Photoreceptor and ganglion cell topographies correlate with information convergence and high acuity regions in the adult pigeon (Columba livia) retina

Abstract

The fovea and area dorsalis are high acuity vision regions in the pigeon retina. However, the degree of neural convergence (an important determinant of acuity) has not been quantified consistently in this bird. The purpose of the study was to determine the topographic density changes and degree of photoreceptor to ganglion cell convergence in the fovea and the area dorsalis. Total photoreceptor and ganglion cell densities were calculated on the horizontal and vertical meridia. In four eyes, retinal topography was mapped for photoreceptors and ganglion cells. Rod density was quantified by counting anti-rod opsin-stained outer segments across the retina. The ratio of cone photoreceptors to ganglion cells, a rough measure of information convergence, was calculated. The fovea and the red field contained significantly higher mean cone and ganglion cell densities compared with the yellow field. Rods were missing from the fovea. Outside the fovea, rods comprised 20% of the photoreceptor population, with no significant density changes across the retina. The ratio of photoreceptors to ganglion cells was highest in the yellow field, suggesting a high degree of information convergence and low acuity. Our data indicate that convergence of cones onto ganglion cells in the red field is similar to that observed in the fovea. Convergence ratios in both the fovea and red field suggest greater visual acuity compared to that of the surrounding yellow field, which is consistent with the higher visual acuities that have been reported in these regions.