Читать книгу Bird Senses - Graham R. Martin - Страница 42

The camera type of eye is found in all vertebrates and in some invertebrates (octopuses, squids). A camera eye is also referred to as a ‘simple’ eye, and this label is not without good reason. Compared with the complexity of the multiple repeated structures which are found in the compound eyes of most invertebrates, camera eyes are structurally and conceptually simple. The important point, however, is that within this simplicity of basic design there is great potential for variation in each of the key components. Both gross and subtle variations in these components can profoundly alter the vision of an animal, and hence change the information that different eyes can extract from the same scene.

The basic structure of a camera eye has just two key functional components, an image-producing system and an image-analysing system (Figure 3.2). Not only can these components show much variation, they can also vary in their characteristics independently of each other. The image in the eye of one species will be different to that of another, as will the ways that these images are analysed. Furthermore, with two eyes in an animal’s head, they can be placed in different positions with respect to each other in the skull. This alters the region about the head from which visual information can be retrieved at any one instant and can profoundly influence what an animal can detect in the world that surrounds it.

These two main functional components of camera eyes are conceptually simple. The optical system produces an image of the world outside the eye, and the analysis system extracts information from that image. These two functional components can be matched in a straightforward manner to the main anatomical parts of an eye (Figure 3.2). Indeed, they can be matched to the key components of any camera, from the camera in your phone to a sophisticated video, single-lens reflex, or plate camera.

The optical system of a camera eye consists of the lens and the cornea. The initial extraction of visual information is carried out by the retina onto which the optical system projects an image of the world. The retina is a very thin structure of immense complexity, made up of layers of specialised neural cells. These include the layer containing the photoreceptor cells, and it these which detect the pattern of light within the image. The neural cells of the retina are anatomically part of the brain to which each eye is connected via the optic nerve.

Although the retina shows immense complexity – indeed, it is composed of many millions of cells – there is only a small number of different cell types. This applies especially to the photoreceptors, whose types are discussed in more detail later in this chapter. The important point to note is that significant differences in vision arise from the ways in which the different photoreceptor types are packed together and arranged across the retinas of different species.

This variation in packing and arranging high numbers of receptor cells of just a few types is not unique to vision. It is what underpins variation in other senses too. This will be discussed in later chapters showing, for example, how variation in touch sensitivity, taste, and smell arise. Each of these senses is based on a relatively small number of receptor types, but marked differences in sensory capacity occur because of their relative numbers, and how the receptors are arranged, in different species. Ultimately, these variations are what underpin the sensory ecology of different species.