Cortical correlates of aberrant vernier acuity in albinism

Albinism is a family of genetic diseases that cause aberrations in melanin synthesis which in turn cause aberrations in retinal development, cone density, retinocortical connectivity, and vernier acuity (VA). In healthy individuals, VA is believed to be limited by neural sampling in V1 (primary visual cortex). So, we asked if albinism-related deficits in VA might similarly reflect aberrations in retinocortical mapping. VA and fMRI data were obtained from 4 albinism patients (1M,3F,ages 18-31) and 4 control subjects (3M,1F,ages 20-41). VA thresholds (minimum angle of resolution, MAR) along the horizontal meridian of the left visual field were measured at 10-11 loci ranging from 0-20 degrees eccentricity using a three-dot vertical vernier stimulus. Cortical magnification (CM) was measured at 1.875x1.875x2.5 mm resolution over the same visual field range using fMRI mapping with checkered rings/wedges and a General Electric 3 Tesla MRI scanner. Overall, albinism patients had higher VA thresholds (mean foveal MAR ± SD: 5.990 ± 7.188 arcmin) than controls (0.498 ± 0.127 arcmin). MAR for controls was a power function of cortical magnification MAR=11.15 x CM-1.192). This was also true for albinism patients, but with much more quantitative variation ranging from virtually normal (MAR = 7.68 x CM-1.373) to grossly aberrant (MAR = 107.3 x CM-1.106). Surprisingly, the two albinism patients with the most disparate VA vs CM functions had nearly identical foveal peak cone densities (46,019 vs 50,402 cones/mm2). Although VA varies widely across patients with albinism; it retains the quantitative relationship (power function) with CM observed in controls, albeit with wide quantitative variation. This latter variation, at least in some cases, is poorly correlated with cone densities thus emphasizing the importance of post-receptoral factors as determinants of overall visual function in albinism.