Resolution, separation of retinal ganglion cells, and cortical magnification in humans

We present direct comparisons of resolution thresholds and quantitative estimates of retinal ganglion cell separation in humans with reported functional magnetic resonance imaging estimates of the human linear cortical magnification factor. Measurements of resolution thresholds (MAR), retinal ganglion cell (GC) densities, and linear cortical magnification factor (M) values were taken from the literature. Our objective was to analyse the apparent overrepresentation of human central vision in the visual cortex and to determine whether the cause of this is an effect of the uneven distribution of GC in the retina and/or that central GC have more devoted cortical area per cell. The reserved amount of cortical distance per retinal unit, i.e. the product of M on the one hand and effective GC separation, MAR, and GC receptive field separation on the other, indicates an overrepresentation of the fovea and immediately surrounding retina in the human striate cortex due to an increase in devoted cortical distance per central GC or resolution unit. This cannot be explained by lateral displacement of foveal ganglion cells nor by peripheral scaling, but rather by an additional magnification in the retino-cortical pathway.