Cortical plasticity in central vision loss: Cortical thickness and neurite structure.

Late-stage macular degeneration (MD) often causes retinal lesions depriving an individual of central vision, forcing them to learn to use peripheral vision for daily tasks. To compensate, many patients develop a preferred retinal locus (PRL), an area of peripheral vision used more often than equivalent regions of spared vision. Thus, associated portions of cortex experience increased use, while portions of cortex associated with the lesion are deprived of sensory input. Prior research has not well examined the degree to which structural plasticity depends on the amount of use across the visual field. Cortical thickness, neurite density, and orientation dispersion were measured at portions of cortex associated with the PRL, the retinal lesion, and a control region in participants with MD as well as age-matched, gender-matched, and education-matched controls. MD participants had significantly thinner cortex in both the cortical representation of the PRL (cPRL) and the control region, compared with controls, but no significant differences in thickness, neurite density, or orientation dispersion were found between the cPRL and the control region as functions of disease or onset. This decrease in thickness is driven by a subset of early-onset participants whose patterns of thickness, neurite density, and neurite orientation dispersion are distinct from matched control participants. These results suggest that people who develop MD earlier in adulthood may undergo more structural plasticity than those who develop it late in life.

A Comprehensive Overview of the Role of Visual Cortex Malfunction in Depressive Disorders: Opportunities and Challenges.

Major depressive disorder (MDD) is a highly heterogeneous mental disorder, and its complex etiology and unclear mechanism are great obstacles to the diagnosis and treatment of the disease. Studies have shown that abnormal functions of the visual cortex have been reported in MDD patients, and the actions of several antidepressants coincide with improvements in the structure and synaptic functions of the visual cortex. In this review, we critically evaluate current evidence showing the involvement of the malfunctioning visual cortex in the pathophysiology and therapeutic process of depression. In addition, we discuss the molecular mechanisms of visual cortex dysfunction that may underlie the pathogenesis of MDD. Although the precise roles of visual cortex abnormalities in MDD remain uncertain, this undervalued brain region may become a novel area for the treatment of depressed patients.

Juvenile depletion of microglia reduces orientation but not high spatial frequency selectivity in mouse V1.

Microglia contain multiple mechanisms that shape the synaptic landscape during postnatal development. Whether the synaptic changes mediated by microglia reflect the developmental refinement of neuronal responses in sensory cortices, however, remains poorly understood. In postnatal life, the development of increased orientation and spatial frequency selectivity of neuronal responses in primary visual cortex (V1) supports the emergence of high visual acuity. Here, we used the colony-stimulating factor 1 receptor (CSF1R) inhibitor PLX5622 to rapidly and durably deplete microglia in mice during the juvenile period in which increased orientation and spatial frequency selectivity emerge. Excitatory and inhibitory tuning properties were measured simultaneously using multi-photon calcium imaging in layer II/III of mouse V1. We found that microglia depletion generally increased evoked activity which, in turn, reduced orientation selectivity. Surprisingly, microglia were not required for the emergence of high spatial frequency tuned responses. In addition, microglia depletion did not perturb cortical binocularity, suggesting normal depth processing. Together, our finding that orientation and high spatial frequency selectivity in V1 are differentially supported by microglia reveal that microglia are required normal sensory processing, albeit selectively.

Visual neuroprosthesis: present and future perspectives.

The purpose of this study is to provide an overview of the replacements used in lost vision in the form of the bionic eye, to show their deficiencies and outline other possibilities for non-invasive stimulation of functional areas of the visual cortex. The review highlights the damage not only to the primary altered cellular structures, but also to all other horizontally and vertically localised structures. Based on the results of a large number of functional magnetic resonance imaging and electrophysiological methods, the authors focus on the pathology of the entire visual pathway in pigmentary retinopathy (PR) and age-related macular degeneration (AMD). This study provides a recent overview of the possible systems used to replace lost vision. These range from stimulation with intraocular implants, through stimulation of the optic nerve and lateral geniculate nucleus to the visual cortex. The second part deals with the design of image processing technology and its transformation into the form of transcranial stimulation of undamaged parts of the brain, which is protected by a patent. This is comprehensive overview of the current possibilities of replacement of lost vision and a proposal for a new non-invasive methods of stimulation of functional neurons of the visual cortex.

Hierarchical and nonhierarchical features of the mouse visual cortical network.

Neocortical computations underlying vision are performed by a distributed network of functionally specialized areas. Mouse visual cortex, a dense interareal network that exhibits hierarchical properties, comprises subnetworks interconnecting distinct processing streams. To determine the layout of the mouse visual hierarchy, we have evaluated the laminar patterns formed by interareal axonal projections originating in each of ten areas. Reciprocally connected pairs of areas exhibit feedforward/feedback relationships consistent with a hierarchical organization. Beta regression analyses, which estimate a continuous hierarchical distance measure, indicate that the network comprises multiple nonhierarchical circuits embedded in a hierarchical organization of overlapping levels. Single-unit recordings in anaesthetized mice show that receptive field sizes are generally consistent with the hierarchy, with the ventral stream exhibiting a stricter hierarchy than the dorsal stream. Together, the results provide an anatomical metric for hierarchical distance, and reveal both hierarchical and nonhierarchical motifs in mouse visual cortex.

Ocular Neurodegenerative Diseases: Interconnection between Retina and Cortical Areas.

The possible interconnection between the eye and central nervous system (CNS) has been a topic of discussion for several years just based on fact that the eye is properly considered an extension of the brain. Both organs consist of neurons and derived from a neural tube. The visual process involves photoreceptors that receive light stimulus from the external environment and send it to retinal ganglionic cells (RGC), one of the cell types of which the retina is composed. The retina, the internal visual membrane of the eye, processes the visual stimuli in electric stimuli to transfer it to the brain, through the optic nerve. Retinal chronic progressive neurodegeneration, which may occur among the elderly, can lead to different disorders of the eye such as glaucoma, age-related macular degeneration (AMD), and diabetic retinopathy (DR). Mainly in the elderly population, but also among younger people, such ocular pathologies are the cause of irreversible blindness or impaired, reduced vision. Typical neurodegenerative diseases of the CSN are a group of pathologies with common characteristics and etiology not fully understood; some risk factors have been identified, but they are not enough to justify all the cases observed. Furthermore, several studies have shown that also ocular disorders present characteristics of neurodegenerative diseases and, on the other hand, CNS pathologies, i.e., Alzheimer disease (AD) and Parkinson disease (PD), which are causes of morbidity and mortality worldwide, show peculiar alterations at the ocular level. The knowledge of possible correlations could help to understand the mechanisms of onset. Moreover, the underlying mechanisms of these heterogeneous disorders are still debated. This review discusses the characteristics of the ocular illnesses, focusing on the relationship between the eye and the brain. A better comprehension could help in future new therapies, thus reducing or avoiding loss of vision and improve quality of life.

LSD degrades hippocampal spatial representations and suppresses hippocampal-visual cortical interactions.

Lysergic acid diethylamide (LSD) produces hallucinations, which are perceptions uncoupled from the external environment. How LSD alters neuronal activities in vivo that underlie abnormal perceptions is unknown. Here, we show that when rats run along a familiar track, hippocampal place cells under LSD reduce their firing rates, their directionality, and their interaction with visual cortical neurons. However, both hippocampal and visual cortical neurons temporarily increase firing rates during head-twitching, a behavioral signature of a hallucination-like state in rodents. When rats are immobile on the track, LSD enhances cortical firing synchrony in a state similar to the wakefulness-to-sleep transition, during which the hippocampal-cortical interaction remains dampened while hippocampal awake reactivation is maintained. Our results suggest that LSD suppresses hippocampal-cortical interactions during active behavior and during immobility, leading to internal hippocampal representations that are degraded and isolated from external sensory input. These effects may contribute to LSD-produced abnormal perceptions.

Neuroglobin effectively halts vision loss in Harlequin mice at an advanced stage of optic nerve degeneration.

Mitochondrial diseases are among the most prevalent groups of inherited neurological disorders, affecting up to 1 in 5000 adults. Despite the progress achieved on the identification of gene mutations causing mitochondrial pathologies, they cannot be cured so far. Harlequin mice, a relevant model of mitochondrial pathology due to apoptosis inducing factor depletion, suffer from progressive disappearance of retinal ganglion cells leading to optic neuropathy. In our previous work, we showed that administering adeno-associated virus encompassing the coding sequences for neuroglobin, (a neuroprotective molecule belonging to the globin family) or apoptosis-inducing factor, before neurodegeneration onset, prevented retinal ganglion cell loss and preserved visual function. One of the challenges to develop an effective treatment for optic neuropathies is to consider that by the time patients become aware of their handicap, a large amount of nerve fibers has already disappeared. Gene therapy was performed in Harlequin mice aged between 4 and 5 months with either a neuroglobin or an apoptosis-inducing factor vector to determine whether the increased abundance of either one of these proteins in retinas could preserve visual function at this advanced stage of the disease. We demonstrated that gene therapy, by preserving the connectivity of transduced retinal ganglion cells and optic nerve bioenergetics, results in the enhancement of visual cortex activity, ultimately rescuing visual impairment. This study demonstrates that: (a) An increased abundance of neuroglobin functionally overcomes apoptosis-inducing factor absence in Harlequin mouse retinas at a late stage of neuronal degeneration; (b) The beneficial effect for visual function could be mediated by neuroglobin localization to the mitochondria, thus contributing to the maintenance of the organelle homeostasis.

Alpha-synuclein activates the classical complement pathway and mediates complement-dependent cell toxicity.

BACKGROUND: Synucleinopathies are characterized by neurodegeneration and deposition of the presynaptic protein α-synuclein in pathological protein inclusions. Growing evidence suggests the complement system not only has physiological functions in the central nervous system, but also is involved in mediating the pathological loss of synapses in Alzheimer's disease. However, it is not established whether the complement system has a similar role in the diseases Parkinson's disease, Dementia with Lewy bodies, and multiple system atrophy (MSA) that are associated with α-synuclein aggregate pathology. METHODS: To investigate if the complement system has a pathological role in synucleinopathies, we assessed the effect of the complement system on the viability of an α-synuclein expressing cell model and examined direct activation of the complement system by α-synuclein in a plate-based activation assay. Finally, we investigated the levels of the initiator of the classical pathway, C1q, in postmortem brain samples from MSA patients. RESULTS: We demonstrate that α-synuclein activates the classical complement pathway and mediates complement-dependent toxicity in α-synuclein expressing SH-SY5Y cells. The α-synuclein-dependent cellular toxicity was rescued by the complement inhibitors RaCI (inhibiting C5) and Cp20 (inhibiting C3). Furthermore, we observed a trend for higher levels of C1q in the putamen of MSA subjects than that of controls. CONCLUSION: α-Synuclein can activate the classical complement pathway, and the complement system is involved in α-synuclein-dependent cellular cytotoxicity suggesting the system could play a prodegenerative role in synucleinopathies.

A convolutional neural network for estimating synaptic connectivity from spike trains.

The recent increase in reliable, simultaneous high channel count extracellular recordings is exciting for physiologists and theoreticians because it offers the possibility of reconstructing the underlying neuronal circuits. We recently presented a method of inferring this circuit connectivity from neuronal spike trains by applying the generalized linear model to cross-correlograms. Although the algorithm can do a good job of circuit reconstruction, the parameters need to be carefully tuned for each individual dataset. Here we present another method using a Convolutional Neural Network for Estimating synaptic Connectivity from spike trains. After adaptation to huge amounts of simulated data, this method robustly captures the specific feature of monosynaptic impact in a noisy cross-correlogram. There are no user-adjustable parameters. With this new method, we have constructed diagrams of neuronal circuits recorded in several cortical areas of monkeys.

GABA levels in ventral visual cortex decline with age and are associated with neural distinctiveness.

Age-related neural dedifferentiation-a decline in the distinctiveness of neural representations in the aging brain-has been associated with age-related declines in cognitive abilities. But why does neural distinctiveness decline with age? Based on prior work in nonhuman primates and more recent work in humans, we hypothesized that the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) declines with age and is associated with neural dedifferentiation in older adults. To test this hypothesis, we used magnetic resonance spectroscopy (MRS) to measure GABA and functional MRI (fMRI) to measure neural distinctiveness in the ventral visual cortex in a set of older and younger participants. Relative to younger adults, older adults exhibited lower GABA levels and less distinct activation patterns for faces and houses in the ventral visual cortex. Furthermore, individual differences in GABA within older adults positively predicted individual differences in neural distinctiveness. These results provide novel support for the view that age-related reductions of GABA contribute to age-related reductions in neural distinctiveness (i.e., neural dedifferentiation) in the human ventral visual cortex.

Neurochemical changes in the primate lateral geniculate nucleus following lesions of striate cortex in infancy and adulthood: implications for residual vision and blindsight.

Following lesions of the primary visual cortex (V1), the lateral geniculate nucleus (LGN) undergoes substantial cell loss due to retrograde degeneration. However, visually responsive neurons remain in the degenerated sector of LGN, and these have been implicated in mediation of residual visual capacities that remain within the affected sectors of the visual field. Using immunohistochemistry, we compared the neurochemical characteristics of LGN neurons in V1-lesioned marmoset monkeys (Callithrix jacchus) with those of non-lesioned control animals. We found that GABAergic neurons form approximately 6.5% of the neuronal population in the normal LGN, where most of these cells express the calcium-binding protein parvalbumin. Following long-term V1 lesions in adult monkeys, we observed a marked increase (~ sevenfold) in the proportion of GABA-expressing neurons in the degenerated sector of the LGN, indicating that GABAergic cells are less affected by retrograde degeneration in comparison with magno- and parvocellular projection neurons. In addition, following early postnatal V1 lesions and survival into adulthood, we found widespread expression of GABA in putative projection neurons, even outside the degenerated sectors (lesion projection zones). Our findings show that changes in the ratio of GABAergic neurons in LGN need to be taken into account in the interpretation of the mechanisms of visual abilities that survive V1 lesions in primates.

Mitochondrial function is impaired in the primary visual cortex in an experimental glaucoma model.

Glaucoma is a neurodegenerative disease that affects eye structures and brain areas related to the visual system. Oxidative stress plays a key role in the development and progression of the disease. The aims of the present study were to evaluate the mitochondrial function and its participation in the brain redox metabolism in an experimental glaucoma model. 3-month-old female Wistar rats were subjected to cauterization of two episcleral veins of the left eye to elevate the intraocular pressure. Seven days after surgery, animals were sacrificed, the brain was carefully removed and the primary visual cortex was dissected. Mitochondrial bioenergetics and ROS production, and the antioxidant enzyme defenses from both mitochondrial and cytosolic fractions were evaluated. When compared to control, glaucoma decreased mitochondrial ATP production (23%, p < 0.05), with an increase in superoxide and hydrogen peroxide production (30%, p < 0.01 and 28%, p < 0.05, respectively), whereas no changes were observed in membrane potential and oxygen consumption rate. In addition, the glaucoma group displayed a decrease in complex II activity (34%, p < 0.01). Moreover, NOX4 expression was increased in glaucoma compared to the control group (27%, p < 0.05). Regarding the activity of enzymes associated with the regulation of the redox status, glaucoma showed an increase in mitochondrial SOD activity (34%, p < 0.05), mostly due to an increase in Mn-SOD (50%, p < 0.05). A decrease in mitochondrial GST activity was observed (11%, p < 0.05). GR and TrxR activity were decreased in both mitochondrial (16%, p < 0.05 and 20%, p < 0.05 respectively) and cytosolic (21%, p < 0.01 and 50%, p < 0.01 respectively) fractions in the glaucoma group. Additionally, glaucoma showed an increase in cytoplasmatic GPx (50%, p < 0.01). In this scenario, redox imbalance took place resulting in damage to mitochondrial lipids (39%, p < 0.01) and proteins (70%, p < 0.05). These results suggest that glaucoma leads to mitochondrial function impairment in brain visual targets, that is accompanied by an alteration in both mitochondrial and cytoplasmatic enzymatic defenses. As a consequence of redox imbalance, oxidative damage to macromolecules takes place and can further affect vital cellular functions. Understanding the role of the mitochondria in the development and progression of the disease could bring up new neuroprotective therapies.

Visual cortex changes in children with sickle cell disease and normal visual acuity: a multimodal magnetic resonance imaging study.

The visual system is primarily affected in sickle cell disease (SCD), and eye examination is recommended starting in late childhood. So far, to our knowledge, all studies have focused on the retina, neglecting the changes that might be present in the cortical portion of the visual system. We performed a multimodal magnetic resonance imaging (MRI) evaluation of the visual cortex in 25 children with SCD (mean age: 12·3 ± 1·9 years) and 31 controls (mean age: 12·7 ± 1·6 years). At ophthalmologic examination, 3/25 SCD children had mild visual acuity deficits and 2/25 had mild tortuosity of the retinal vessels. None showed optic pathway infarcts at MRI or Transcranial Doppler abnormal blood velocities, and 6/25 disclosed posterior cerebral artery stenosis (five mild and one severe) at MR-angiography. Compared to controls, SCD children had increased posterior pericalcarine cortical thickness, with a different trajectory of cortical maturation and decreased connectivity within medial and ventral visual neural networks. Our findings suggest that SCD affects the development and the tuning of the visual cortex, leading to anatomical and functional changes in childhood even in the absence of retinopathy, and set the basis for future studies to determine if these changes can represent useful predictors of visual impairment in adulthood, biomarkers of disease progression or treatment response.

Distinct Laminar and Cellular Patterns of GABA Neuron Transcript Expression in Monkey Prefrontal and Visual Cortices.

The functional output of a cortical region is shaped by its complement of GABA neuron subtypes. GABA-related transcript expression differs substantially between the primate dorsolateral prefrontal cortex (DLPFC) and primary visual (V1) cortices in gray matter homogenates, but the laminar and cellular bases for these differences are unknown. Quantification of levels of GABA-related transcripts in layers 2 and 4 of monkey DLPFC and V1 revealed three distinct expression patterns: 1) transcripts with higher levels in DLPFC and layer 2 [e.g., somatostatin (SST)]; 2) transcripts with higher levels in V1 and layer 4 [e.g., parvalbumin (PV)], and 3) transcripts with similar levels across layers and regions [e.g., glutamic acid decarboxylase (GAD67)]. At the cellular level, these patterns reflected transcript- and cell type-specific differences: the SST pattern primarily reflected differences in the relative proportions of SST mRNA-positive neurons, the PV pattern primarily reflected differences in PV mRNA expression per neuron, and the GAD67 pattern reflected opposed patterns in the relative proportions of GAD67 mRNA-positive neurons and in GAD67 mRNA expression per neuron. These findings suggest that differences in the complement of GABA neuron subtypes and in gene expression levels per neuron contribute to the specialization of inhibitory neurotransmission across cortical circuits.

Visual System Impairment in a Mouse Model of Krabbe Disease: The Twitcher Mouse.

Krabbe disease (KD, or globoid cell leukodystrophy; OMIM #245200) is an inherited neurodegenerative condition belonging to the class of the lysosomal storage disorders. It is caused by genetic alterations in the gene encoding for the enzyme galactosylceramidase, which is responsible for cleaving the glycosydic linkage of galatosylsphingosine (psychosine or PSY), a highly cytotoxic molecule. Here, we describe morphological and functional alterations in the visual system of the Twitcher (TWI) mouse, the most used animal model of Krabbe disease. We report in vivo electrophysiological recordings showing defective basic functional properties of the TWI primary visual cortex. In particular, we demonstrate a reduced visual acuity and contrast sensitivity, and a delayed visual response. Specific neuropathological alterations are present in the TWI visual cortex, with reduced myelination, increased astrogliosis and microglia activation, and around the whole brain. Finally, we quantify PSY content in the brain and optic nerves by high-pressure liquid chromatography-mass spectrometry methods. An increasing PSY accumulation with time, the characteristic hallmark of KD, is found in both districts. These results represent the first complete characterization of the TWI visual system. Our data set a baseline for an easy testing of potential therapies for this district, which is also dramatically affected in KD patients.

Mesoscopic cortical network reorganization during recovery of optic nerve injury in GCaMP6s mice.

As the residual vision following a traumatic optic nerve injury can spontaneously recover over time, we explored the spontaneous plasticity of cortical networks during the early post-optic nerve crush (ONC) phase. Using in vivo wide-field calcium imaging on awake Thy1-GCaMP6s mice, we characterized resting state and evoked cortical activity before, during, and 31 days after ONC. The recovery of monocular visual acuity and depth perception was evaluated in parallel. Cortical responses to an LED flash decreased in the contralateral hemisphere in the primary visual cortex and in the secondary visual areas following the ONC, but was partially rescued between 3 and 5 days post-ONC, remaining stable thereafter. The connectivity between visual and non-visual regions was disorganized after the crush, as shown by a decorrelation, but correlated activity was restored 31 days after the injury. The number of surviving retinal ganglion cells dramatically dropped and remained low. At the behavioral level, the ONC resulted in visual acuity loss on the injured side and an increase in visual acuity with the non-injured eye. In conclusion, our results show a reorganization of connectivity between visual and associative cortical areas after an ONC, which is indicative of spontaneous cortical plasticity.

Evidence of cortical thickness reduction and disconnection in high myopia.

High myopia (HM) is associated with impaired long-distance vision. accumulating evidences reported that abnormal visual experience leads to dysfunction in brain activity in HM even corrected. However, whether the long-term of abnormal visual experience lead to neuroanatomical changes remain unknown, the aim at this study is to investigate the alternation of cortical surface thickness in HM patients. 82 patients with HM (HM groups), 57 healthy controls (HC groups) were recruited. All participants underwent high-resolution T1 and resting-state functional magnetic resonance imaging (MRI) scans. The cortical thickness analysis was preformed to investigate the neuroanatomical changes in HM patients using computational anatomy toolbox (CAT 12) toolbox. Compare with HCs, HM patients showed decreased the cortical surface thickness in the left middle occipital gyrus (MOG), left inferior parietal lobule (IPL), right inferior temporal gyrus (ITG), right precuneus, right primary visual area 1 (V1), right superior temporal gyrus (STG), right superior parietal lobule (SPL), right occipital pole, and right the primary motor cortex (M1), and increased to the parietal operculum (OP4) (P < 0.01, FWE-corrected), the mean cortical thickness of right orbitofrontal cortex (OFC), right dorsolateral prefrontal cortex (DLPFC) and right subcallosal cortex showed negatively correlation between clinical variables (axis length (ALM), the average macular thickness (AMT), keratometer (KER) 1, KER2, the mean KER, the mean macular fovea thickness (MFK), the refractive diopter) in HM patients. Our result mainly provided an evidence of cortical thickness reduction and disconnection in visual center and visual processing area, and cortical thickness increase in left multimodal integration region in HM patients. This may provide important significance of the study of the neural mechanism of HM.

The Retinal Inner Plexiform Synaptic Layer Mirrors Grey Matter Thickness of Primary Visual Cortex with Increased Amyloid ß Load in Early Alzheimer's Disease.

The retina may serve as putative window into neuropathology of synaptic loss in Alzheimer's disease (AD). Here, we investigated synapse-rich layers versus layers composed by nuclei/cell bodies in an early stage of AD. In addition, we examined the associations between retinal changes and molecular and structural markers of cortical damage. We recruited 20 AD patients and 17 healthy controls (HC). Combining optical coherence tomography (OCT), magnetic resonance (MR), and positron emission tomography (PET) imaging, we measured retinal and primary visual cortex (V1) thicknesses, along with V1 amyloid ß (Aß) retention ([11C]-PiB PET tracer) and neuroinflammation ([11C]-PK11195 PET tracer). We found that V1 showed increased amyloid-binding potential, in the absence of neuroinflammation. Although thickness changes were still absent, we identified a positive association between the synapse-rich inner plexiform layer (IPL) and V1 in AD. This retinocortical interplay might reflect changes in synaptic function resulting from Aß deposition, contributing to early visual loss.