Age-dependent changes in visual acuity and retinal morphology in pigeons.

The visual acuities of 17 pigeons that ranged in age from 2 to 17 years were tested with high-contrast, square-wave gratings. A systematic decline in visual acuity was observed that was well described by a logarithmic function. Pupillary diameter also declined with age, which decreased retinal illumination, but increased depth of focus. A small amount of presbyopia also was observed. Both the decrease in retinal illumination and the presbyopia accounted for only a trivial proportion of the acuity loss. No relationship between corneal or lenticular density and age was observed. Ophthalmoscopic examination of the optic media revealed no abnormalities associated with age. Microscopic examination of the area dorsalis of the retina (the high-density region specialized for frontal vision) revealed age-related losses of up to 33% of photoreceptors and 23% of cells in the ganglion-cell layer. A study of the photoreceptor layer within area dorsalis indicated that single-cone densities were unaffected by aging whereas the double-cone densities, which are the predominant photoreceptor type in the area dorsalis, were reduced in number by about one third. Calculation of the Nyquist limit both for photoreceptors and ganglion cells suggested that the decreased retinal density together with the decreased retinal illumination and presbyopia could not account for all of the observed acuity loss.