Abstract
Carriers of blue cone monochromacy have fewer cone photoreceptors than normal. Here we examine how this disruption at the level of the retina affects visual function and cortical organization in these individuals. Visual resolution and contrast sensitivity was measured at the preferred retinal locus of fixation and visual resolution was tested at two eccentric locations (2.5° and 8°) with spectacle correction only. Adaptive optics corrected resolution acuity and cone spacing were simultaneously measured at several locations within the central fovea with adaptive optics scanning laser ophthalmoscopy (AOSLO). Fixation stability was assessed by extracting eye motion data from AOSLO videos. Retinotopic mapping using fMRI was carried out to estimate the area of early cortical regions, including that of the foveal confluence. Without adaptive optics correction, BCM carriers appeared to have normal visual function, with normal contrast sensitivity and visual resolution, but with AO-correction, visual resolution was significantly worse than normal. This resolution deficit is not explained by cone loss alone and is suggestive of an associated loss of retinal ganglion cells. However, despite evidence suggesting a reduction in the number of retinal ganglion cells, retinotopic mapping showed no reduction in the cortical area of the foveal confluence. These results suggest that ganglion cell density may not govern the foveal overrepresentation in the cortex. We propose that it is not the number of afferents, but rather the content of the information relayed to the cortex from the retina across the visual field that governs cortical magnification, as under normal viewing conditions this information is similar in both BCM carriers and normal controls.
Highlights
IntroductionPeak cone density varies over a factor of 3 in normal eyes [1], with the ratio of long- (L-) to middle- (M-) wavelength sensitive cones varying from between 1:1 to 17:1 in persons with normal color vision [2,3]
The normal human visual system is extremely variable in both structure and function
We previously examined the topography of the cone mosaics of female carriers of BCM using adaptive optics (AO) retinal imaging methods and demonstrated that cone density was reduced and mosaic regularity was disrupted in these individuals, to a variable degree [21]
Summary
Peak cone density varies over a factor of 3 in normal eyes [1], with the ratio of long- (L-) to middle- (M-) wavelength sensitive cones varying from between 1:1 to 17:1 in persons with normal color vision [2,3] This variability extends to subcortical areas, with lateral geniculate nucleus (LGN) volume showing a 2–3 fold range [4,5], and to the cortex, with the surface area of primary visual cortex (V1) varying by as much as a factor of three [6,7]. It has been hypothesized that the cone mosaic may be the key factor that leads to the variability observed in higher structures [6,12] In this model, it is hypothesized that cone density governs retinal ganglion cell (RGC) density, which in turn determines the retinotopic organization of the cortex. This is consistent with a model put forth by Wassle et al, who stated that ganglion cell density drives the retinotopic organization of the cortex [13,14] and is in contrast with models that invoke independent factors to regulate the size of the cortical representation [15,16]
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