Retinal Structure of Birds of Prey Revealed by Ultra-High Resolution Spectral-Domain Optical Coherence Tomography

Birds and particularly birds of prey are known for their unique retinal structures. Although the arrangement of retinal layers in birds is the same as in other vertebrates,1 variations in the morphology of the retina, the regions of highest visual acuity and the retinal vascularization are significant.1,2 The visual acuity of birds is improved by the structure of the retina itself, in which the density of functional photoreceptors (number of cones and rods per unit area) can be several times that in humans.3–6 The relative number of cones per rod is usually higher in diurnal species. Therefore, the retina of diurnal birds is consistently rich in cones and exhibits higher visual acuity. In contrast, the retinas of nocturnal birds have a greater relative density of rod photoreceptors, which implies high visual sensitivity.7,8 Many avian species have regions within the retina in which the density of the functional photoreceptors is higher and that contain a predominance of cones over rods. These areas of the retina are known as foveae, and they are specialized for increased visual acuity.1,9,10 Two foveas are present in various diurnal birds, including birds of prey (hawks, eagles, and falcons): a deep fovea (nasal/central region) and a shallow fovea (temporal region).1,11–14 The central fovea presents a higher density of photoreceptors and generally a steeper and deeper depression compared with the temporal fovea.2,10,11 Findings in studies have suggested a greater visual acuity at the central fovea than at the shallow fovea. Some exceptions to bifoveality occur in species of birds of prey such as owls, which have only one fovea, located in the temporal region. Another unique feature of the foveae of owls is that they present a relatively higher density of rod photoreceptors, which is a distinctive feature of nocturnal avian species. The retinas of birds are anangiotic, and nourishment is mainly supplied from a highly vascularized body known as the pecten oculi, whereas oxygenation is received from the choriocapillaris.8 The pecten extends from the optic nerve head (ONH) into the vitreous body of the eye, and this vascular structure is larger and more elaborated in diurnal birds than in nocturnal ones.2,8 The avascular structure of the retina provides a low-scattering passage for light to travel from the retinal surface to the photoreceptors and thus ensures the sharpest vision. Although examinations of the eye of birds of prey have been performed with various methods,15 information about the anatomic structure of the retina of birds in general is relatively modest compared with that for mammalian retina. Since the retina of birds is considered one of the most specialized of any animal species,2,8 in vivo examination of its structure has significant importance for vision research. Although optical coherence tomography (OCT) has been used to image retinas in animals,16–19 high-resolution spectral-domain OCT (SD-OCT) imaging has not been reported in avian species. In the present study, we demonstrated the application of high-resolution SD-OCT for noninvasive, noncontact, in vivo retinal imaging of birds of prey.