Loss and enhancement of layer-selective signals in geniculostriate and corticotectal pathways of adult human amblyopia

Analyses of eye movement parameters in children with anisometropic amblyopia

OBJECTIVE

To investigate the characteristics of eye movement in children with anisometropic amblyopia, and to compare those characteristics with eye movement in a control group.

METHODS

31 children in the anisometropic amblyopia group (31 amblyopic eyes in group A, 31 contralateral eyes in group B) and 24 children in the control group (48 eyes in group C). Group A was subdivided into groups Aa (severe amblyopia) and Ab (mild-moderate amblyopia). The overall age range was 6-12 years (mean, 7.83 ± 1.79 years). All children underwent ophthalmic examinations; eye movement parameters including saccade latency and amplitude were evaluated using an Eyelink1000 eye tracker. Data Viewer and MATLAB software were used for data analysis.

RESULTS

Mean and maximum saccade latencies, as well as mean and maximum saccade amplitudes, were significantly greater in group A than in groups B and C before and after treatment (P < 0.05). Mean and maximum saccade latencies were significantly different among groups Aa, Ab, and C (P < 0.05). Pupil trajectories in two detection modes suggested that binocular fixation was better than monocular fixation.

CONCLUSIONS

Eye movement parameters significantly differed between contralateral normal eyes and control eyes. Clinical evaluation of children with anisometropic amblyopia should not focus only on static visual acuity, but also on the assessment of eye movement.

Deficits of the "Good" Eye in Amblyopia: Processing Geometric Properties

Purpose

Although fellow eyes of amblyopia are typically considered normal, recent studies have revealed impairments in certain aspects of vision. However, it remains unclear at which level of object processing these impairments occur. This study aims to investigate the functional level of visual perception impairment in the fellow eye of children and adults with amblyopia using the geometric functional hierarchy discrimination task based on Klein Mathematics methodology.

Methods

Seventy-six patients with amblyopia (40 children and 36 adults) and 77 age-matched healthy controls (40 children and 37 adults) were recruited for this study. The participants completed four sets of geometric hierarchies (in ascending order of stability: Euclidean, affine, projective, and topology) and one set of color discrimination tasks. They were instructed to rapidly and accurately select a distinct shape from the four quadrants.

Results

The participants' performance was evaluated using the inverse efficiency (IE) score (IE = response time (RT)/accuracy). The results of IEs show that the fellow eye of children with amblyopia exhibits normal topological processing, yet displays higher IEs in other geometric properties and color processing, suggesting impairments in these specific discrimination abilities. However, adults with amblyopia did not show deficits on any discrimination types compared with adult controls.

Conclusions

The lack of compromised topological processing suggests that amblyopia may not have inflicted any damage to the subcortical visual pathways. Furthermore, these deficits observed in the fellow eye tend to diminish significantly during adulthood, implying that amblyopia may potentially hinder the maturation process of the fellow eye.

Evidence for adaptive myelination of subcortical shortcuts for visual motion perception in healthy adults

Conscious visual motion information follows a cortical pathway from the retina to the lateral geniculate nucleus (LGN) and on to the primary visual cortex (V1) before arriving at the middle temporal visual area (MT/V5). Alternative subcortical pathways that bypass V1 are thought to convey unconscious visual information. One flows from the retina to the pulvinar (PUL) and on to medial temporal visual area (MT); while the other directly connects the LGN to MT. Evidence for these pathways comes from non-human primates and modest-sized studies in humans with brain lesions. Thus, the aim of the current study was to reconstruct these pathways in a large sample of neurotypical individuals and to determine the degree to which these pathways are myelinated, suggesting information flow is rapid. We used the publicly available 7T (N = 98; 'discovery') and 3T (N = 381; 'validation') diffusion magnetic resonance imaging datasets from the Human Connectome Project to reconstruct the PUL-MT (including all subcompartments of the PUL) and LGN-MT pathways. We found more fibre tracts with greater density in the left hemisphere. Although the left PUL-MT path was denser, the bilateral LGN-MT tracts were more heavily myelinated, suggesting faster signal transduction. We suggest that this apparent discrepancy may be due to 'adaptive myelination' caused by more frequent use of the LGN-MT pathway that leads to greater myelination and faster overall signal transmission.

Structural and functional alterations in the brains of patients with anisometropic and strabismic amblyopia: a systematic review of magnetic resonance imaging studies

Amblyopia is the most common cause of vision loss in children and can persist into adulthood in the absence of effective intervention. Previous clinical and neuroimaging studies have suggested that the neural mechanisms underlying strabismic amblyopia and anisometropic amblyopia may be different. Therefore, we performed a systematic review of magnetic resonance imaging studies investigating brain alterations in patients with these two subtypes of amblyopia; this study is registered with PROSPERO (registration ID: CRD42022349191). We searched three online databases (PubMed, EMBASE, and Web of Science) from inception to April 1, 2022; 39 studies with 633 patients (324 patients with anisometropic amblyopia and 309 patients with strabismic amblyopia) and 580 healthy controls met the inclusion criteria (e.g., case-control designed, peer-reviewed articles) and were included in this review. These studies highlighted that both strabismic amblyopia and anisometropic amblyopia patients showed reduced activation and distorted topological cortical activated maps in the striate and extrastriate cortices during task-based functional magnetic resonance imaging with spatial-frequency stimulus and retinotopic representations, respectively; these may have arisen from abnormal visual experiences. Compensations for amblyopia that are reflected in enhanced spontaneous brain function have been reported in the early visual cortices in the resting state, as well as reduced functional connectivity in the dorsal pathway and structural connections in the ventral pathway in both anisometropic amblyopia and strabismic amblyopia patients. The shared dysfunction of anisometropic amblyopia and strabismic amblyopia patients, relative to controls, is also characterized by reduced spontaneous brain activity in the oculomotor cortex, mainly involving the frontal and parietal eye fields and the cerebellum; this may underlie the neural mechanisms of fixation instability and anomalous saccades in amblyopia. With regards to specific alterations of the two forms of amblyopia, anisometropic amblyopia patients suffer more microstructural impairments in the precortical pathway than strabismic amblyopia patients, as reflected by diffusion tensor imaging, and more significant dysfunction and structural loss in the ventral pathway. Strabismic amblyopia patients experience more attenuation of activation in the extrastriate cortex than in the striate cortex when compared to anisometropic amblyopia patients. Finally, brain structural magnetic resonance imaging alterations tend to be lateralized in the adult anisometropic amblyopia patients, and the patterns of brain alterations are more limited in amblyopic adults than in children. In conclusion, magnetic resonance imaging studies provide important insights into the brain alterations underlying the pathophysiology of amblyopia and demonstrate common and specific alterations in anisometropic amblyopia and strabismic amblyopia patients; these alterations may improve our understanding of the neural mechanisms underlying amblyopia.

Contour integration deficits at high spatial frequencies in children treated for anisometropic amblyopia

Purpose

This study was conducted to reexamine the question of whether children treated for anisometropic amblyopia have contour integration deficits. To do so, we used psychophysical methods that require global contour processing while minimizing the influence of low-level deficits: visibility, shape perception, and positional uncertainty.

Methods

Thirteen children with anisometropic amblyopia (age: 10.1 ± 1.8 years) and thirteen visually normal children (age: 10.8 ± 2.0 years) participated in this study. The stimuli were closed figures made up of Gabor patches either in noise or on a blank field. The contrast thresholds to detect a circular contour on a blank field, as well as the thresholds of aspect ratio and contour element number to discriminate a circular or elliptical contour in noise, were measured at Gabor spatial frequencies of 1.5, 3, and 6 cpd for amblyopic eyes (AEs), fellow eyes (FEs), and normal control eyes. Visual acuities and contrast sensitivity functions for AEs and FEs and the Randot stereoacuity were measured before testing.

Results

The AEs showed contrast deficits and degraded shape perception compared to the FEs at higher spatial frequencies (6 cpd). When the influence of abnormal contrast sensitivity and shape perception were minimized, the AEs showed contour integration deficits at spatial frequencies 3 and 6 cpd. These deficits were not related to basic losses in contrast sensitivity and acuity, stereoacuity, and visual crowding. Besides, no significant difference was found between the fellow eyes of the amblyopic children and the normal control eyes in the performance of contour integration.

Conclusion

After eliminating or compensating for the low-level deficits, children treated for anisometropic amblyopia still show contour integration deficits, primarily at higher spatial frequencies, which might reflect the deficits in global processing caused by amblyopia. Contour integration deficits are likely independent of spatial vision deficits. Refractive correction and/or occlusion therapies may not be sufficient to fully restore contour integration deficits, which indicates the need for the development of clinical treatments to recover these deficits.

White matter BOLD signals at 7 Tesla reveal visual field maps in optic radiation and vertical occipital fasciculus

There is growing evidence that blood-oxygen-level-dependent (BOLD) activity in the white matter (WM) can be detected by functional magnetic resonance imaging (fMRI). However, the functional relevance and significance of WM BOLD signals remain controversial. Here we investigated whether 7T BOLD fMRI can reveal fine-scale functional organizations of a WM bundle. Population receptive field (pRF) analyses of the 7T retinotopy dataset from the Human Connectome Project revealed clear contralateral retinotopic organizations of two visual WM bundles: the optic radiation (OR) and the vertical occipital fasciculus (VOF). The retinotopic maps of OR are highly consistent with post-mortem dissections and diffusion tractographies, while the VOF maps are compatible with the dorsal and ventral visual areas connected by the WM. Similar to the grey matter (GM) visual areas, both WM bundles show over-representations of the central visual field and increasing pRF size with eccentricity. Hemodynamic response functions of visual WM were slower and wider compared with those of GM areas. These findings clearly demonstrate that WM BOLD at 7 Tesla is closely coupled with neural activity related to axons, encoding highly specific information that can be used to characterize fine-scale functional organizations of a WM bundle.

Cognitive dysfunctions in high myopia: An overview of potential neural morpho-functional mechanisms

Dementia and cognitive impairment (CIM) carry high levels of mortality. Visual impairment (VI) is linked with CIM risk. High myopia (HM) is a chronic disease frequently leading to irreversible blindness. Current opinion has shifted from retinal injury as the cause of HM to the condition being considered an eye-brain disease. However, the pathogenesis of this disease and the manner in which neural structures are damaged are poorly understood. This review comprehensively discusses the relationship between HM, the central nervous system, and CIM, together with the novel concept of three visual pathways, and possible research perspectives.

Amblyopia: progress and promise of functional magnetic resonance imaging

Amblyopia is a neurodevelopmental disorder characterized by functional deficits in the visual cortex. Functional magnetic resonance imaging (fMRI) is the most commonly used neuroimaging technique for investigating amblyopia. Herein, we systematically searched a PubMed database from inception to December 2021 to highlight the current progress and promises about fMRI technology in amblyopia; amblyopia's neural mechanism, the comparison of different types of amblyopia, and the evaluation of the therapeutic effect were explored. Relevant articles published in English and appropriate cross-references were considered for inclusion, including basic studies, imaging techniques, clinical diagnostic and therapeutic studies, case series, and reviews.

Modulation of mean luminance improves binocular balance across spatial frequencies in amblyopia

Amblyopia is a visual impairment that perturbs binocular balance at high spatial frequencies in favor of the fellow eye. Studies reveal that amblyopes who had been treated with monocular therapies still show imbalance. Binocular balance is achieved when both eyes’ inputs are weighed equally. A reduced light can diminish the dimmed eye's weight in binocular combination. In this study, we examined if binocular balance across spatial frequencies could be improved by reducing the luminance of the fellow eye in adult amblyopes. By doing so, we relieved their binocular imbalance across spatial frequencies. Also, normal observers showed amblyopic binocular imbalance when the dominant eye’s light level was dimmed. Therefore, reducing the luminance in the unaffected eye in amblyopia mitigated the binocular imbalance, whereas doing so in normal adults simulated the amblyopic imbalance across spatial frequencies.

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Binocular balance is impaired in amblyopia

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Reduced luminance of the fellow eye can improve it across spatial frequencies in amblyopia

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Reduced luminance of one eye from normal observers simulates amblyopic imbalance

Psychophysical Reverse Correlation Revealed Broader Orientation Tuning and Prolonged Reaction Time in Amblyopia

Purpose

Neural selectivity of orientation is a fundamental property of visual system. We aim to investigate whether and how the orientation selectivity changes in amblyopia.

Methods

Seventeen patients with amblyopia (27.1 ± 7.1 years) and 18 healthy participants (25.1 ± 2.7 years) took part in this study. They were asked to continuously detect vertical gratings embedded in a stream of randomly oriented gratings. Using a technique of subspace reverse correlation, the orientation-time perceptive field (PF) for the atypical grating detection task was derived for each participant. Detailed comparisons were made between the PFs measured with the amblyopic and healthy eyes.

Results

The PF of the amblyopic eyes showed significant differences in orientation and time domain compared with that of the normal eyes (cluster-based permutation test, ps < 0.05), with broader bandwidth of orientation tuning (31.41 ± 10.59 degrees [mean ± SD] vs. 24.76 ± 6.85 degrees, P = 0.039) and delayed temporal dynamics (483 ± 68 ms vs. 425 ± 58 ms, P = 0.015). None of the altered PF properties correlated with the contrast sensitivity at 1 cycle per degree (c/deg) in amblyopia. No difference in PFs between the dominant and non-dominant eyes in the healthy group was found.

Conclusions

The altered orientation-time PF to the low spatial frequency and high contrast stimuli suggests amblyopes had coarser orientation selectivity and prolonged reaction time. The broader orientation tuning probably reflects the abnormal lateral interaction in the primary visual cortex, whereas the temporal delay might indicate a high level deficit.