Animal Eyes

From a Light-Sensitive Surface to a Camera-Type Eye

Notably, the most important features in eye design are aimed at gaining spatial resolution. Such resolution can be achieved by curving the photosensitive surface. In a flat surface, photoreactive cells absorb photons from different directions in space equally well. This is the case for light-sensitive spots of a limpet. In this animal, only light from beneath is shielded through the presence of screening pigments. If the surface curves inward (concave), as for example is the case for the slit shell mollusk Pleurotomaria, individual cells start to become selectively more activated by light from specific directions. This is referred to as spatial resolution, and this property tends to improve with increased curvature and a narrowing of the rim area of the eye pit. In photographic terms, the narrowing of the rim corresponds to reduction of the aperture. The resulting eye type, referred to as a pinhole eye, functions on the same principles as a pinhole camera and is exemplified in the cephalo-pod mollusk Nautilus.

Although pinhole eyes can yield good spatial resolution, their small aperture means that they perform poorly at low light levels. To increase [Page 55]light capture, the pinhole or aperture has to be widened, which consequently results in a dramatic loss of spatial resolution. The solution in photography and animal eye evolution alike is the introduction of a lens, which allows capture of much more light while using the focusing power of the lens to maintain good resolution. The resulting “camera” or “single chamber” eye (Figure 1a) is found in vertebrates. Although most vertebrate eyes look fairly similar, organizational details do vary with species. For example, although mammals focus images by changing the shape (and hence the refractive power) of the lens, fish and certain amphibians adjust the focus by changing the location of the lens. The organization of the retina also differs in animals, often because of the animal's ecology or environment. For example, for us, the frontal visual space is particularly important, and we humans accordingly have a single fovea (area of increased visual acuity) within our binocular visual fields. Certain birds of prey need to simultaneously keep track of visual stimuli in the front and on the side. Accordingly, they have a second fovea within the monocular visual field of each eye. Another example is that animals living in flat environments, whether fish, mammals, or birds, tend to have horizontally oriented high-acuity areas, which allow them to see particularly well along the horizon.

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