Abstract
The eye disease macular degeneration (MD) is a leading cause of blindness worldwide. There is no cure for MD, but several promising treatments aimed at restoring vision at the level of the retina are currently under investigation. These treatments assume that the patient's brain can still process appropriately the retinal input once it is restored, but whether this assumption is correct has yet to be determined. We used functional magnetic resonance imaging (fMRI) and connective field modelling to determine whether the functional connectivity between the input-deprived portions of primary visual cortex (V1) and early extrastriate areas (V2/3) is still retinotopically organised. Specifically, in both patients with juvenile macular degeneration and age-matched controls with simulated retinal lesions, we assessed the extent to which the V1-referred connective fields of extrastriate voxels, as estimated on the basis of spontaneous fMRI signal fluctuations, adhered to retinotopic organisation. We found that functional connectivity between the input-deprived portions of visual areas V1 and extrastriate cortex is still largely retinotopically organised in MD, although on average less so than in controls. Patients with stable fixation exhibited normal retinotopic connectivity, however, suggesting that for the patients with unstable fixation, eye-movements resulted in spurious, homogeneous signal modulations across the entire input-deprived cortex, which would have hampered our ability to assess their spatial structure of connectivity. Despite the prolonged loss of visual input due to MD, the cortico-cortical connections of input-deprived visual cortex remain largely intact. This suggests that the restoration of sight in macular degeneration can rely on a largely unchanged retinotopic representation in early visual cortex following loss of central retinal function.
Highlights
Macular degeneration (MD) is an eye disease causing a progressive degeneration of the photoreceptor cells in the centre of the retina, and results in foveal vision loss
We first evaluated whether connective field modelling was capable of tracing retinotopic connectivity within the input-deprived portions of visual cortex in healthy controls with simulated lesions
If input-deprived V1 exhibits polarangle preserving connectivity, the connective field centres of voxels within the dorsal-right extrastriate ROI should be located in dorsal right V1, whereas the connective field centres of voxels within the ventral-right extrastriate ROI should be located in ventral right V1, and a similar pattern should be observed in the left hemisphere
Summary
Macular degeneration (MD) is an eye disease causing a progressive degeneration of the photoreceptor cells in the centre of the retina, and results in foveal vision loss. It has been suggested that visual processing in input-deprived visual cortex undergoes large-scale reorganisation in some, but not all patients.[2] That is, in the prolonged absence of visual stimulation, cortical neurons would shift their receptive fields toward the portions of the visual field that are still intact, thereby regaining visual sensitivity. While it is largely reassuring that deprived primary visual cortex is generally not remapped,[8,9,10] the reported anatomical changes in early visual cortex could still have adversely affected the functional cortico-cortical connections of the deprived cortex to areas downstream This raises the question whether the visual brain would still be able to process appropriately retinal input—were this to be restored
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