Problems of deep foveas

Abstract

Deep or convexiclivate foveas occur in some birds, including raptors, some lizards and certain deep-sea fishes. Theories on their function are reviewed. Common to raptor and deep-sea fish foveas is a radial fibre lining, dark staining so probably optically dense, adjacent to the less refractile vitreous. The foveal curvatures and size are remarkably similar in a wide taxonomic and size range of birds and fishes.Ray plotting through traced foveal outlines suggests that sharp images will be formed beneath the centre and shoulders, with the centre image enlarged enough to account for the high acuity of raptors. Deep foveas will also exaggerate eccentricity of off-centre images of a point source, such as deep-sea fishes may meet.Despite similarities in foveal shape, the receptors differ widely between species. Raptors, and notosudid fishes, have short cones. Searsid fishes have long single rods. Howella and Bajacalifornia have multiple bank rods, more in the fovea than the periphery. Those of Howella are shown to be multiple inner-outer segment complexes rather than interrupted single rods. Implications of foveal and receptor .features are discussed.

Eye Size, Fovea, and Foraging Ecology in Accipitriform Raptors

“…The possible optical function of the shape of the deep avian fovea has been debated for decades [Walls, 1937;Pumphrey, 1948]. Walls [1937] and Locket [1992] suggested that steep slopes of the fovea and the difference between the refractive indices of the vitreous and the retina may magnify the image. Pumphrey [1948] considered both the steep slopes and the concave bottom of the fovea to contribute to this function.…”

Foveate vision in deep–sea teleosts: a comparison of primary visual and olfactory inputs

“…Species within the genus Conocara have received particular attention, where a deep foveal pit located in the temporal retina is lined with radial ¢bres (MÏller cells) and possesses a centroperipheral gradient of retinal ganglion cells of up to 37:1 (Locket 1992;Collin & Partridge 1996;Wagner et al 1998). Although the refractive index of the MÏller cell lining needs to be measured, its thickness and staining properties suggest it may refract light, distorting or magnifying the image of an object passing over the fovea.…”

The eyes of deep–sea fishes and the changing nature of visual scenes with depth

“…A further bene¢t may also come from the pitted shapes of many bathypelagic foveae. Recent evidence suggests that this shape is ideally suited for estimating the distance of point sources in darkness (Locket 1992). For humans, with an eye separation of 6 cm and foveal ganglion cell receptive ¢elds in the order of 0.5 arcmin, image disparities give reliable distance estimations up to 30 m away.…”