Morphometric Changes of the Corpus Callosum in Congenital Blindness

We examined the effects of visual deprivation at birth on the development of the corpus callosum in a large group of congenitally blind individuals. We acquired high-resolution T1-weighted MRI scans in 28 congenitally blind and 28 normal sighted subjects matched for age and gender. There was no overall group effect of visual deprivation on the total surface area of the corpus callosum. However, subdividing the corpus callosum into five subdivisions revealed significant regional changes in its three most posterior parts. Compared to the sighted controls, congenitally blind individuals showed a 12 reduction in the splenium, and a 20 increase in the isthmus and the posterior part of the body. A shape analysis further revealed that the bending angle of the corpus callosum was more convex in congenitally blind compared to the sighted control subjects. The observed morphometric changes in the corpus callosum are in line with the well-described cross-modal functional and structural neuroplastic changes in congenital blindness

Optic radiations representing different eccentricities age differently

The neural pathways that carry information from the foveal, macular, and peripheral visual fields have distinct biological properties. The optic radiations (OR) carry foveal and peripheral information from the thalamus to the primary visual cortex (V1) through adjacent but separate pathways in the white matter. Here, we perform white matter tractometry using pyAFQ on a large sample of diffusion MRI (dMRI) data from subjects with healthy vision in the U.K. Biobank dataset (UKBB; N = 5382; age 45-81). We use pyAFQ to characterize white matter tissue properties in parts of the OR that transmit information about the foveal, macular, and peripheral visual fields, and to characterize the changes in these tissue properties with age. We find that (1) independent of age there is higher fractional anisotropy, lower mean diffusivity, and higher mean kurtosis in the foveal and macular OR than in peripheral OR, consistent with denser, more organized nerve fiber populations in foveal/parafoveal pathways, and (2) age is associated with increased diffusivity and decreased anisotropy and kurtosis, consistent with decreased density and tissue organization with aging. However, anisotropy in foveal OR decreases faster with age than in peripheral OR, while diffusivity increases faster in peripheral OR, suggesting foveal/peri-foveal OR and peripheral OR differ in how they age

Simultaneous Assessment of White Matter Changes in Microstructure and Connectedness in the Blind Brain

Magnetic resonance imaging (MRI) of the human brain has provided converging evidence that visual deprivation induces regional changes in white matter (WM) microstructure. It remains unclear how these changes modify network connections between brain regions. Here we used diffusion-weighted MRI to relate differences in microstructure and structural connectedness of WM in individuals with congenital or late-onset blindness relative to normally sighted controls. Diffusion tensor imaging (DTI) provided voxel-specific microstructural features of the tissue, while anatomical connectivity mapping (ACM) assessed the connectedness of each voxel with the rest of the brain. ACM yielded reduced anatomical connectivity in the corpus callosum in individuals with congenital but not late-onset blindness. ACM did not identify any brain region where blindness resulted in increased anatomical connectivity. DTI revealed widespread microstructural differences as indexed by a reduced regional fractional anisotropy (FA). Blind individuals showed lower FA in the primary visual and the ventral visual processing stream relative to sighted controls regardless of the blindness onset. The results show that visual deprivation shapes WM microstructure and anatomical connectivity, but these changes appear to be spatially dissociated as changes emerge in different WM tracts. They also indicate that regional differences in anatomical connectivity depend on the onset of blindness

Impact of ocular disease on circadian rhythms and brain connectivity

Investigation into the impact of ocular disease on sleep and mood has shown that in humans eyes have an important role, and that absence of eyes or interference with light reaching the retina can have deleterious effects. Light is the main zeitgeber ‘time-giver’ used by most species for the regulation of circadian rhythms and is detected by rods, cones and photosensitive retinal ganglion cells (pRGCs) in mammals. The aims of this research project were to investigate this from three different perspectives. Three prospective studies were undertaken. The first, studied the impact of ocular disease on the sleep/wake cycle in diabetic retinopathy (DR) and in bilateral anophthalmia. There was no significant difference found between the severity of DR and global sleep scores, however the acquired anophthalmic groups have significantly raised global sleep scores compared to controls and the congenital anophthalmic group. Both anophthalmic groups had varying sleep/wake cycles on their actograms depending on the lifestyle (independent of the urinary melatonin). All the anophthalmic participants showed a non 24 hour sleep-wake rhythm disorder after melatonin profiling. The second study investigates the evidence for the presence of extraocular circadian photoreceptors (EOCP) in participants with anophthalmia and sighted controls. Changes in brain activity using a functional MRI scan was assessed, when a bright white light is shining in different locations. This study did not reveal any evidence of EOCP.Finally, structural brain MRI differences in anophthalmic groups were investigated. While similar changes in structural reorganisation occur in all anophthalmic groups in the occipital cortex, the acquired anophthalmic groups show an inverse relation with the time since becoming anophthalmic and the volume of optic radiation and optic nerve volume. The acquired anophthalmic group did not show increase in hippocampal volume (memory areas) or in the precuneus (spatial navigation) contrast to the congenital anophthalmic groups

Visual callosal topography in the absence of retinal input.

Using probabilistic diffusion tractography, we examined the retinotopic organization of splenial callosal connections within early blind, anophthalmic, and control subjects. Early blind subjects experienced prenatal retinal "waves" of spontaneous activity similar to those of sighted subjects, and only lack postnatal visual experience. In anophthalmia, the eye is either absent or arrested at an early prenatal stage, depriving these subjects of both pre- and postnatal visual input. Therefore, comparing these two groups provides a way of separating the influence of pre- and postnatal retinal input on the organization of visual connections across hemispheres. We found that retinotopic mapping within the splenium was not measurably disrupted in early blind or anophthalmic subjects compared to visually normal controls. No significant differences in splenial volume were observed across groups. No significant differences in diffusivity were found between early blind subjects and sighted controls, though some differences in diffusivity were noted between anophthalmic subjects and controls. These results suggest that neither prenatal retinal activity nor postnatal visual experience plays a role in the large-scale topographic organization of visual callosal connections within the splenium

Visual callosal topography in the absence of retinal input.

Using probabilistic diffusion tractography, we examined the retinotopic organization of splenial callosal connections within early blind, anophthalmic, and control subjects. Early blind subjects experienced prenatal retinal "waves" of spontaneous activity similar to those of sighted subjects, and only lack postnatal visual experience. In anophthalmia, the eye is either absent or arrested at an early prenatal stage, depriving these subjects of both pre- and postnatal visual input. Therefore, comparing these two groups provides a way of separating the influence of pre- and postnatal retinal input on the organization of visual connections across hemispheres. We found that retinotopic mapping within the splenium was not measurably disrupted in early blind or anophthalmic subjects compared to visually normal controls. No significant differences in splenial volume were observed across groups. No significant differences in diffusivity were found between early blind subjects and sighted controls, though some differences in diffusivity were noted between anophthalmic subjects and controls. These results suggest that neither prenatal retinal activity nor postnatal visual experience plays a role in the large-scale topographic organization of visual callosal connections within the splenium

Neural and socio-cognitive sequelae of congenital visual impairment during midchildhood

Previous studies identified cognitive difficulties in individuals with congenital visual impairment that significantly impacted on wellbeing and educational attainment. However, factors leading to adverse outcome remained unclear. The current study aimed to identify associations and mechanisms of specific cognitive deficits associated with visual impairment from a neurodevelopmental perspective. Based on recent theoretical advances (Johnson, 2011), it was assumed that visual impairment leads to differences in cognition by influencing experience-driven brain maturational processes, which support the integration between cortical areas to support cognitive processes. In order to explore this hypothesis, children with visual impairment due to disorders that were thought to only affect peripheral sensory parts of the visual system were assessed on neuropsychological instruments covering a range of functional domains. Further, structural and functional neurophysiological methods were employed to establish the impact of visual impairment on brain organisation. The results of the present work confirm earlier reports of specific deficits in spatial memory, social understanding, and aspects of executive function, despite typical performance in other domains. In addition, the current study is the first study to indicate dosage-dependence with some aspects of social communication being even impaired in children with only mild to moderate visual impairment, while aspects of executive function and spatial memory were only found to be deficient in children with more severe forms of visual impairment. Further, neurophysiological investigations indicated differences in structural and functional brain organisation in children with VI that related to differences in behavioural performance. In general, the results of the present study suggest that visual sensory impairment impacts on brain and cognitive development with important implications for education and clinical treatment of children with visual impairment