (basal ganglia and thalamus) in this evolutionarily significant group of mammals. In this study, we performed mapping of brain regions and connections in a diprotodontid marsupial from data obtained from an excised brain scanned in high-field (9.4 T) microstructural magnetic resonance imaging (MRI) instrument. The analysis was based on two MRI methodologies. First, high-resolution structural scans were used to map MRI visible brain regions from T1w and T2w images. Second, extensive diffusion tensor imaging (DTI) data were obtained to elucidate connectivity between brain areas using deterministic diffusion tracking of neuronal brain fibers. From the data, we were able to identify corticostriate connections between the frontal association and dorsomedial isocortex and the head of the caudate, and between the lateral somatosensory cortex and the putamen. We were also able to follow the olfactory and limbic connections by tracing fibers in the fornix, cingulum, intrabulbar part of the anterior commissure, and lateral olfactory tract. There was segregation of fibers in the anterior commissure such that olfactory connections passed through the rostroventral part and successively more dorsal cortical areas connected through more dorsal parts of the commissure. Our findings confirm a common pattern of cortical connectivity in therian mammals, even where brain expansion has occurred independently in diverse groups.

Altered Regional Homogeneity in Patients With Congenital Blindness: A Resting-State Functional Magnetic Resonance Imaging Study

In patients with congenital blindness (CB), the lack of any visual experience may affect brain development resulting in functional, structural, or even psychological changes. Few studies to date have addressed or focused on the synchronicity of regional brain activity in patients with CB. Our study aimed to investigate regional brain activity in patients with CB in a resting state and try to explain the possible causes and effects of any anomalies. Twenty-three CB patients and 23 healthy control (HC) volunteers agreed to undergo resting state functional magnetic resonance imaging (fMRI) scans. After the fMRI data were preprocessed, regional homogeneity (ReHo) analysis was conducted to assess the differences in brain activity synchronicity between the two groups. Receiver operating characteristic (ROC) curve analysis was used to explore whether the brain areas with statistically significant ReHo differences have diagnostic and identification values for CB. All CB patients were also required to complete the Hospital Anxiety and Depression Scale (HADS) to evaluate their anxiety and depression levels. The results showed that in CB patients mean ReHo values were significantly lower than in HCs in the right orbital part of the middle frontal gyrus (MFGorb), bilateral middle occipital gyrus (MOG), and the right dorsolateral superior frontal gyrus (SFGdl), but significantly higher in the left paracentral lobule (PCL), right insula and bilateral thalamus. The ReHo value of MFGorb showed a negative linear correlation with both the anxiety score and the depression score of the HADS. ROC curve analysis revealed that the mean ReHo values which differed significantly between the groups have excellent diagnostic accuracy for CB (especially in the left PCL and right SFGdl regions). Patients with CB show abnormalities of ReHo values in several specific brain regions, suggesting potential regional structural changes, functional reorganization, or even psychological effects in these patients. FMRI ReHo analysis may find use as an objective method to confirm CB for medical or legal purposes.

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.

Structural and white matter changes associated with duration of Braille education in early and late blind children

In early (EB) and late blind (LB) children, vision deprivation produces cross-modal plasticity in the visual cortex. The progression of structural- and tract-based spatial statistics changes in the visual cortex in EB and LB, as well as their impact on global cognition, have yet to be investigated. The purpose of this study was to determine the cortical thickness (CT), gyrification index (GI), and white matter (WM) integrity in EB and LB children, as well as their association to the duration of blindness and education. Structural and diffusion tensor imaging data were acquired in a 3T magnetic resonance imaging in EB and LB children (n = 40 each) and 30 sighted controls (SCs) and processed using CAT12 toolbox and FSL software. Two sample t-test was used for group analyses with P < 0.05 (false discovery rate-corrected). Increased CT in visual, sensory-motor, and auditory areas, and GI in bilateral visual cortex was observed in EB children. In LB children, the right visual cortex, anterior-cingulate, sensorimotor, and auditory areas showed increased GI. Structural- and tract-based spatial statistics changes were observed in anterior visual pathway, thalamo-cortical, and corticospinal tracts, and were correlated with education onset and global cognition in EB children. Reduced impairment in WM, increased CT and GI and its correlation with global cognitive functions in visually impaired children suggests cross-modal plasticity due to adaptive compensatory mechanism (as compared to SCs). Reduced CT and increased FA in thalamo-cortical areas in EB suggest synaptic pruning and alteration in WM integrity. In the visual cortical pathway, higher education and the development of blindness modify the morphology of brain areas and influence the probabilistic tractography in EB rather than LB.

Developmental plasticity of texture discrimination following early vision loss in the marsupial Monodelphis domestica

Behavioral strategies that depend on sensory information are not immutable; rather they can be shaped by the specific sensory context in which animals develop. This behavioral plasticity depends on the remarkable capacity of the brain to reorganize in response to alterations in the sensory environment, particularly when changes in sensory input occur at an early age. To study this phenomenon, we utilize the short-tailed opossum, a marsupial that has been a valuable animal model to study developmental plasticity due to the extremely immature state of its nervous system at birth. Previous studies in opossums have demonstrated that removal of retinal inputs early in development results in profound alterations to cortical connectivity and functional organization of visual and somatosensory cortex; however, behavioral consequences of this plasticity are not well understood. We trained early blind and sighted control opossums to perform a two-alternative forced choice texture discrimination task. Whisker trimming caused an acute deficit in discrimination accuracy for both groups, indicating the use of a primarily whisker-based strategy to guide choices based on tactile cues. Mystacial whiskers were important for performance in both groups; however, genal whiskers only contributed to behavioral performance in early blind animals. Early blind opossums significantly outperformed their sighted counterparts in discrimination accuracy, with discrimination thresholds that were lower by ∼75 μm. Our results support behavioral compensation following early blindness using tactile inputs, especially the whisker system.

Aberrant development of excitatory circuits to inhibitory neurons in the primary visual cortex after neonatal binocular enucleation

The development of GABAergic interneurons is important for the functional maturation of cortical circuits. After migrating into the cortex, GABAergic interneurons start to receive glutamatergic connections from cortical excitatory neurons and thus gradually become integrated into cortical circuits. These glutamatergic connections are mediated by glutamate receptors including AMPA and NMDA receptors and the ratio of AMPA to NMDA receptors decreases during development. Since previous studies have shown that retinal input can regulate the early development of connections along the visual pathway, we investigated if the maturation of glutamatergic inputs to GABAergic interneurons in the visual cortex requires retinal input. We mapped the spatial pattern of glutamatergic connections to layer 4 (L4) GABAergic interneurons in mouse visual cortex at around postnatal day (P) 16 by laser-scanning photostimulation and investigated the effect of binocular enucleations at P1/P2 on these patterns. Gad2-positive interneurons in enucleated animals showed an increased fraction of AMPAR-mediated input from L2/3 and a decreased fraction of input from L5/6. Parvalbumin-expressing (PV) interneurons showed similar changes in relative connectivity. NMDAR-only input was largely unchanged by enucleation. Our results show that retinal input sculpts the integration of interneurons into V1 circuits and suggest that the development of AMPAR- and NMDAR-only connections might be regulated differently.

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

Altered white matter integrity in patients with monocular blindness: A diffusion tensor imaging and tract-based spatial statistics study

BACKGROUND

Visual deprivation can lead to abnormal and plastic changes in the brain's visual system and other systems. Although the secondary changes of gray matter in patients have been well studied, the study of white matter is rare. In fact, subtle changes in white matter may be revealed by diffusion tensor imaging, and tract-based spatial statistics can be used to analyze DTI image data.

PURPOSE

In the present study, diffusion tensor imaging (DTI) and tract-based spatial statistics (TBSS) were used to investigate abnormal structural changes in the white matter (WM) of patients with monocular blindness (MB).

METHODS

We recruited 16 healthy controls (HC) (fourteen males and two females) and 16 patients (fifteen males and one female) with right-eye blindness (without differences in left-eye vision). All patients were of similar age. Data acquisition was performed using magnetic resonance imaging (MRI) and DTI. Voxel-based whole brain comparisons of fractional anisotropy (FA) and radial diffusivity (RD) of WM fibers in patients and HC were performed using the TBSS method. The mean FA and RD values for altered brain regions in MB patients were analyzed via the receiver operating characteristic (ROC) curve. Correlation analysis was performed to investigate the relationships between the average FA (RD) value of the whole brain and anxiety score, depression score, and visual function questionnaire score in MB patients.

RESULTS

In MB patients, the mean FA of the whole brain was decreased versus HC. Moreover, the FA values of the corpus callosum, the corona radiata, the posterior thalamic radiation, and the right retrolenticular part of internal capsule were significantly decreased. In addition, the average RD value of the whole brain in MB patients was higher than that observed in HC. The mean FA and RD values of brain regions were analyzed using the ROC curve, and the results showed that the area under the ROC curve was more accurate. Furthermore, the average FA and RD values of the whole brain were significantly correlated with anxiety score, depression score, and visual function-related quality of life score.

CONCLUSION

DTI and TBSS may be useful in examining abnormal spontaneous alterations in the WM of MB patients. The observed changes in FA and RD values may imply the larvaceous neurological mechanism involved in MB.

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.

Comparison of intrinsic brain activity in individuals with low/moderate myopia versus high myopia revealed by the amplitude of low-frequency fluctuations

Background

Previous neuroimaging studies demonstrated that individuals with high myopia are associated with abnormalities in anatomy of the brain.

Purpose

The purpose of this study was to explore alterations in the intrinsic brain activity by studying the amplitude of low-frequency fluctuations.

Material and Methods

A total of 64 myopia individuals (41 with high myopia with a refractive error <–600 diopter [D], 23 with low/moderate myopia with a refractive error between –100 and –600 D, and similarly 59 healthy controls with emmetropia closely matched for age) were recruited. The amplitude of low-frequency fluctuations method was conducted to investigate the difference of intrinsic brain activity across three groups.

Results

Compared with the healthy controls, individuals with low/moderate myopia showed significantly decreased amplitude of low-frequency fluctuation values in the bilateral rectal gyrus, right cerebellum anterior lobe/calcarine, and bilateral thalamus and showed significantly increased amplitude of low-frequency fluctuation values in left white matter (optic radiation), right prefrontal cortex, and left primary motor cortex (M1)/primary somatosensory cortex (S1). In addition, individuals with high myopia showed significantly decreased amplitude of low-frequency fluctuation values in the right cerebellum anterior lobe/calcarine/bilateral parahippocampal gyrus, bilateral posterior cingulate cortex, and bilateral middle cingulate cortex and significantly increased amplitude of low-frequency fluctuation values in left white matter (optic radiation), bilateral frontal parietal cortex, and left M1/S1. Moreover, we found that the amplitude of low-frequency fluctuation values of the different brain areas was closely related to the clinical features in the high myopia group.

Conclusion

Our results demonstrated that individuals with low/moderate myopia and high myopia had abnormal intrinsic brain activities in various brain regions related to the limbic system, default mode network, and thalamo-occipital pathway.

Brain structure in children with congenital visual disorders and visual impairment

AIM

To examine if congenital visual impairment is associated with differences in brain anatomy in children.

METHOD

Ten children (8-12y) with congenital disorders of the peripheral visual system with severe visual impairment (SVI; >0.8 logMAR) or mild-to-moderate visual impairment (MVI; 0.6-0.8 logMAR) were compared to 21 typically sighted comparison (TSC) children. Thalamus volume, grey matter density, white matter microstructure, and integrity of visual tracts were investigated in SVI, MVI, and TSC groups with anatomical and diffusion-weighted magnetic resonance imaging.

RESULTS

Compared to the TSC group, the SVI group had lower white matter integrity in tracts of the visual system (optic radiations: SVI 0.35±0.015, TSC 0.39±0.007 [p=0.022]; posterior corpus callosum: SVI 0.37±0.019; TSC 0.42±0.009 [p=0.033]) and lower left thalamus volume (SVI 4.37±0.087; TSC 4.99±0.339 [p=0.015]). Neuroanatomical differences were greater in the SVI group, while no consistent differences between the MVI and TSC group were observed.

INTERPRETATION

Posterior tracts of the visual system are compromised in children with congenital visual impairment versus those who are typically sighted. The severity of visual input appears to have affected neuroanatomical development as significant reductions were only found in the SVI group.

WHAT THIS PAPER ADDS

Severe visual impairment in mid-childhood is associated with reduced integrity of visual pathways and reduced thalamus volume.

Influence of Visual Deprivation on Auditory Spectral Resolution, Temporal Resolution, and Speech Perception

We evaluated whether blind subjects have advantages in auditory spectral resolution, temporal resolution, and speech perception in noise compared with sighted subjects. We also compared psychoacoustic performance between early blind (EB) subjects and late blind (LB) subjects. Nineteen EB subjects, 16 LB subjects, and 20 sighted individuals were enrolled. All subjects were right-handed with normal and symmetric hearing thresholds and without cognitive impairments. Three psychoacoustic measurements of the subjects’ right ears were performed via an inserted earphone to determine spectral-ripple discrimination (SRD), temporal modulation detection (TMD), and speech recognition threshold (SRT) in noisy conditions. Acoustic change complex (ACC) responses were recorded during passive listening to standard ripple-inverted ripple stimuli. EB subjects exhibited better SRD than did LB (p = 0.020) and sighted (p = 0.003) subjects. TMD was better in EB (p < 0.001) and LB (p = 0.007) subjects compared with sighted subjects. SRD was positively correlated with the duration of blindness (r = 0.386, p = 0.024). Acoustic change complex data for ripple noise change at the Cz and Fz electrodes showed trends toward significant correlations with the behavioral results. In conclusion, compared with sighted subjects, EB subjects showed advantages in terms of auditory spectral and temporal resolution, while LB subjects showed an advantage in temporal resolution exclusively. These findings suggest that it might take longer for auditory spectral resolution to functionally enhance following visual deprivation compared to temporal resolution. Alternatively, a critical period of very young age may be required for auditory spectral resolution to improve following visual deprivation.

A thalamocortical pathway for fast rerouting of tactile information to occipital cortex in congenital blindness

In congenitally blind individuals, the occipital cortex responds to various nonvisual inputs. Some animal studies raise the possibility that a subcortical pathway allows fast re-routing of tactile information to the occipital cortex, but this has not been shown in humans. Here we show using magnetoencephalography (MEG) that tactile stimulation produces occipital cortex activations, starting as early as 35 ms in congenitally blind individuals, but not in blindfolded sighted controls. Given our measured thalamic response latencies of 20 ms and a mean estimated lateral geniculate nucleus to primary visual cortex transfer time of 15 ms, we claim that this early occipital response is mediated by a direct thalamo-cortical pathway. We also observed stronger directed connectivity in the alpha band range from posterior thalamus to occipital cortex in congenitally blind participants. Our results strongly suggest the contribution of a fast thalamo-cortical pathway in the cross-modal activation of the occipital cortex in congenitally blind humans.

In congenitally blind people, tactile stimuli can activate the occipital (visual) cortex. Here, the authors show using magnetoencephalography (MEG) that occipital activation can occur within 35 ms following tactile stimulation, suggesting the existence of a fast thalamocortical pathway for touch in congenitally blind humans.

How Areal Specification Shapes the Local and Interareal Circuits in a Macaque Model of Congenital Blindness

There is little understanding of the structural underpinnings of the functional reorganization of the cortex in the congenitally blind human. Taking advantage of the extensive characterization of the macaque visual system, we examine in macaque the influence of congenital blindness resulting from the removal of the retina during in utero development. This effectively removes the normal influence of the thalamus on cortical development leading to an induced hybrid cortex (HC) combining features of primary visual and extrastriate cortex. Retrograde tracers injected in HC reveal a local, intrinsic connectivity characteristic of higher order areas and show that the HC receives a uniquely strong, purely feedforward projection from striate cortex but no ectopic inputs, except from subiculum, and entorhinal cortex. Statistical modeling of quantitative connectivity data shows that HC is relatively high in the cortical hierarchy and receives a reinforced input from ventral stream areas while the overall organization of the functional streams are conserved. The directed and weighted anophthalmic cortical graph from the present study can be used to construct dynamic and structural models. These findings show how the sensory periphery governs cortical phenotype and reveal the importance of developmental arealization for understanding the functional reorganization in congenital blindness.

Development of the thalamus: From early patterning to regulation of cortical functions

The thalamus is a brain structure of the vertebrate diencephalon that plays a central role in regulating diverse functions of the cerebral cortex. In traditional view of vertebrate neuroanatomy, the thalamus includes three regions, dorsal thalamus, ventral thalamus, and epithalamus. Recent molecular embryological studies have redefined the thalamus and the associated axial nomenclature of the diencephalon in the context of forebrain patterning. This new view has provided a useful conceptual framework for studies on molecular mechanisms of patterning, neurogenesis and fate specification in the thalamus as well as the guidance mechanisms for thalamocortical axons. Additionally, the availability of genetic tools in mice has led to important findings on how thalamic development is linked to the development of other brain regions, particularly the cerebral cortex. This article will give an overview of the organization of the embryonic thalamus and how progenitor cells in the thalamus generate neurons that are organized into discrete nuclei. I will then discuss how thalamic development is orchestrated with the development of the cerebral cortex and other brain regions. This article is categorized under: Nervous System Development > Vertebrates: Regional Development Nervous System Development > Vertebrates: General Principles.

Altered intra- and inter-regional functional connectivity of the visual cortex in individuals with peripheral vision loss due to retinitis pigmentosa

This study investigated changes in intra- and inter-regional functional connectivity (FC) in individuals with retinitis pigmentosa (RP) by using regional homogeneity (ReHo) and FC methods. Sixteen RP individuals and 14 healthy controls (HCs) underwent resting-state functional magnetic resonance imaging scans (fMRI). A combined ReHo and FC method was conducted to evaluate synchronization of brain activity. Compared with HCs, RP individuals had significantly lower ReHo values in the bilateral lingual gyrus/cerebellum posterior lobe (LGG/CPL). In FC analysis, the RP group showed decreased positive FC relative to the HC group, from bilateral LGG/CPL to bilateral LGG/cuneus (CUN) and to left postcentral gyrus (PosCG). In contrast, the RP group showed increased negative FC relative to the HC group, from bilateral LGG/CPL to bilateral thalamus, and decreased negative FC from bilateral LGG/CPL to right middle frontal gyrus (MFG), and to left inferior parietal lobule (IPL). Moreover, ReHo values of the bilateral LGG/CPL showed negative correlations with the duration of RP. FC values of the bilateral LGG/CPL-left IPL showed negative correlations with best-corrected visual acuity (BCVA) of the right eye and left eye in RP individuals. Our results reveal reduced synchronicity of neural activity changes in the primary visual area in RP individuals. Moreover, RP individuals showed intrinsic visual network disconnection and reorganization of the retino-thalamocortical pathway and dorsal visual stream, suggesting impaired visuospatial and stereoscopic vision.

Altered whole-brain gray matter volume in primary angle closure glaucoma patients: a voxel-based morphometry study

The study aimed to compare the whole-brain gray matter volume (GMV) and white matter volume (WMV) difference between primary angle closure glaucoma (PACG) patients and health controls (HCs) using a voxel-based morphometry method. A total of 27 patients with PACG (17 males and 10 females) and 27 HCs (17 males and 10 females), closely matched for age and education, were enrolled in the study. All subjects underwent magnetic resonance imaging (MRI) scans. The MRI data were processed using SPM8 software in voxel-based morphometry 8 toolbox. The relationship between the mean GMV values of brain regions and the clinical features including psychological testing and mean retinal nerve fiber layer (RNFL) thickness in PACG groups were analyzed by using Pearson correlation. Compared with HCs, PACG patients showed significantly decreased GMV values in the left cerebellum posterior lobe (CPL), right extra-nuclear, and right superior temporal gyrus. In contrast, PACG patients showed significantly increased GMV values in the left CPL, right CPL, right superior temporal gyrus, right thalamus and right insula (P<0.01). Moreover, in the PACG group, the left mean RNFL showed a positive correlation with the mean GMV values of the left CPL (r=0.719; P<0.001) and the right mean RNFL showed a positive correlation with the mean GMV values of the left CPL (r=0.721; P<0.001). The Hamilton depression score showed a positive correlation with the mean GMV values of right insula (r=0.897; P<0.001). Our results demonstrated that PACG patients showed altered brain structure in various regions related to visuomotor function, thalamocortical pathway, and emotion function, which might provide a useful informations to understanding the anatomy neural mechanisms of deficit in vision loss and depression in PACG.

Precision in the development of neocortical architecture: From progenitors to cortical networks

Of all brain regions, the 6-layered neocortex has undergone the most dramatic changes in size and complexity during mammalian brain evolution. These changes, occurring in the context of a conserved set of organizational features that emerge through stereotypical developmental processes, are considered responsible for the cognitive capacities and sensory specializations represented within the mammalian clade. The modern experimental era of developmental neurobiology, spanning 6 decades, has deciphered a number of mechanisms responsible for producing the diversity of cortical neuron types, their precise connectivity and the role of gene by environment interactions. Here, experiments providing insight into the development of cortical projection neuron differentiation and connectivity are reviewed. This current perspective integrates discussion of classic studies and new findings, based on recent technical advances, to highlight an improved understanding of the neuronal complexity and precise connectivity of cortical circuitry. These descriptive advances bring new opportunities for studies related to the developmental origins of cortical circuits that will, in turn, improve the prospects of identifying pathogenic targets of neurodevelopmental disorders.

Not all cortical expansions are the same: the coevolution of the neocortex and the dorsal thalamus in mammals

A central question in comparative neurobiology concerns how evolution has produced brains with expanded neocortices, composed of more areas with unique connectivity and functional properties. Some mammalian lineages, such as primates, exhibit exceptionally large cortices relative to the amount of sensory inputs from the dorsal thalamus, and this expansion is associated with a larger number of distinct cortical areas, composing a larger proportion of the cortical sheet. We propose a link between the organization of the neocortex and its expansion relative to the size of the dorsal thalamus, based on a combination of work in comparative neuroanatomy and experimental research.

Alterations in cortical and thalamic connections of somatosensory cortex following early loss of vision

Early loss of vision produces dramatic changes in the functional organization and connectivity of the neocortex in cortical areas that normally process visual inputs, such as the primary and second visual area. This loss also results in alterations in the size, functional organization, and neural response properties of the primary somatosensory area, S1. However, the anatomical substrate for these functional changes in S1 has never been described. In the present investigation, we quantified the cortical and subcortical connections of S1 in animals that were bilaterally enucleated very early in development, prior to the formation of retino-geniculate and thalamocortical pathways. We found that S1 receives dense inputs from novel cortical fields, and that the density of existing cortical and thalamocortical connections was altered. Our results demonstrate that sensory systems develop in tandem and that alterations in sensory input in one system can affect the connections and organization of other sensory systems. Thus, therapeutic intervention following early loss of vision should focus not only on restoring vision, but also on augmenting the natural plasticity of the spared systems.

The Combinatorial Creature: Cortical Phenotypes within and across Lifetimes

The neocortex is one of the most distinctive structures of the mammalian brain, yet also one of the most varied in terms of both size and organization. Multiple processes have contributed to this variability, including evolutionary mechanisms (i.e., alterations in gene sequence) that alter the size, organization, and connections of neocortex, and activity dependent mechanisms that can also modify these same features. Thus, changes to the neocortex can occur over different time-scales, including within a single generation. This combination of genetic and activity dependent mechanisms that create a given cortical phenotype allows the mammalian neocortex to rapidly and flexibly adjust to different body and environmental contexts, and in humans permits culture to impact brain construction.

The pial vasculature of the mouse develops according to a sensory-independent program

The cerebral vasculature is organized to supply the brain’s metabolic needs. Sensory deprivation during the early postnatal period causes altered neural activity and lower metabolic demand. Neural activity is instructional for some aspects of vascular development, and deprivation causes changes in capillary density in the deprived brain region. However, it is not known if the pial arteriole network, which contains many leptomeningeal anastomoses (LMAs) that endow the network with redundancy against occlusions, is also affected by sensory deprivation. We quantified the effects of early-life sensory deprivation via whisker plucking on the densities of LMAs and penetrating arterioles (PAs) in anatomically-identified primary sensory regions (vibrissae cortex, forelimb/hindlimb cortex, visual cortex and auditory cortex) in mice. We found that the densities of penetrating arterioles were the same across cortical regions, though the hindlimb representation had a higher density of LMAs than other sensory regions. We found that the densities of PAs and LMAs, as well as quantitative measures of network topology, were not affected by sensory deprivation. Our results show that the postnatal development of the pial arterial network is robust to sensory deprivation.

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.

Neural Coding of Whisker-Mediated Touch in Primary Somatosensory Cortex Is Altered Following Early Blindness

Sensory systems do not develop and function independently of one another, yet they are typically studied in isolation. Effects of multisensory interactions on the developing neocortex can be revealed by altering the ratios of incoming sensory inputs associated with different modalities. We investigated neural responses in primary somatosensory cortex (S1) of short-tailed opossums (Monodelphis domestica; either sex) after the elimination of visual input through bilateral enucleation very early in development. To assess the influence of tactile experience after vision loss, we also examined naturally occurring patterns of exploratory behavior. In early blind (EB) animals, overall levels of tactile experience were similar to those of sighted controls (SC); locomotor activity was unimpaired and accompanied by whisking. Using extracellular single-unit recording techniques under anesthesia, we found that EB animals exhibited a reduction in the magnitude of neural responses to whisker stimuli in S1, coupled with spatial sharpening of receptive fields, in comparison to SC animals. These alterations manifested as two different effects on sensory processing in S1 of EB animals: the ability of neurons to detect single whisker stimulation was decreased, whereas their ability to discriminate between stimulation of neighboring whiskers was enhanced. The increased selectivity of S1 neurons in EB animals was reflected in improved population decoding performance for whisker stimulus position, particularly along the rostrocaudal axis of the snout, which aligns with the primary axis of natural whisker motion. These findings suggest that a functionally distinct form of somatosensory plasticity occurs when vision is lost early in development.SIGNIFICANCE STATEMENT After sensory loss, compensatory behavior mediated through the spared senses could be generated entirely through the recruitment of brain areas associated with the deprived sense. Alternatively, functional compensation in spared modalities may be achieved through a combination of plasticity in brain areas corresponding to both spared and deprived sensory modalities. Although activation of neurons in cortex associated with a deprived sense has been described frequently, it is unclear whether this is the only substrate available for compensation or if plasticity within cortical fields corresponding to spared modalities, particularly primary sensory cortices, may also contribute. Here, we demonstrate empirically that early loss of vision alters coding of sensory inputs in primary somatosensory cortex in a manner that supports enhanced tactile discrimination.

Early sensory experience influences the development of multisensory thalamocortical and intracortical connections of primary sensory cortices

The nervous system integrates information from multiple senses. This multisensory integration already occurs in primary sensory cortices via direct thalamocortical and corticocortical connections across modalities. In humans, sensory loss from birth results in functional recruitment of the deprived cortical territory by the spared senses but the underlying circuit changes are not well known. Using tracer injections into primary auditory, somatosensory, and visual cortex within the first postnatal month of life in a rodent model (Mongolian gerbil) we show that multisensory thalamocortical connections emerge before corticocortical connections but mostly disappear during development. Early auditory, somatosensory, or visual deprivation increases multisensory connections via axonal reorganization processes mediated by non-lemniscal thalamic nuclei and the primary areas themselves. Functional single-photon emission computed tomography of regional cerebral blood flow reveals altered stimulus-induced activity and higher functional connectivity specifically between primary areas in deprived animals. Together, we show that intracortical multisensory connections are formed as a consequence of sensory-driven multisensory thalamocortical activity and that spared senses functionally recruit deprived cortical areas by an altered development of sensory thalamocortical and corticocortical connections. The functional-anatomical changes after early sensory deprivation have translational implications for the therapy of developmental hearing loss, blindness, and sensory paralysis and might also underlie developmental synesthesia.

Early blindness is associated with increased volume of the uncinate fasciculus

Growing evidence demonstrates dramatic structural and functional neuroplastic changes in individuals born with early-onset blindness. For example, cross-modal sensory processing at the level of the occipital cortex appears to be associated with adaptive behaviors in the blind. However, detailed studies examining the structural properties of key white matter pathways in other regions of the brain remain limited. Given that blind individuals rely heavily on their sense of hearing, we examined the structural properties of two important pathways involved with auditory processing, namely the uncinate and arcuate fasciculi. High angular resolution diffusion imaging (HARDI) tractography was used to examine structural parameters (i.e., tract volume and quantitative anisotropy, or QA) of these two fasciculi in a sample of 13 early blind individuals and 14 normally sighted controls. Compared to controls, early blind individuals showed a significant increase in the volume of the left uncinate fasciculus. A small area of increased QA was also observed halfway along the right arcuate fasciculus in the blind group. These findings contribute to our knowledge regarding the broad neuroplastic changes associated with profound early blindness.

Preserved sleep microstructure in blind individuals

The loss of vision, particularly when it occurs early in life, is associated with compensatory cortical plasticity not only in the visual cortical areas, but throughout the entire brain. The absence of visual input to the retina can also induce changes in entrainment of the circadian rhythm, as light is the primary zeitgeber of the master biological clock found in the suprachiasmatic nucleus of the hypothalamus. In addition, a greater number of sleep disturbances is often reported in blind individuals. Here, we examined various electroencephalographic microstructural components of sleep, both during rapid-eye-movement (REM) sleep and non-REM (NREM) sleep, between blind individuals, including both of early and late onset, and normal-sighted controls. During wakefulness, occipital alpha oscillations were lower, or absent in blind individuals. During sleep, differences were observed across electrode derivations between the early and late blind samples, which may reflect altered cortical networking in early blindness. Despite these differences in power spectra density, the electroencephalography microstructure of sleep, including sleep spindles, slow wave activity, and sawtooth waves, remained present in the absence of vision.

Cortical and thalamic connectivity to the second auditory cortex of the cat is resilient to the onset of deafness

It has been well established that following sensory loss, cortical areas that would normally be involved in perceiving stimuli in the absent modality are recruited to subserve the remaining senses. Despite this compensatory functional reorganization, there is little evidence to date for any substantial change in the patterns of anatomical connectivity between sensory cortices. However, while many auditory areas are contracted in the deaf, the second auditory cortex (A2) of the cat undergoes a volumetric expansion following hearing loss, suggesting this cortical area may demonstrate a region-specific pattern of structural reorganization. To address this hypothesis, and to complement existing literature on connectivity within auditory cortex, we injected a retrograde neuronal tracer across the breadth and cortical thickness of A2 to provide the first comprehensive quantification of projections from cortical and thalamic auditory and non-auditory regions to the second auditory cortex, and to determine how these patterns are affected by the onset of deafness. Neural projections arising from auditory, visual, somatomotor, and limbic cortices, as well as thalamic nuclei, were compared across normal hearing, early-deaf, and late-deaf animals. The results demonstrate that, despite previously identified changes in A2 volume, the pattern of projections into this cortical region are unaffected by the onset of hearing loss. These results fail to support the idea that crossmodal plasticity reflects changes in the pattern of projections between cortical regions and provides evidence that the pattern of connectivity that supports normal hearing is retained in the deaf brain.

Thalamocortical Connectivity and Microstructural Changes in Congenital and Late Blindness

There is ample evidence that the occipital cortex of congenitally blind individuals processes nonvisual information. It remains a debate whether the cross-modal activation of the occipital cortex is mediated through the modulation of preexisting corticocortical projections or the reorganisation of thalamocortical connectivity. Current knowledge on this topic largely stems from anatomical studies in animal models. The aim of this study was to test whether purported changes in thalamocortical connectivity in blindness can be revealed by tractography based on diffusion-weighted magnetic resonance imaging. To assess the thalamocortical network, we used a clustering method based on the thalamic white matter projections towards predefined cortical regions. Five thalamic clusters were obtained in each group representing their cortical projections. Although we did not find differences in the thalamocortical network between congenitally blind individuals, late blind individuals, and normal sighted controls, diffusion tensor imaging (DTI) indices revealed significant microstructural changes within thalamic clusters of both blind groups. Furthermore, we find a significant decrease in fractional anisotropy (FA) in occipital and temporal thalamocortical projections in both blind groups that were not captured at the network level. This suggests that plastic microstructural changes have taken place, but not in a degree to be reflected in the tractography-based thalamocortical network.

Prenatal thalamic waves regulate cortical area size prior to sensory processing

The cerebral cortex is organized into specialized sensory areas, whose initial territory is determined by intracortical molecular determinants. Yet, sensory cortical area size appears to be fine tuned during development to respond to functional adaptations. Here we demonstrate the existence of a prenatal sub-cortical mechanism that regulates the cortical areas size in mice. This mechanism is mediated by spontaneous thalamic calcium waves that propagate among sensory-modality thalamic nuclei up to the cortex and that provide a means of communication among sensory systems. Wave pattern alterations in one nucleus lead to changes in the pattern of the remaining ones, triggering changes in thalamic gene expression and cortical area size. Thus, silencing calcium waves in the auditory thalamus induces Rorβ upregulation in a neighbouring somatosensory nucleus preluding the enlargement of the barrel-field. These findings reveal that embryonic thalamic calcium waves coordinate cortical sensory area patterning and plasticity prior to sensory information processing.

How sensory maps are formed in the brain is only partially understood. Here the authors describe spontaneous calcium waves that propagate across different sensory nuclei in the embryonic thalamus; disrupting the wave pattern triggers thalamic gene expression changes and eventually alters the size of cortical areas.

State-dependent modulation of functional connectivity in early blind individuals

Resting-state functional connectivity (RSFC) studies have provided strong evidences that visual deprivation influences the brain's functional architecture. In particular, reduced RSFC coupling between occipital (visual) and temporal (auditory) regions has been reliably observed in early blind individuals (EB) at rest. In contrast, task-dependent activation studies have repeatedly demonstrated enhanced co-activation and connectivity of occipital and temporal regions during auditory processing in EB. To investigate this apparent discrepancy, the functional coupling between temporal and occipital networks at rest was directly compared to that of an auditory task in both EB and sighted controls (SC). Functional brain clusters shared across groups and cognitive states (rest and auditory task) were defined. In EBs, we observed higher occipito-temporal correlations in activity during the task than at rest. The reverse pattern was observed in SC. We also observed higher temporal variability of occipito-temporal RSFC in EB suggesting that occipital regions in this population may play the role of a multiple demand system. Our study reveals how the connectivity profile of sighted and early blind people is differentially influenced by their cognitive state, bridging the gap between previous task-dependent and RSFC studies. Our results also highlight how inferring group-differences in functional brain architecture solely based on resting-state acquisition has to be considered with caution.

Congenital Anophthalmia and Binocular Neonatal Enucleation Differently Affect the Proteome of Primary and Secondary Visual Cortices in Mice

In blind individuals, visually deprived occipital areas are activated by non-visual stimuli. The extent of this cross-modal activation depends on the age at onset of blindness. Cross-modal inputs have access to several anatomical pathways to reactivate deprived visual areas. Ectopic cross-modal subcortical connections have been shown in anophthalmic animals but not in animals deprived of sight at a later age. Direct and indirect cross-modal cortical connections toward visual areas could also be involved, yet the number of neurons implicated is similar between blind mice and sighted controls. Changes at the axon terminal, dendritic spine or synaptic level are therefore expected upon loss of visual inputs. Here, the proteome of V1, V2M and V2L from P0-enucleated, anophthalmic and sighted mice, sharing a common genetic background (C57BL/6J x ZRDCT/An), was investigated by 2-D DIGE and Western analyses to identify molecular adaptations to enucleation and/or anophthalmia. Few proteins were differentially expressed in enucleated or anophthalmic mice in comparison to sighted mice. The loss of sight affected three pathways: metabolism, synaptic transmission and morphogenesis. Most changes were detected in V1, followed by V2M. Overall, cross-modal adaptations could be promoted in both models of early blindness but not through the exact same molecular strategy. A lower metabolic activity observed in visual areas of blind mice suggests that even if cross-modal inputs reactivate visual areas, they could remain suboptimally processed.

Species-dependent role of crossmodal connectivity among the primary sensory cortices

When a major sense is lost, crossmodal plasticity substitutes functional processing from the remaining, intact senses. Recent studies of deafness-induced crossmodal plasticity in different subregions of auditory cortex indicate that the phenomenon is largely based on the "unmasking" of existing inputs. However, there is not yet a consensus on the sources or effects of crossmodal inputs to primary sensory cortical areas. In the present review, a rigorous re-examination of the experimental literature indicates that connections between different primary sensory cortices consistently occur in rodents, while primary-to-primary projections are absent/inconsistent in non-rodents such as cats and monkeys. These observations suggest that crossmodal plasticity that involves primary sensory areas are likely to exhibit species-specific distinctions.

Reading in the dark: neural correlates and cross-modal plasticity for learning to read entire words without visual experience

Cognitive neuroscience has long attempted to determine the ways in which cortical selectivity develops, and the impact of nature vs. nurture on it. Congenital blindness (CB) offers a unique opportunity to test this question as the brains of blind individuals develop without visual experience. Here we approach this question through the reading network. Several areas in the visual cortex have been implicated as part of the reading network, and one of the main ones among them is the VWFA, which is selective to the form of letters and words. But what happens in the CB brain? On the one hand, it has been shown that cross-modal plasticity leads to the recruitment of occipital areas, including the VWFA, for linguistic tasks. On the other hand, we have recently demonstrated VWFA activity for letters in contrast to other visual categories when the information is provided via other senses such as touch or audition. Which of these tasks is more dominant? By which mechanism does the CB brain process reading? Using fMRI and visual-to-auditory sensory substitution which transfers the topographical features of the letters we compare reading with semantic and scrambled conditions in a group of CB. We found activation in early auditory and visual cortices during the early processing phase (letter), while the later phase (word) showed VWFA and bilateral dorsal-intraparietal activations for words. This further supports the notion that many visual regions in general, even early visual areas, also maintain a predilection for task processing even when the modality is variable and in spite of putative lifelong linguistic cross-modal plasticity. Furthermore, we find that the VWFA is recruited preferentially for letter and word form, while it was not recruited, and even exhibited deactivation, for an immediately subsequent semantic task suggesting that despite only short sensory substitution experience orthographic task processing can dominate semantic processing in the VWFA. On a wider scope, this implies that at least in some cases cross-modal plasticity which enables the recruitment of areas for new tasks may be dominated by sensory independent task specific activation.

Cross-modal plasticity in sensory deprived animal models: From the thalamocortical development point of view

Over recent decades, our understanding of the plasticity of the central nervous system has expanded enormously. Accordingly, it is now widely accepted that the brain can adapt to changes by reorganizing its circuitry, both in response to external stimuli and experience, as well as through intrinsic mechanisms. A clear example of this is the activation of a deprived sensory area and the expansion of spared sensory cortical regions in individuals who suffered peripheral sensory loss. Despite the efforts to understand these neuroplastic changes, the mechanisms underlying such adaptive remodeling remains poorly understood. Progress in understanding these events may be hindered by the highly varied data obtained from the distinct experimental paradigms analyzed, which include different animal models and neuronal systems, as well as studies into the onset of sensory loss. Here, we will establish the current state-of-the-art describing the principal observations made according to the time of sensory deprivation with respect to the development of the thalamocortical connectivity. We will review the experimental data obtained from animal models where sensory deprivation has been induced either before or after thalamocortical axons reach and invade their target cortical areas. The anatomical and functional effects of sensory loss on the primary sensory areas of the cortex will be presented. Indeed, we consider that the comparative approach of this review is a necessary step in order to help deciphering the processes that underlie sensory neuroplasticity, for which studies in animal models have been indispensable. Understanding these mechanisms will then help to develop restorative strategies and prostheses that will overcome the functional loss.