Abstract
Most individuals with congenital achromatopsia (ACHM) carry mutations that affect the retinal phototransduction pathway of cone photoreceptors, fundamental to both high acuity vision and colour perception. As the central fovea is occupied solely by cones, achromats have an absence of retinal input to the visual cortex and a small central area of blindness. Additionally, those with complete ACHM have no colour perception, and colour processing regions of the ventral cortex also lack typical chromatic signals from the cones. This study examined the cortical morphology (grey matter volume, cortical thickness, and cortical surface area) of multiple visual cortical regions in ACHM (n = 15) compared to normally sighted controls (n = 42) to determine the cortical changes that are associated with the retinal characteristics of ACHM. Surface-based morphometry was applied to T1-weighted MRI in atlas-defined early, ventral and dorsal visual regions of interest. Reduced grey matter volume in V1, V2, V3, and V4 was found in ACHM compared to controls, driven by a reduction in cortical surface area as there was no significant reduction in cortical thickness. Cortical surface area (but not thickness) was reduced in a wide range of areas (V1, V2, V3, TO1, V4, and LO1). Reduction in early visual areas with large foveal representations (V1, V2, and V3) suggests that the lack of foveal input to the visual cortex was a major driving factor in morphological changes in ACHM. However, the significant reduction in ventral area V4 coupled with the lack of difference in dorsal areas V3a and V3b suggest that deprivation of chromatic signals to visual cortex in ACHM may also contribute to changes in cortical morphology. This research shows that the congenital lack of cone input to the visual cortex can lead to widespread structural changes across multiple visual areas.
Highlights
Congenital achromatopsia (ACHM; known as rod monochromacy) is a largely stationary, genetically heterogeneous and predominantly autosomal recessive retinal disorder with a prevalence of approximately ∼1 in 30,000 people (Francois, 1961; Kohl et al, 2004; Aboshiha et al, 2016)
It appears that ventral visual area V4 is disproportionately affected by reductions in surface area and volume compared to the dorsal pathway areas V3a and V3b
We found no significant differences in cortical thickness in ACHM in any visual area
Summary
Congenital achromatopsia (ACHM; known as rod monochromacy) is a largely stationary, genetically heterogeneous and predominantly autosomal recessive retinal disorder with a prevalence of approximately ∼1 in 30,000 people (Francois, 1961; Kohl et al, 2004; Aboshiha et al, 2016). The functional integrity of cone photoreceptors is fundamental for the mediation of photopic (bright light) vision, high visual acuity and colour perception. In the normally-sighted, the central fovea of the retina is composed exclusively of cone photoreceptors providing high visual acuity. Individuals with complete ACHM have a central scotoma where rods are absent and a complete loss of colour vision from birth, along with reduced visual acuity (Haegerstrom-Portnoy et al, 1996a,b; Remmer et al, 2015; Hirji et al, 2018). In normally-sighted individuals, the foveal region of the visual field dominated by cones is overrepresented in the visual cortex (cortical magnification). This means that in ACHM a disproportionately large area of the visual cortex receives atypical input due to the defective cone photoreceptors. It is possible that the lack of visual input to foveal representations within visual regions throughout the cortex may influence the structural characteristics of the visual cortex within this patient population
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