The Organization of the Visual Cortex in Patients with Scotomata Resulting from Lesions of the Central Retina

Cerebral Modifications and Visual Pathway Reorganization in Maculopathy: A Systematic Review

Background

Macular degeneration (MD) is one of the most frequent causes of visual deficit, resulting in alterations affecting not only the retina but also the entire visual pathway up to the brain areas. This would seem related not just to signal deprivation but also to a compensatory neuronal reorganization, having significant implications in terms of potential rehabilitation of the patient and therapeutic perspectives.

Objective

This paper aimed to outline, by analyzing the existing literature, the current understanding of brain structural and functional changes detected with neuroimaging techniques in subjects affected by juvenile and age-related maculopathy.

Methods

Articles using various typologies of central nervous system (CNS) imaging in at least six patients affected by juvenile or age-related maculopathy were considered. A total of 142 were initially screened. Non-pertinent articles and duplicates were rejected. Finally, 19 articles, including 649 patients, were identified.

Results

In these sources, both structural and functional modifications were found in MD subjects' CNS. Changes in visual cortex gray matter volume were observed in both age-related MD (AMD) and juvenile MD (JMD); in particular, an involvement of not only its posterior part but also the anterior one suggests further causes besides an input-deprivation mechanism only. White matter degeneration was also found, more severe in JMD than in AMD. Moreover, functional analysis revealed differences in cortical activation patterns between MD and controls, suggesting neuronal circuit reorganization. Interestingly, attention and oculomotor training allowed better visual performances and correlated to a stronger cortical activation, even of the area normally receiving inputs from lesioned macula.

Conclusion

In MD, structural and functional changes in cerebral circuits and visual pathway can happen, involving both cerebral volume and activation patterns. These modifications, possibly due to neuronal plasticity (already observed and described for several brain areas), can allow patients to compensate for macular damage and gives therapeutic perspectives which could be achievable through an association between oculomotor training and biochemical stimulation of neuronal plasticity.

Enhanced Visual Attentional Modulation in Patients with Inherited Peripheral Retinal Degeneration in the Absence of Cortical Degeneration

The role of attentional mechanisms in peripheral vision loss remains an outstanding question. Our study was aimed at determining the effect of genetically determined peripheral retinal dystrophy caused by Retinitis Pigmentosa (RP) on visual cortical function and tested the recruitment of attentional mechanisms using functional magnetic resonance imaging (fMRI). We included thirteen patients and twenty-two age- and gender-matched controls. We analyzed cortical responses under attentional demands and passive viewing conditions while presenting a visual stimulus covering the central and paracentral visual field. Brain activity was studied in visual areas V1, V2, and V3 as well as in cortical regions of interest corresponding to the preserved and the damaged visual field. The influence of visual field extent and age of disease onset were also investigated. Cortical thickness of visual areas was also measured. We found that cortical visual responses under attentional demands were increased in patients with larger degeneration of visual field, as demonstrated by significant interaction effects between group and task conditions. Moreover, activation during the task condition was increased for patients in two cortical regions of interest corresponding to the preserved and damaged visual field, specifically in patients with severe visual field loss. These findings were observed in the presence of preserved visual cortical structure. We conclude that RP patients have enhanced visual attention recruitment despite their retinal degeneration, while cortical structure and overall response levels remain intact. The unmasking of feedback signals from higher level visual regions involved in attentional processes may explain the increased cortical responses. These findings are relevant for the design of strategies for treating retinal diseases, based on attentional cuing.

fMRI with Central Vision Loss: Effects of Fixation Locus and Stimulus Type

PURPOSE

In patients with central visual field scotomata, a large part of visual cortex is not adequately stimulated. Patients often use a new eccentric fixation area on intact peripheral retina ("preferred retinal locus"-PRL) that functions as a pseudo-fovea. We used functional magnetic resonance imaging (fMRI) to examine whether stimulating this pseudo-fovea leads to increased activation or altered activation patterns in visual cortex in comparison to stimulating a comparable peripheral area in the opposite hemifield (OppPRL).

METHODS

Nineteen patients with binocular central scotomata caused by hereditary retinal dystrophies and an age-matched control group were tested. The center of the visual field, PRL, and OppPRL were stimulated with flickering checkerboard stimuli and object pictures during fMRI measurement.

RESULTS

Results show that stimulation with pictures of everyday objects led to overall larger BOLD (blood oxygen level dependent) responses in visual cortex compared to that evoked by stimulation with flickering checkerboards. Patients showed this enhancement as early as in V1. When the PRL was directly stimulated with object pictures, the central representation area in early visual cortex was coactivated in the patients but not in the controls. In higher visual areas beyond retinotopic cortex, BOLD responses to stimulation of the PRL with object pictures were significantly enhanced in comparison to stimulation of the OppPRL area. Highly stable eccentric fixation with the PRL was associated with a higher BOLD signal in visual cortex in patients, and this effect was most pronounced in the conditions with object picture stimulation.

CONCLUSIONS

The observed results suggest that naturalistic images are more likely to trigger top-down processes that regulate activation in early visual cortex in patients with central vision loss.

Primary visual cortical remapping in patients with inherited peripheral retinal degeneration

Human studies addressing the long-term effects of peripheral retinal degeneration on visual cortical function and structure are scarce. Here we investigated this question in patients with Retinitis Pigmentosa (RP), a genetic condition leading to peripheral visual degeneration. We acquired functional and anatomical magnetic resonance data from thirteen patients with different levels of visual loss and twenty-two healthy participants to study primary (V1) visual cortical retinotopic remapping and cortical thickness. We identified systematic visual field remapping in the absence of structural changes in the primary visual cortex of RP patients. Remapping consisted in a retinotopic eccentricity shift of central retinal inputs to more peripheral locations in V1. Importantly, this was associated with changes in visual experience, as assessed by the extent of the visual loss, with more constricted visual fields resulting in larger remapping. This pattern of remapping is consistent with expansion or shifting of neuronal receptive fields into the cortical regions with reduced retinal input. These data provide evidence for functional changes in V1 that are dependent on the magnitude of peripheral visual loss in RP, which may be explained by rapid cortical adaptation mechanisms or long-term cortical reorganization. This study highlights the importance of analyzing the retinal determinants of brain functional and structural alterations for future visual restoration approaches.

Organization of the Central Visual Pathways Following Field Defects Arising from Congenital, Inherited, and Acquired Eye Disease

Visual field defects that arise from eye disease are increasing as human life spans lengthen. The consequences of visual field defects on the central visual pathways are important to assess, particularly in light of potential treatments of eye disease that restore function to the retina. For individuals with field defects arising from congenital eye disease, primary visual cortex (V1) appears to remap, whereas this form of reorganization is not present in individuals with field defects that arise later in life as a result of inherited or acquired eye disease. However, research has revealed that the areas of V1 that normally map the visual field defect are active under specific circumstances. This review attempts to resolve whether or not this activity reflects reorganization of the central visual pathways. Alongside the measures of function are measures of anatomical properties of the human visual pathway, which demonstrate transneuronal degeneration in individuals with eye disease. These results are concerning because degeneration of the central visual pathways may ultimately limit the success of sight-restoring treatments of eye disease.

Neural correlates of visual search in patients with hereditary retinal dystrophies

In patients with central visual field scotomata a large part of visual cortex is not adequately stimulated. We investigated evidence for possible upregulation in cortical responses in 22 patients (8 females, 14 males; mean age 41.5 years, range 12-65 years) with central visual field loss due to hereditary retinal dystrophies (Stargardt's disease, other forms of hereditary macular dystrophies and cone-rod dystrophy) and compared their results to those of 22 age-matched controls (11 females, 11 males; mean age, 42.4 years, range, 13-70 years). Using functional magnetic resonance imaging (fMRI) we recorded differences in behavioral and BOLD signal distribution in retinotopic mapping and visual search tasks. Patients with an established preferred retinal locus (PRL) exhibited significantly higher activation in early visual cortex during the visual search task, especially on trials when the target stimuli fell in the vicinity of the PRL. Compared with those with less stable fixation, patients with stable eccentric fixation at the PRL exhibited greater performance levels and more brain activation.

Mapping the visual brain: how and why

Over the past 15 years, techniques for identifying visual areas using magnetic resonance imaging (MRI) in human subjects have been applied widely to multiple populations. This review will cover the basic techniques of using functional MRI and very high-resolution structural MRI to determine boundaries between different areas of the visual cortex. Recent applications of these methods to ophthalmological patient populations are discussed, and the future potential applications of very high field strength MRI are considered.