Anatomical and behavioral correlates of visual acuity in the great horned owl

Abstract

Light microscopy of the Great Horned Owl retina has revealed a well-defined convexiclivate fovea which contains both rods and cones. Comparative cell-density counts of rod and cone nuclei, cells of the inner nuclear layer, and ganglion cells for both foveal and peripheral nasal sections of retina indicate a marked decrease in convergence ratio and a small but significant increase in cones, foveally. Rods occur with a much higher density than cones in both retinal areas, but show no local increase in density across the center of the fovea. Behavioral, psychophysical studies of visual acuity over a wide range of luminances have indicated a maximum resolution in this species of 4–5 min of arc between 0–1.0 log ml. Although evidence was obtained for two different mechanisms mediating visual acuity at low vs. high luminance levels, both components of the acuity function 'appear to have a spectral sensitivity function similar to that of rhodopsin. Alternative hypotheses concerning the role of foveal rods vs. extrafoveal rods and foveal rods vs. foveal cones are suggested to account for the results obtained.