Peter G.C. Bedford, BVetMed, PhD, DVOphthal, DECVO, FHEA, FRCVS
It is without thought that we automatically and somewhat mechanically put a pen to paper or even dive to the depths with an aqualung. We accept our daily actions as being the norm and I suppose the same can be said for those abilities that we possess which are collectively called the "special senses". And so it is with sight, that we part our eyelids when we rise and the pictures just happen. We expect a coloured and perfectly focused panorama to greet our awakening and perhaps we really do not think too often about how this particular miracle comes to pass. What we see is what we call vision and it is simply part of normal function: how we see only really occupies the thoughts of those who research and those who treat disease of the eye. We perhaps assume that the animal kingdom around us has the same facility for we see lions stalking and birds feeding, but on occasion the owners of companion animals and perhaps students of the veterinary art wonder about the type of vision those animals which share our lives possess. Can my dog watch television? What colours does my dog see? Why does my horse falter? These are questions that all of us as ophthalmologists are asked from time to time. And these questions are more than matters of intellectual interest because the visual capabilities of our companion animals can directly affect their contribution to the quality of human society in terms of both work and play. The process by which a single light sensitive pigment spot has evolved into a complex biological camera specifically adapted to the environmentally driven visual requirements of today's fish, bird and mammal is a fascinating one. As the primeval single cell organism has evolved into the spectrum of animal life that exists in today's world that pigment spot has become a retina encased within a specialised globe in which the development of both cornea and lens has added to the overall visual acuity of the organ we call the eye. And there are many species differences in terms of the details of structure and physiological functions, those differences arising as solutions to varying environmental needs.
Descriptions of the adaption and variation in the process of sight seen in vertebrates is complicated because of the many factors involved. The detection of light or colour and the parameters involved in visual acuity are basic to the concept of vision, but in the vertebrate world all of these factors can vary from species to species and the complete visual experience in any one species is a synthesis of these many constituent parts. Many basic features are understood, but sadly that understanding is not always complete and furthermore any description of vision in any non-human species tends to be defined in terms of human visual capability: as such it may not be an absolutely accurate representation of how vision really is in the non-human vertebrate world.
Phototransduction, the conversion of light into an action potential, is fundamental to the process of sight. It requires the absorption of a photon of light by a photopigment within a cellular unit referred to as a photoreceptor. In most species there are basically two types of photoreceptor, the so-called rods and cones: different photopigments are present in different species, the apoprotein opsin component of the photopigment differing between the rods and cones and between the different types of cones. It is the opsin which determines which wavelength of light the photopigment will absorb and therefore dictates the sensitivity of the retina to light. For example the minimum threshold of light for vision in the cat is approximately 6 times lower than that for normal human beings. The development of a tapetum in many species is an important adjunct to vision in low levels of illumination, allowing the animal to utilise reflected light. It is the photoisomerisation of the second component of a photopigment which is responsible for the generation of the neuronal signal, a signal which is further and variously modified on a species basis as it passes through the retina to ganglion cells and via their axons to the visual cortex. It is primarily the study of rhodopsin, the photopigment of rods, which has revealed most about phototransduction, the process of isomerisation of its chromophore, 11-CIS-retinaldehyde or retinal, to all-trans-retinaldehyde providing the specific detail required. Cone photopigments have been more difficult to study but they do appear to be similar in both structure and function to rhodopsin. Most mammals would appear to possess at least two cone pigments, one sensitive to one short wavelength and one sensitive to longer wavelengths. Whereas maximal sensitivity for rhodopsin is around 500nm in the dog, it is 555nm for the cone chromophore in this species and 560nm for the cat. But there are many species differences even at this level of the sight process: for example in some fish and amphibians the same photopigment, retinal2, can be found in both the rods and the cones.
Not only do the photoreceptor types vary, but their distribution in the retina varies between the vertebrate species: for example, in most mammals the rod photoreceptor outnumbers its cone companion, with nocturnal animals possessing a greater proportion of rods and the diurnal species having a greater proportion of cones. Cone numbers tend to increase in the predator species, whereas their prey has more rods. In man and many bird species there is a cone-only area of the retina, the fovea, lateral to the optic disc whereas in other species in a similarly situated cone rich zone, the area centralis, the rods outnumber the cones. In general the cone is twice as thick as its rod counterpart, but in the cone rich areas they become thin to allow a higher density of their population.
Species differences are found in the organisation of the other neuroretinal layers. In some species including the cat an interplexiform neuron which synapses directly with both amacrine and bipolar cells and indirectly with the photoreceptors has been found which probably modulates photoreceptor output. More than 20 types of amacrine cell have been seen in the mammalian retina, all modulating ganglion cell activity and contributing to motion, direction and contrast. The number of types of ganglion cell also varies with the ratio of bipolar cells to the β ganglion cell, approaching 1.1 in species with high resolution vision.
Whilst the structure and function of the retina is probably the most significant factor in visual acuity, the optical structures of the eye do play a significant part in the vision process. The ability of the cornea to transmit light and the degree of accommodation through the lens vary considerably throughout and within the species. In man 80% of light in the 450-550nm range will pass through the cornea whereas in the rabbit it is nearly 90%. Whereas in humans accommodation is achieved by the lens, rodents and ungulates lack any significant facility of accommodation. In the young primate the accommodative power may be as much as 34D whereas in the horse it is limited to 1 or 2D. In the chicken significant accommodation is achieved through both the lens and the cornea and in this species, together with other birds and reptiles, the two eyes may accommodate independently of one another. Because of a very small difference between the refractive indices of water and cornea, the fish cornea has no power of refraction. As such the spherical shaped lens in this species has a very high refractive index, due to its shape and its constituent protein types. Diving birds which are emmetropic in the air overcome their underwater hypermetropia by increasing the accommodative ability of the lens to about 60D.
The many factors which are involved in visual sensation across the vertebrate species will be discussed in the lecture under the headings of sensitivity to light, sensitivity to motion, sensitivity to flicker, visual perspective, visual field of view, depth perception, visual acuity, form perception and colour perception. Given that vision starts with phototransduction it is the environmental requirements which have produced the many structural and functional differences in the visual systems that we see throughout the vertebrate world. We simply open our eyes to see but we need to open our minds to understand.