{"id":36981,"date":"2015-03-11T20:53:58","date_gmt":"2015-03-11T20:53:58","guid":{"rendered":"http:\/\/www.labri.fr\/perso\/barla\/blog\/?p=36981"},"modified":"2015-03-11T21:50:52","modified_gmt":"2015-03-11T21:50:52","slug":"animal-eyes","status":"publish","type":"post","link":"https:\/\/www.labri.fr\/perso\/barla\/blog\/?p=36981","title":{"rendered":"Animal Eyes"},"content":{"rendered":"<p id=\"top\" \/>by Michael Land and Dan-Eric Nilsson<\/p>\n<p><b>The origin of Vision<\/b><\/p>\n<p>Four basic classes (Nilsson 2009)\u00a0: 1. Behaviours controlled by non-directional monitoring of ambient light. Examples are the control of circadian rhythms, light-avoidance responses for protection against harmful levels of short wavelength light, shadow responses to avoid predation, and surface detection for burrowing animals. 2. Behaviours based on directional light sensitivity. Examples are phototaxis, control of body posture (optical statocysts), and alarm responses for approaching predators. 3. Visual tasks based on low spatial resolution. Examples are detection of selfmotion, object avoidance responses (anti-collision), habitat selection, and orientahon to coarse landmarks or major celestial objects such as the sun or moon. 4. Visual tasks based on high spatial resolution. Examples are detection and pursuit of prey, predator detection and evasion, mate detection and eval- I uation, orientation to fine landmarks, visual communication, and recognition of individuals. \u2013 p.11<\/p>\n<p>The common theme throughout this entire sequence is that the acquisition of spatial information is continuously increasing. For most behavioural tasks there is a minimum amount of spatial resolution needed to perform the task to a degree that increases fitness. In most cases the performance of the task can be further improved by access to more information.\u00a0 The amount of spatial information may eventually exceed the minimum requirement for another task, that will then take over and keep up selection for even higher spatial resolution. This way, eye evolution will go on for as long as the fitness is increased by adding or improving visually guided behaviours. \u2013 p.14<\/p>\n<p><b>What makes a good eye?<\/b><\/p>\n<p>Spherical aberration is the name given to the blurring that occurs because a simple spherical surface does not bring all rays together at a single focus. Rays furthest from the axis of the lens are refracted too much, and finish up in front of the focus for rays near the axis, again resulting in a blur circle which is larger than the Airy disc. This is potentially a serious problem for biological lenses, but animals get round it in one of two ways. They may make the optical surfaces non-spherical, and indeed the human cornea is not spherical but hyperbolic in shape to avoid just this problem. A common alternative is to make a lens which is not optically homogeneous, as glass is, but which has a gradient of refractive index from the centre (high) to the periphery (low). The result is that the outer zones of the lens refract less than they would in a glass lens, and with the correct gradient of refractive index all imaging rays can be brought to a single point. That fish lenses have this construction, and correspondingly excellent optics, has been known since the studies of Matthiessen in the 1880s. The human lens has inherited this design from our fishy ancestors, and corrects its own spherical aberration this way. Thus the human eye has both non-spherical (cornea) and inhomogeneous (lens) correction mechanisms. \u2013 p.56<\/p>\n<p>Chromatic aberration is caused because short wavelength blue light is refracted more strongly than long wavelength red light. This occurs in bio- I logical materials just as in glass, and means that the blue image in the human eye is almost 0.5 mm in front of the red image. No animal eye seems to have emulated the achievement of the early telescope makers in making an achromatic lens by using a combination of materials. However, there are other solutions. Humans partially evade the problem by using only a relatively narrow range of wavelengths in the middle of the spectrum for high-acuity vision, so that our resolution in the blue end of the spectrum is poor-little more than a \u2018colour wash\u2019. Fish and some other vertebrates I are a little more subtle, and have lenses with multiple focal lengths. This ensures that each cone type has an in-focus image for at least a proportion of the light reaching it. \u2013 p.57<\/p>\n<p>Unlike diffraction, where eye size is a virtue, these other problems get worse as eyes get bigger. No insect needs a mechanism for focusing its eyes. \u2013 p.57<\/p>\n<p>This leads to a very important conclusion: eyes are \u2018photon starved\u2019 &#8211; in the sense that they are unable to exploit their potential capabilities &#8211; at all light levels except bright daylight. \u2013 p.66<\/p>\n<p>The clever way of managing this trade-off between resolution and sensitivity, so that the eye has the best resolution available to it at different light levels, is to have small receptors, but to pool them into larger assemblages in darker conditions. There is a good deal of evidence to suggest that this is what occurs in the eyes of vertebrates and in some arthropods. \u2013 p.70<\/p>\n<p><b>Aquatic Eyes : The evolution of the lens<\/b><\/p>\n<p>In terrestrial animals it is the curved air-fluid interface of the cornea that performs most of the optical work of bringing light to a focus, but in aquatic animals this surface has no optical function. It exists of course, but with fluid on both sides it has no capacity to refract light. Thus, with rare exceptions, the lens is the only optical structure capable of producing an image in water. \u2013 p.72<\/p>\n<p>Why do animals have graded-index lenses with potentially excellent imaging properties, when the retina is placed much too close to the lens to be in focus? We see from this that the primary benefit of introducing a weak lens is improved sensitivity rather than improved resolution, and this will result in better contrast sensitivity, faster vision, or vision at lower intensities. Thus an eye with a weak lens has a considerable advantage over a lens-less cup eye, where image brightness must always be traded for resolution. \u2013 p.78 <i>The design of a lens must always be compared to the specificities of the environment.<\/i><\/p>\n<p>The reason that under-focused lens eyes are not more common than they are is probably that the path to focused lens eyes and high-resolution I vision is quite straightforward. By making the lens more powerful, it is possible to gain resolution with a maintained brightness of the retinal image.\u00a0 \u2013 p. 78 <i>Thus with a change of environment more complex (focused) eyes will be highly likely to evolve.<\/i><\/p>\n<p>There are at least two reasons for filling the retinal cup. One is to cover the photoreceptor cells with a filter protecting against photo-damage by short wavelength light. Another is to provide a mechanical support around which the retina can grow to form a stable cup. It is thus probable that ocular lenses have a complex evolutionary history, starting with non-optical functions, followed by a role for increasing sensitivity, and finally allowing for high spatial resolution. \u2013 p.79 <i>Has this evolutionary process also occurred outside of oceans?<\/i><\/p>\n<p>It turns out that a lens made simply of a glass-like material (dry crystalline protein for example) will not produce an image of good enough quality, nor have a focal length short enough to be really useful. Clearly, a spherical lens made of homogeneous protein does not fit with what we know of fish lenses, namely that they are of excellent optical quality and short focal length. \u2013 p.79<\/p>\n<p>What does the refractive index gradient achieve? In the first place it changes the pattern of refraction from a discrete bending of the rays at each interface to one in which rays are bent continuously within the body of the lens. The effect on spherical aberration is that the outermost rays, which travel shorter distances within the lens, are bent relatively less than I they are at the interfaces of the homogeneous lens (Fig. 4.5b). The other important defect of biological lenses in general is chromatic aberration, in which light of shorter wavelengths is brought to a focus closer to the lens than longer wavelength light. The solution is to produce lenses with multiple focal lengths, brought about by variations in the basic Matthiessen gradient (Kroger et at 1999). The way this works is shown in Fig. 4.6. For light of a single wavelength, the inner zones of the lens bring light to a closer focus than the outer zones (this effectively means that the lens is over-. J corrected for spherical aberration). However, for white light with a range of wavelengths the images from inner and outer zones have a spread of focal lengths, because of chromatic aberration. This means that the position of the image for short wavelengths formed by the outer zone can be made to coincide with the image for long wavelengths formed by the inner zone. This in turn allows cones with different wavelengths of maximum sensitivity to receive in-focus images in the same plane. \u2013 p.80<\/p>\n<p>Because cephalopods and vertebrates have very separate evolutionary origins, we can be certain that the similarities exist because both groups have J hit upon, and perfected, the same engineering solution to the problem of seeing well in the marine environment. \u2013 p.83<\/p>\n<p>Large eyes have to be stabilized, or motion blur will wreck the excellent resolution obtained by having a fine-grain retina and a long focal length. Similarly, depth of focus becomes smaller as eyes get bigger, making focusing mechanisms essential. However, the eye has to move from time to time, and in both cephalopods and vertebrates this is achieved by a fast flick-like movement known as a saccade, during which the eye is I effectively blind. In both cases the strategy seems to be to minimize the time that the eye is moving relative to the surroundings, with consequent blurring of the image. \u2013 p.85 <i>The stream of consciousness of mollusks and arthropods would thus also needs to cope with visual interruptions!<\/i><\/p>\n<p>Large size can buy either high acuity or high light-gathering power, and it seems here that it must be the latter, because no marine animal exploits resolution anywhere near the diffraction limit of the spherical lens. \u2013 p.86<\/p>\n<p>The requirements of flexibility and economy of space limit the number of optic nerve fibres to about a million (compared with something like a hundred times as many photoreceptors), making this a real bottleneck in the visual pathway. This is why there is need for economy in the ganglion cell distribution, with the greatest numbers associated with parts of the image where there is the most information. \u2013 p.87 <i>So there needs to be some high\/low resolution regions and front-end processing to take advantage of the photoreceptor-to-ganglion fibers ratio.<\/i><\/p>\n<p>&nbsp;<\/p>\n<p><b>Lens eyes on land<\/b><\/p>\n<p>In water the cornea has little or no optical effect, because it has a fluid of the same refractive index on both sides. On land, however, the front surface is in air, so there is now a large refractive index difference, across which rays are bent by refraction. It turns out that the ray-bending power of a fish lens and a cornea in air are quite similar. \u2013 p. 94<\/p>\n<p>An eye with both a cornea and a fish-type lens has too much focusing power, and if the first proto-amphibian to come on land had done nothing about this it would have been very myopic. The blurring would be comparable to what happens to our vision when we go swimming without goggles; in this case, however, we lose the power of the cornea (which now has fluid on both sides) and become hyperopic, which means that we do not have clear vision at any distance. \u2013 p.95<\/p>\n<p>Similar model eyes have been devised for a number of vertebrates, including the goldfish, frog, turtle, pigeon, rat, cat, and monkey. The Gullstrand model consists of a cornea and a lens, all having radii of curvature close to those of real eyes (see fig. 5.5a). There are thus three surfaces to consider. (The human lens, like fish lenses, is not homogeneous, but Gullstrand chose a single refractive index of 1.413 that would provide a homogeneous lens with the same power as a real lens.) \u2013 p.100 <i>Box 5.1 provide a sufficient approximation to the optical system of a wide range of animal eyes!<\/i><\/p>\n<p>To solve the problem of excessive optical power, land vertebrates could have done a number of things. They might have abandoned the lens altogether and adopted the cornea as the sale image-forming structure, or they could have kept the lens and flattened the cornea so that it had no power, or they could have retained both but shrunk the eye to fit the shorter focal length of the combined system. The last of these possibilities, or something like it, does occur in nocturnal mammals; but most of the reptiles, birds and mammals have opted for a compromise, in which the lens is retained, but with much less power than the ancestral fish lens. The lens and cornea then divide the optical power between them: in humans this ratio is about 1 to 2. In optical technology it is usually a good idea to split the required refraction between several surfaces, because the optical defects (aberrations) of several weakly curved surfaces are usually less than those of a single surface of much stronger curvature. Retaining a weaker, flatter lens, together with a not too curved cornea, may have been a way of obtaining images of high quality. \u2013 p.104 <i>These are like hardware adaptations to better serve a task-specific software given physical constraints.<\/i><\/p>\n<p>Good resolution requires a large eye to provide a long focal length, so that the angle between receptors is as small as possible. I This has to be matched by good image quality, which requires a large lens , to provide a small diffraction blur-circle. \u2013 p.105<\/p>\n<p>Generally speaking the power of the cornea is relatively more important in diurnal eyes, and the lens in nocturnal eyes. \u2013 p.107<\/p>\n<p>In fish, with a spherical lens of fixed focal length, the only available mechanism for focusing is for the lens to move bodily towards or away from the retina, just as in a camera. In mammals, birds and reptiles, however, the lens is deformable and so can change its focal length. \u2013 p.108<\/p>\n<p>However away from this axis the cornea presents a tilted profile and the image quality gets rapidly worse. A consequence of this is the highly centred visual system of primates, including man, where \u2018good\u2019 vision is concentrated in a central foveal region only 1\u00b0 across. To use this effectively we have a very sophisticated eye movement system that finds objects of interest in the periphery and centres them for foveal scrutiny (see Chapter 9). Fixation of this kind is relatively uncommon, even amongst mammals. \u2013 p.110<\/p>\n<p>It seems that the lens in man corrects its own aberration in the same way as the lenses of fish, by having a refractive index gradient (Millodot and Sivak 1979). It is, after all, their descendant. So each refracting structure looks after itself: the cornea by being aspheric and the lens by being inhomogeneous. Something rather more interesting occurs in the rat, and other mammals with nocturnal-type eyes in which the large spherical lens forces the cornea to be more or less spherical too (Chaudhuri et at 1983). The cornea has thus little scope for an aspheric correction-which might in any case be inappropriate because most of these animals need a : large field of view without major variations in image quality across it. Instead the lens has a gradient that makes it over-corrected, like a fish lens but more so, so that it corrects not only its own aberration but that of the cornea as well. And because the system is spherical the correction works across the whole field. \u2013 p.110<\/p>\n<p>It seems that the function of the pupil in man is not so much to compensate for changes in brightness as to obtain the best compromise between resolution and sensitivity. In bright light it also limits the cone of light reaching the retina to match the acceptance angle of the cones. \u2013 p.111<\/p>\n<p>The muscular mechanics of the circular pupil mean that it cannot close down beyond a certain limit, and the alternative is a slit pupil, which can close much further. \u2013 p.112<\/p>\n<p>The other function of a slit pupil, mentioned in Chapter 4, is to assist in the correction for chromatic aberration. This correction involves the use of a rnultifocal lens in which different zones have different focal lengths, so that wavelengths refracted by different amounts because of optical dispersion can all be brought to a common focus (Fig. 4.5). A slit pupil, even when partially closed, allows all lens zones to be sampled, whereas a circular pupil cuts out the outer zones, and so undermines this correction. &#8211; p.113<\/p>\n<p>Animals I that live a life dominated by activity around the horizon, for example, predators like the cheetah and herbivores such as rabbits and ungulates, have a narrow horizontal strip through the retina where the ganglion cell density is I very high-the \u2018visual streak\u2019 (Fig. 5.13). Animals from a more three-dimensional environment, such as forest, either have a uniform retina, or one with a more or less circular \u2018area centralis\u2019 where ganglion cells are concentrated (Hughes 1977). A similar situation occurs in fishes from different underwater niches. In primates this \u2018area\u2019 concentrates to a 1\u00b0 central spot, the fovea centralis, with exceptionally high numbers of ganglion cells associated with it-one per cone, about 150 x 103 per square millimeter. This compares with about 6 x 103 in the centre of the area centralis of the rat retina. Birds often have two foveas, one looking out laterally, and the other, situated at the rear (temporal region) of the retina, imaging the region of the bill where the bird pecks at food (as in the pigeon retina in Fig. 5.13). \u2013 p.115 <i>In studying animal eyes, one should thus consider acuity modes and distributions where relevant information is expected to be available.<\/i><\/p>\n<p>However, in mammals and some birds a pattern emerges in which the eyes of predatory species, such as cats, dogs, hawks, and owls, have their axes directed much further forward (fig. 5.14). This gives them a larger frontal region of binocular overlap, and correspondingly a blind region behind. In carnivores the significance of this overlap is probably that it provides a better signal, in terms of both resolution and photon catch, in the hunting direction. In primates, which are not primarily carnivorous, it provides a basis for stereoscopic vision based on the disparity between the images in the two eyes, and this is of great value when looking for and manipulating objects within a range of up to a few metres. \u2013 p.117<\/p>\n<p>p.121 <i>Fig 5.18 recalls the ambient optic array. A full description would require distributions of photo-receptors, ganglion cells and maybe even saccades.<\/i><\/p>\n<p>&nbsp;<\/p>\n<p><b>Mirrors in animals<\/b><\/p>\n<p>The mirror design has not been popular. Although it has the advantage of compactness and high light-gathering power, it does have a very serious weakness: it inevitably produces a low-contrast image. The light reaching the image (in scallop eyes) has already passed through the retina unfocused before the mirror returns it as a focused image. This reduces the image contrast to roughly one-half that in an equivalent lens eye, which would be like looking through a fog. \u2013 p.136<\/p>\n<p>A great many eyes have mirrors behind the retina,. But unlike the scallop mirror their function is not to form an image. Their function is to reflect the light already focused by the lens, and return it through the retina, giving the retina a second chance of capturing photons missed on the first pass. Because the tapetum is in the focal plane of the lens it has no effect on the optical system of the eye, and the reflected light is returned I through the lens as a narrow beam (Fig. 6.8a), visible only from the direcbon of the original illumination. \u2013 p.139<\/p>\n<p><b>\u00a0<\/b><\/p>\n<p><b>Apposition compound eyes<\/b><\/p>\n<p>In apposition eyes, such as those of most diurnal insects, each of the lenses does form a tiny image (although this is not what the animal actually sees). \u2013 p.157<\/p>\n<p>p.158 Fig 7.1 <i>The geometric optic system may thus also be extended to apposition eyes.<\/i><\/p>\n<p>What, then does the insect see? Do the receptors (typically eight) beneath each I lens resolve the inverted images (as Hollywood would like us to believe), or do they just indicate the average intensity across the field of view of the ommatidium? An ommatidium is the \u2018unit\u2019 of a compound eye, consisting of the lens, receptors, and associated structures. See Fig. 7.3.) Remarkably, the answer depends on the animal. \u2013 p.161<\/p>\n<p>\u00b7 Having a slightly higher refractive index than its surroundings, the rhabdom behaves as a light guide, so that the light that enters its distal tip travels down the structure, trapped by total internal reflection. Any spatial information in the image that enters the rhabdom tip is lost, scrambled by the multiple reflections within the light guide, so that the rhabdom itself acts as a photocell that averages all the light that enters it. \u2013 p.161<\/p>\n<p>The description of apposition optics given above holds for most diurnal insects and crustaceans (bees, grasshoppers, water fleas, crabs, etc.) but it does not apply to the true (two-winged) flies. Thus each point in space is viewed by seven rhabdomeres in seven adjacent ommatidia. The net result of this impressive feat of neural I knitting (indicated in fig. 7.4c) is that all the axons that \u2018look at\u2019 the same point in space finish up making connections with the same cells in the lamina. Thus, as far as the lamina is concerned, the image is exactly the same as it would be in a conventional apposition eye, except that the signal, in terms of photon captures, is seven times stronger. One advantage of the extra signal is that it provides flies with a short period at dawn and dusk when they can see well, but the eyesight of their predators and competitors is less sensitive and so less effective at detecting small objects.\u00a0 \u2013 p.163<b><\/b><\/p>\n<p>It implies that apposition eyes tend to under-sample the image slightly, or put another way, they \u00b7 operate at levels of contrast in the image considerably higher than those experienced by the human eye at its resolution limit. \u2013 p.167<\/p>\n<p>The very short focal length of the facet lenses of compound eyes, 100 Jim or less, ensures that such defects as spherical and chromatic aberration, which are troublesome in camera-type eyes, are negligible in compound eyes. Similarly the depth of field is enormous, extending to infinity from as close as an insect ever needs to see. \u2013 p.168<\/p>\n<p>The serious point is that because of diffraction compound eyes are stuck up an evolutionary blind alley. For a single-lens camera-type eye only one lens needs to be made larger to improve resolution, but for a compound eye all have to be enlarged and the numbers have to increase correspondingly. The net result is that the size of camera eyes increases linearly with resolution, but compound eye size increases as the square of resolution. \u2013 p.170<\/p>\n<p>Thus the range of illumination conditions over which an insect with an apposition eye can operate is similar to that of a mammal using its cone system. \u2013 p.170<\/p>\n<p>Insects and crustaceans with light-coloured apposition eyes have an easily visible dark spot which has the alarming I property that it moves across the eye as the observer rotates around the l animal (Fig. 7.10). It seems as though one is being watched. In fact this is a passive optical phenomenon that has nothing to do with the visual process I itself. Whatever the background colour of the eye, the region that images the observer must look dark because it absorbs photons from the observer\u2019s direction. The dark spot (the pseudopupil) moves with the viewer because different parts of the eye image different directions in space; it is almost disappointingly simple. \u2013 p.173<\/p>\n<p>We can learn a great deal from the pseudopupil (Stavenga 1979). Its form reveals structures in the ommatidium, without recourse to histology. \u2013 p.175<\/p>\n<p>Many insects and crustaceans have a forward or upward-pointing region of high acuity, related either to the capture of other insect prey, or to the pursuit in flight of females by males. Where both sexes have the specialization (mantids, dragonflies, robber-flies, hyperiid amphipods) predation is the reason, but more commonly it is only the male that has the acute zone (simuliid midges, hoverflies, mayflies, drone bees) indicating a role in sexual pursuit. The acute zones vary considerably. \u2013 p.180 <i>Thus foci of vision are bimodal in these insects. In contrast, human vision is unimodal and a lot more kurtotic, but probably also exhibits more saccades as processing is deferred to the visual cortex.<\/i><\/p>\n<p><b>\u00a0<\/b><\/p>\n<p><b>Superposition eyes<\/b><\/p>\n<p>All superposition eyes produce a single deep-lying erect image in the vicinity of the retina. Not only does this distinguish them from apposition eyes, which have multiple inverted images, but also from camera-type eyes where the image is inverted. Clearly we are dealing here with something quite out of the ordinary. \u2013 p.191<\/p>\n<p>These three ways of achieving the \u2018dog-leg\u2019 ray-paths required for superposition imagery (refracting, reflecting, and mixed or parabolic superposition) are shown in Fig. 8.4. Size for size, superposition eyes are more sensitive than apposition eyes, which is why they are most commonly encountered in animals such as moths and fireflies that are active at night, or in marine crustaceans from the mid-water depths where the light regime is similar to moonlight on the surface. The result is that the superposition eye is a hundred times more sensitive than a similar sized apposition eye, and in truly nocturnal moths and beetles, which have even larger superposition pupils, the sensitivity can be I ten times higher again. \u2013 p.198<\/p>\n<p>What we have seen is that butterfly eyes behave as apposition eyes, because light entering a single facet is received by a single rhabdom. They I are called \u2018afocal\u2019 because light is not focused on the rhabdom tip as in most apposition eyes, but enters the rhabdom as a parallel beam. In their fundamental optical design, however, these ommatidia remain of the superposition type, constructed from two-lens telescopes. This makes it easy to understand how different lepidopteran groups managed to switch readily from the diurnal (apposition) version of the afocal eye to the nocturnal (superposition) version. To become nocturnal, the powers of the distal and proximal lenses must become more equal, the receptor layer moves to a deeper location, and gradually more and more facets contribute to the image. There are no blind intermediaries. \u2013 p.207<\/p>\n<p><b>\u00a0<\/b><\/p>\n<p><b>Movements of the eyes<\/b><\/p>\n<p>Thus, paradoxically, eye movements are just as concerned with keeping gaze still as they are with changing its direction. The underlying reason for this problem is that the photoreceptors themselves are quite slow: it takes 10 milliseconds or more for a receptor to respond fully to a change in light intensity, and this means that changes in the image that occur faster than this are lost. Just as in photography where it is important to avoid blur by keeping the camera still, so with eyes. \u2013 p.217<\/p>\n<p>However, as an animal turns as it moves through the environment, stabilization alone is , not enough; the eyes must move to re-centre gaze from time to time or they I will finish up in one or other extreme position. Saccades are the means of achieving this. Their impressive speed reflects the need to keep the time they blur the image to a minimum. Humans spend about 10 per cent of their waking hours engaged in saccades, during which vision is degraded, either through blur or \u2018saccadic suppression\u2019 when vision is actively suppressed. Amazingly, this amounts to about one and a half hours of near I blindness each day. \u2013 p.219<\/p>\n<p>Humans and other primates, but probably not many other vertebrates, have the ability to track objects smoothly, provided they do not move too fast or too unpredictably. \u2013 p.219 <i>Is this related to the ability to catch and grasp objects?<\/i><\/p>\n<p>Rather like insects, birds also make head saccades, and these are a very obvious feature of their visual behavior. Birds do have eye movements, but their main function seems to be to \u2018sharpen up\u2019 the head saccades. As the head turns the eyes counter-rotate briefly, then flick to the new position and again counter-rotate until the head completes its saccade. But the eyes do not seem to make saccades on their own. \u2013 p.225<\/p>\n<p>For a light insect not attached to the ground there is nothing to prevent it using body manoeuvres to move its eyes. \u2013 p.226<\/p>\n<p>There are situations, however, when it would be useful to prevent movement of the lateral field of view without impeding locomotion. The best examples are seen in ground feeding birds, which need a clear field of view to the side while foraging, in order to recognize small items of food. Birds achieve this by \u2018head-bobbing\u2019 in which the head is thrust forward, and then held \u2018 I still in space by a backward movement of the neck while the body continues to move forward underneath (Fig 9.7). \u2013 p.227<\/p>\n<p>Many other animals, from insects to mammals, adopt a style of movement in which they run in short fast bursts and then freeze, and it could be argued that the effect of this is functionally similar to head-bobbing: providing interludes of clear sight between periods when vision is compromised by motion. Here the reason seems to be defense against predators rather than food seeking, allowing periods of vigilance and relative invisibility between movements in which they are more visible and vulnerable. \u2013 p.228<\/p>\n<p>What are the arguments against allowing continuous image motion, and in favour of sampling via a series of more or less stationary images? Three seem the most persuasive: A. Resolution is lost if motion blurs the image. B. It is easier to see motion of foreground objects if the retinal image of the background is stationary. C. It is easier to obtain heading and distance information from the pattern I of motion on the retina that results from locomotion, if the rotational motion has already been removed. \u2013 p.228<\/p>\n<p>There are good reasons why fast motion must degrade an image, and they are to do with the rather slow rate at which photoreceptors respond to changes in intensity. In other words, poorly resolving systems with large receptor acceptance angles can tolerate higher velocities than better resolving eyes. \u2013 p.229<\/p>\n<p>Significant blurring of an image occurs at angular velocities that exceed one receptor acceptance angle per response time. \u2013 p.230<\/p>\n<p>Interestingly, vision also fades when the image is kept very well stabilized, and so some low-speed image motion is actually desirable. \u2013 p.230<\/p>\n<p>Thus insects such as bees and flies should be able to tolerate angular velocities of at least 1000s-1 without significant resolution loss. \u2013 p.230<\/p>\n<p>Keeping the eyes totally still should thus be a good way to ensure maximum detection of objects that move. Interestingly, this may be even more effective than one might expect because, in humans at least, a totally still image fades in a few seconds, and when this has occurred the only detectable objects are those that move. We cannot, in normal life, keep our eyes still enough to lose the image of the overall scene, but it may well be that other animals can. \u2013 p.231<\/p>\n<p>The focus of expansion is no longer a point but a blurred line that cannot be used by the animal to determine its heading, and all the retinal vectors are distorted, making distance judgements harder. An effective cure for this is simply not to let the eye rotate, by applying gaze stabilizing reflexes (vestibule-ocular and optokinetic). Then, between saccades at least, the eye will see an almost undistorted translational flow field. With rotation out of the way, retinal velocities can be read as distances. \u2013 p.232 <i>This could explain the weird sensation of immediacy one experiences in \u201cride\u201d movies?<\/i><\/p>\n<p>Before they jump, locusts frequently make side-to-side movements of their heads (peering). These movements are pure lateral translations, without rotation, which makes them ideal for registering the distances of objects by their image motion. \u2013 p.233<\/p>\n<p>Whatever the mixture of reasons, however, one thing is clear: no animal should make smooth rotational eye movements, except in the special (and not very common) circumstance of tracking a moving target. \u2013 p.235<\/p>\n<p>The result of this visual schizophrenia is a repertoire of eye movements unlike anything else in the animal kingdom (Land et at 1990). In addition to \u2018normal\u2019 eye movements (fast saccades, tracking and optokinetic stabilizing movements) there is a special class of frequent, small (c. 10\u00b0) and relatively slow (40oS-1) movements (Fig. 9.l2c), which give . the animal a strange inquisitive appearance, perhaps because they resemble human saccades in their frequency of occurrence. They are, however, not saccades, which are much faster. These movements are typically at right angles to the band, and the only plausible explanation is that they are the scanning movements the animal uses to pick out relevant coloured or polarized features in the surroundings. \u2013 p.237<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n","protected":false},"excerpt":{"rendered":"<p>by Michael Land and Dan-Eric Nilsson The origin of Vision Four basic classes (Nilsson 2009)\u00a0: 1. Behaviours controlled by non-directional monitoring of ambient light. Examples are the control of circadian rhythms, light-avoidance responses for protection against harmful levels of short wavelength light, shadow responses to avoid predation, and surface detection for burrowing animals. 2. Behaviours &#8230; <a title=\"Animal Eyes\" class=\"read-more\" href=\"https:\/\/www.labri.fr\/perso\/barla\/blog\/?p=36981\" aria-label=\"Read more about Animal Eyes\">Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[621],"tags":[],"class_list":["post-36981","post","type-post","status-publish","format-standard","hentry","category-books"],"_links":{"self":[{"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/posts\/36981","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=36981"}],"version-history":[{"count":4,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/posts\/36981\/revisions"}],"predecessor-version":[{"id":37001,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/posts\/36981\/revisions\/37001"}],"wp:attachment":[{"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=36981"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=36981"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=36981"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}