Mechanisms of Seeing 36-1 The sensation of color In discussing the sense of sight, we have to realize that(outside of a gallery 36-1 The sensation of color of modern art! )one does not see random spots of color or spots of light. When 36-2 The physiology of the eye we look at an object we see a man or a thing; in other words, the brain interprets what we see. How it does that, no one knows, and it does it, of course, at a very 36-3 The rod cells high level. Although we evidently do learn to recognize what a man looks like after much experience, there are a number of features of vision which are more 36-4 The compound (insect)ey elementary but which also involve combining information from different parts 36-5 Other eyes what we see. To help us understand how we make an interpretation of an entire image, it is worth while to study the earliest stages of the putting together of in 36-6 Neurology of vision formation from the different retinal cells. In the present chapter we shall concen trate mainly on that aspect of vision, although we shall also mention a number of side issues as we go along An example of the fact that we have an accumulation, at a very elementary level, of information from several parts of the eye at the same time, beyond our voluntary control or ability to learn, was that blue shadow which was produced by white light when both white and red were shining on the same screen. This effect at least involves the knowledge that the background of the screen is pink, even though, when we are looking at the blue shadow, it is only"white"light coming into a particular spot in the eye; somewhere, pieces of information have been put together. The more complete and familiar the context is, the more the eye will make corrections for peculiarities. In fact, Land has shown that if we mix that apparent blue and the red in various proportions, by using two photographic transparencies with absorption in front of the red and the white in different pro- portions, it can be made to represent a real scene, with real objects, rather faithfully In this case we get a lot of intermediate apparent colors too, analogous to what we would get by mixing red and blue-green; it seems to be an almost complete set of olors, but if we look very hard at them, they are not so very good. Even so, it is surprising how much can be obtained from just red and white. The more the scene looks like a real situation, the more one is able to compensate for the fact that all the light is actually nothing but pink! Another example is the appearance of"colors"in a black-and-white rotating isc, whose black and white areas are as shown in Fig. 36-1. when the disc is rotated, the variations of light and dark at any one radius are exactly the same; it g.36-1 Sc only the background that is different for the two kinds of"stripes. "Yet one of above is spun, colors appear in the"rings"appears colored with one color and the other with another. No one of the two darker"rings. "If yet understands the reason for those colors, but it is clear that information is being direction is reversed, the colors m卿m put together at a very elementary level, in the eye itself, most likely in the other ring Almost all present-day theories of color vision agree that the color-mixing data indicate that there are only three pigments in the cones of the eye, and that it is the spectral absorption in those three pigments that fundamentally produces he color sense. But the total sensation that is associated with the absorption characteristics of the three pigments acting together is not necessarily the sum of the individual sensations. We all agree that yellow simply does not seem to be reddish green; in fact it might be a tremendous surprise to most people to discover that light is, in fact, a mixture of colors, because presumably the sensation The colors depend on speed of rotation, on the brightness of illumination, and to some extent on who looks at them and how intently he stares at them
is due to some other process than a simple mixture like a chord in music, where the three notes are there at the same time and if we listen hard we can hear them individually. We cannot look hard and see the red and the green The earliest theories of vision said that there are three pigments and three kinds of cones, each kind containing one pigment; that a nerve runs from each cone to the brain, so that the three pieces of information are carried to the brain and then in the brain, anything can happen. This, of course, is an incomplete idea it does no good to discover that the information is carried along the optic nerve to the brain, because we have not even started to solve the problem. We must ask more basic questions: Does it make any difference where the information is put together? Is it important that it be carried right up into the brain in the optic nerve, or could the retina do some analysis first? We have seen a picture of the retina as an extremely complicated thing with lots of interconnections(Fig. 35-2) and it might make some analyses As a matter of fact, people who study anatomy and the development of the eye have shown that the retina is, in fact, the brain: in the development of the em- bryo, a piece of the brain comes out in front, and long fibers grow back, con- necting the eyes to the brain. The retina is organized in just the way the brain is organized and, as someone has beautifully put it, "The brain has developed a way to look out upon the world. The eye is a piece of brain that is touching light, so to speak, on the outside. So it is not at all unlikely that some analysis of the color has already been made in the retina This gives us a very interesting opportunity. None of the other senses involves uch a large amount of calculation, so to speak, before the signal gets into a nerve that one can make measurements on. The calculations for all the rest of the senses usually happen in the brain itself, where it is very difficult to get at specific places to make Its, because there are so many interconnections. Here, with the light, three layers of cells ma the results of the calculations being transmitted through the optic nerve. So we have the first chance to observe physiologically how, perhaps, the first layers Photochemical Absorptions of the brain work in their first steps. It is thus of double interest, not simply interesting for vision, but interesting to the whole problem of physiology yk1《r-2 The fact that there are three pigments does not mean that there must be three kinds of sensations. One of the other theories of color vision has it that there are really opposing color schemes( Fig. 36-2). That is, one of the nerve fibers carries a lot of impulses if there is yellow being seen, and less than usual for blue. Another Fig. 36-2. Neural connections ac- nerve fiber carries green and red information in the way, and another, white cording to an opponent "theory of and black. In other words, in this theory someone has already started to make a color vision guess as to the system of wiring, the method of calculation The problems we are trying to solve by guessing at these first calculations are questions about the apparent colors that are seen on a pink background, what hap- pens when the eye is adapted to different colors, and also the so-called psychological phenomena. The psychological phenomena are of the nature for instance, that hite does not"feel"like red and yellow and blue, and this theory was advanced because the psychologists say that there are four apparent pure colors: There are four stimuli which have a remarkable capacity to evoke psychologically simple blue, yellow, green, and red hues respectively. Unlike sienna, magenta, purple, or most of the discriminable colors, these simple hues are unmixed in the sense that none partakes of the nature of the other; specifically, blue is not yellowish, reddish or greenish, and so on; these are psychologically primary hues. That is a psycho- logical fact, so-called. To find out from what evidence this psychological fact was deduced, we must search very hard indeed through all the literature: In the modern literature all we find on the subject are repeats of the same statement, or of one by a german psychologist, who uses as f his authorities Leonardo da vinci who of course, we all know was a great artist. He says, "Leonardo thought there were five colors. "Then, looking still further, we find, in a still older book, the evidence for the subject. The book says something like this: "Purple is reddish-blue, orange is reddish-yellow, but can red be seen as purplish- orange? Are not red and yellow more unitary than purple or orange? The average person, asked to state which
colors are unitary, names red, yellow, and blue, these three and some observers add a fourth, green. Psychologists are accustomed to accept the four as salient hues. So that is the situation in the psychological analysis of this matter: if every- body says there are three, and somebody says there are four, and they want it to be four, it will be four. That shows the difficulty with psychological researches It is clear that we have such feelings, but it is very difficult to obtain much informa tion about them So the other direction to go is the physiological direction, to find out experi mentally what actually happens in the brain, the eye the retina, or wherever, and perhaps to discover that some combinations of impulses from various cells move along certain nerve fibers. Incidentally, primary pigments do not have to be in separate cells; one could have cells in which are mixtures of the various pigments cells with the red and the green pigments, cells with all three(the information of all three is then white information), and so on. There are many ways of hooking the system up, and we have to find out which way nature has used. It would be hoped, ultimately, that when we understand the physiological connections we will have a little bit of understanding of some of those aspects of the psychology, so we look in that direction 36-2 The physiology of the eye We begin by talking not only about color vision, but about vision in general just to remind ourselves about the interconnections in the retina, shown in Fi 35-2. The retina is really like the surface of the brain. Although the actual picture through a microscope is a little more complicated looking than this somewhat schematized drawing, by careful analysis one can see all these interconnections There is no question that one part of the surface of the retina is connected to other parts, and that the information that comes out on the long axons, which produce the optic nerve, are combinations of information from many cells. There are three layers of cells in the succession of function there are retinal cells which are the ones that the light affects, an intermediate cell which takes information from a single or a few retinal cells and gives it out again to several cells in a third layer of cells and carries it to the brain. There are all kinds of cross connections between cells in the layers We now turn to some aspects of the structure and performance of the eye (see Fig 35-1). The focusing of the light is accomplished mainly by the cornea by the fact that it has a curved surface which"bends"the light. This is why we cannot see clearly under water, because we then do not have enough difference between the index of the cornea, which is 1.37, and that of the water. which is 1.33 Behind the cornea is water, practically, with an index of 1.33, and behind that is a lens which has a very interesting structure: it is a series of layers, like an onion, except that it is all transparent, and it has an index of 1. 40 in the middle and 1.38 at the outside. (It would be nice if we could make optical glass in which we could adjust the index throughout, for then we would not have to curve it as much as we do when we have a uniform index. ) Furthermore, the shape of the cornea is not that of a sphere. A spherical lens has a certain amount of spherical aberration The cornea is"flatter"at the outside than is a sphere, in just such a manner that he spherical aberration is less for the cornea than it would be if we put a spherical lens in there! The light is focused by the cornea-lens system onto the retina. As we look at things that are closer and farther away, the lens tightens and loosens and changes the focus to adjust for the different distances. To adjust for the total ￠ amount of light there is the iris, which is what we call the color of the eye, brown or blue, depending on who it is; as the amount of light increases and decreases, the iris moves in and out Fig. 36-3. The neural interconnect- of the lens, the motion of the eye, the muscles which turn the eye in the socket, and the eyes *he mechanical operation of Let us now look at the neural machinery for controlling the accommodation tions for the iris, shown schematically in Fig. 36-3. Of all the information that comes out of the optic nerve A, the great majority is divided into one of two bundles(which we will talk about later)and thence to the brain. but there are a few fibers, of 36-3
interest to us now, which do not run directly to the visual cortex of the brain where we"see""the images, but instead go into the mid-brain H. These are the fibers which measure the average light and make adjustment for the iris; or, if the image looks foggy, they try to correct the lens; or, if there is a double image, they try to adjust the eye for binocular vision. At any rate, they go through the mid- brain and feed back into the eye. At K are the muscles which run the accommoda tion of the lens and at L another one that runs into the iris the iris has two muscle systems. One is a circular muscle L which, when it is excited, pulls in and closes down the iris; it acts very rapidly and the nerves are directly connected from the brain through short axons into the iris. The opposite muscles are radial muscles so that, when the things get dark and the circular muscle relaxes, these radial muscles pull out. Here we have, as in many places in the body, a pair of muscles hich work in opposite directions, and in ry such which control the two are very delicately adjusted, so that when signals are sent in tighten one, signals are automatically sent in to loosen the other. The iris is a eculiar exception: the nerves which make the iris contract are the ones we have already described, but the nerves which make the iris expand come out from no one knows exactly where, go down into the spinal cord back of the chest, into the ODO thoracic sections, out of the spinal cord, up through the neck ganglia, and all the way around and back up into the head in order to run the other end of the iris in fact, the signal goes through a completely different nervous system, not the central nervous system at all, but the sympathetic nervous system, so it is a very of making We have already emphasized another strange thing about the eye, that the light-sensitive cells are on the wrong side, so that the light has to go through several vers of other cells before it gets to the receptors -it is built inside out! So some Fig 36-4.The of the features are wonderful and some are apparently stupid connections Figure 36-4 shows the connections of the eye to the part of the brain which is from the eyes to the most directly concerned with the visual process. The optic nerve fibers run into a certain area just beyond D, called the lateral geniculate, whereupon they run out to a section of the brain called the visual cortex. notice that some of the fibers from complete. The optic nerves from the left side of the right eye run across the optic chiasma B, while the ones on the left side of the left eye come around and go thi same way. So the left side of the brain receives all the information which comes from the left side of the eyeball of each eye, i.e., on the right side of the visual field, while the right side of the brain sees the left side of the visual field. This is the manner in which the information from each of the two eyes is put together in order to tell how far away things are. This is the system of binocular vision The connections between the retina and the visual cortex are interesting. If a spot in the retina is excised or destroyed in any way then the whole fiber will die and we can thereby find out where it is connected. It turns out that, essentially, the connections are one to one-for each spot in the retina there is one spot in the visual cortex-and spots that are very close together in the retina are very close together in the visual cortex. So the visual cortex still represents the spatial arrangement of the rods and cones, but of course much distorted. Things which are in the center of the field, which occupy a very small part of the retina, are expanded over many, many cells in the visual cortex. It is clear that it is useful to have things which are originally close together, still close together. The most remarkable aspect of the matter, however, is the following. The place where one would think it would be most important to have things close right in the middle of the eld. Believe it or not, the our visual field as we look at something is of such a nature that the information from all the points on the right side of that line is going into the left side of the brain and information from the points on the left side is going into the right side of the brain, and the way this area is made, there is a cut right down through the middle so that the things that are very close together right in the middle are very far apart in the brain! Somehow, the information has to go from one side of the brain to the other through some other channels, which is quite surprising
he question of how this network ever gets"wired" together is very interesting The problem of how much is already wired and how much is learned is an old one It used to be thought long ago that perhaps it does not have to be wired carefully at all, it is only just roughly interconnected, and then, by experience, the young child learns that when a thing is "up there "it produces some sensation in the brain Doctors always tell us what the young child"feels, but how do they know what a child feels at the age of one?)The child, at the age of one, supposedly sees that n object is"up there, gets a certain sensation, and learns to reach"there because when he reaches"here, "it does not work. That approach probably is not correct, because we already see that in many cases there are these special detailed interconnections. More illuminating are some most remarkable experiments done ith a salamander. (Incidentally, with the salamander there is a direct crossover connection, without the optic chiasma, because the eyes are on each side of the head and have no common area. Salamanders do not have binocular vision. The experi ment is this. We can cut the optic nerve in a salamander and the nerve will grow out from the eyes again. Thousands and thousands of cell fibers will thus re-establish themselves. Now, in the optic nerve the fibers do not stay adjacent to each other-it is like a great, sloppily made telephone cable, all the fibers twisting and turning, but when it gets to the brain they are all sorted out again. When we cut the optic nerve of the salamander, the interesting question is, will it ever get straightened out? The answer is remarkable: yes. If we cut the optic nerve of the salamander and it grows back, the salamander has good visual acuity again. However, if we cut the optic nerve and turn the eye upside down and let it grow back again, it has good isual acuity all right, but it has a terrible error: when the salamander sees a fly ap here, it jumps at it"down there, and it never learns. Therefore there is some mysterious way by which the thousands and thousands of fibers find their right places in the brail This problem of how much is wired in, and how much is not, is an important problem in the theory of the development of creatures. The answer is not known but is being studied intensively The same experiment in the case of a goldfish shows that there is a terrible not, like a great scar or complication, in the optic nerve where we cut it, but in spite of all this the fibers grow back to their right places in the brain. In order to do this, as they grow into the old channels of the optic nerve they must make several decisions about the direction in which they should grow. How do they do this? There seem to be chemical clues that different fibers respond to differently. Think of the enormous number of growing fibers, each of which is an ndividual differing in some way from its neighbors; in responding to whatever the chemical clues are, it responds in a unique enough way to find its proper place for ultimate connection in the brain! This is an interesting-a fantastic-thing It is one of the great recently discovered phenomena of biology and is undoubtedly connected to many older unsolved problems of growth, organization, and develop ment of organisms, and particularly of embryos One other interesting phenomenon has to do with the motion of the eye. The eyes must be moved in order to make the two images coincide in different cumstances. These motions are of different kinds: one is to follow something, which requires that both eyes must go in the same direction, right or left, and the othel point them toward the same place at various distances away, which requires that they must move oppositely. The nerves going into the muscles of the eye are already wired up for just such purposes. There is one set of nerves which will pull the muscles on the inside of one eye and the outside of the other, and relax the opposite muscles, so that the two eyes move together. There is another center where an excitation will cause the eyes to move in toward each other from parallel Either eye can be turned out to the corner if the other eye moves toward the nose, but it is impossible consciously or unconsciously to turn both eyes out at the same time, not because there are no muscles, but because there is no way to send a signal to turn both eyes out, unless we have had an accident or there is something the matter, for instance if a nerve has been cut. Although the muscles of certainly steer that eye about, not even a Yogi is able to move both eyes out freely 36-5
under voluntary control, because there does not seem to be any way to do it. We are already wired to a certain extent. This is an important point, because most of e earlier books on anatomy and psychology, and so on, do not appreciate or do not emphasize the fact that we are so completely wired already-they say that everything is just learned 36-3 The rod cells Let us now examine in more detail what happens in the rod cells. Figure 36-5 shows an electron micrograph of the middle of a rod cell(the rod cell keeps going p out of the field). There are layer after layer of plane structures, shown magnified at the right, which contain the substance rhodopsin (visual purple), the dye, or pigment, which produces the effects of vision in the rods. The rhodopsin, which is the pigment, is a big protein which contains a special group called retinene which can be taken off the protein, and which is, undoubtedly, the main cause of the absorption of light. We do not understand the reason for the planes, but it is very likely that there is some reason for holding all the rhodopsin molecules parallel. The chemistry of the thing has been worked out to a large extent, but there might be some physics to it. It may be that all of the molecules are arranged in some kind of a row so that when one is excited an electron which is generated, say, may Fig. 36-5. Electron micrograph of a run all the way down to some place at the end to get the signal out, or something This subject is very important, and has not been worked out. Itis a field in which both biochemistry and solid state physics, or something like it, will ultimately be used This kind of a structure, with layers, appears in other circumstances where /NNM photosynthesis. If we magnify those, we find the same thing with almost the same kind of layers, but there we have chlorophyll, of course, instead of retinene. The hemical form of retinene is shown in Fig. 36-6. It has a series of alternate double bonds along the side chain, which is characteristic of nearly all strongly absorbing rganic substances, like chlorophyll, blood, and so on. This substance is impossible Fig. 36-6. The structure of retinene human beings to manufacture in their own cells-we have to eat it. So we eat it in the form of a special substance, which is exactly the same as retinene except that there is a hydrogen tied on the right end it is called vitamin A, and if we do not eat enough of it, we do not get a supply of retinene, and the eye becomes what we call night blind, because there is then not enough pigment in the rhodopsin he rods at nigh The reason why such a series of double bonds absorbs light very strongly is also known. We may just give a hint: The alternating series of double bonds is called a conjugated double bond; a double bond means that there is an extra electron there, and this extra electron is easily shifted to the right or left. When light strike this molecule, the electron of each double bond is shifted over by one step. All the electrons in the whole chain shift, like a string of dominoes falling over, and though each one moves only a little distance(we would expect that, in a single atom, we could move the electron only a little distance), the net effect is the same as though the one at the end was moved over to the other end! It is the same though one electron went the whole distance back and forth, and so, in this manner we get a much stronger absorption under the influence of the electric field, than if ye could only move the electron a distance which is associated with one atom So, since it is easy to move the electrons back and forth, retinene absorbs light very strongly; that is the machinery of the physical-chemical end of it. 36-4 The compound (insect) eye et u return to biology. The human eye is not the only kind of eye. In the vertebrates, almost all eyes are essentially like the human eye. However, in the lower animals there are many other kinds of eyes: eye spots, various eye cups, and other less sensitive things, which we have no time to discuss. But there is one other highly developed eye among the invertebrates, the compound eye of the insect (Most insects having lal also have va eyes as well. )A bee is an insect whose vision has been studied very carefully. It is 36-6
easy to study the properties of the vision of bees because they are attracted to honey, and we can make experiments in which we identify the honey by putting it on blue aper or red paper, and see which one they come to. By this method some very interesting things have been discovered about the vision of the bee In the first place, in trying to measure how acutely bees could see the color difference between two pieces of"white "paper, some researchers found they were not very good, and others found they were fantastically good. Even if the two pieces of white paper were almost exactly the same, the bees could still tell the difference.The experimenters used zinc white for one piece of paper and lead d although these look exactly the same the bee could easily distinguish them, because they reflect a different amount in the ultraviolet In this way it was discovered that the bee is sensitive over a wider range of the spectrum than is our own. Our eye works from 7000 angstroms to 4000 ang stroms, from red to violet, but the bee's can see down to 3000 angstroms into the ultraviolet! This makes for a number of different interesting effects. In the first place, bees can distinguish between many flowers which to us look alike. Of course, we must realize that the colors of flowers are not designed for our eyes, but for the bee; they are signals to attract the bees to a specific flower. We all know that there are many"white"flowers. Apparently white is not very interesting to the bees, because it turns out that all of the white flowers have different proportions of reflection in the ultraviolet; they do not reflect one hundred percent of the ultra- violet as would a true white. All the light is not coming back, the ultraviolet is missing, and that is a color, just as, for us, if the blue is missing, it comes out ellow. So all the flowers are colored for the bees. However, we also know that red cannot be seen by bees. Thus we might expect that all red flowers should look black to the bee. Not so! A careful study of red flowers shows, first, that even with our own eye we can see that a great majority of red flowers have a bluish tinge be cause they are mainly reflecting an additional amount in the blue, which is the part that the bee sees. Furthermore, experiments also show that flowers vary in their reflection of the ultraviolet over different parts of the petals, and so on. So if w could see the flowers as bees see them they would be even more beautiful and varied It has been shown, however that there are a few red flowers which do not re- fect in the blue or in the ultraviolet, and would, therefore, appear black to the bee! This was of quite some concern to the people who worry about this matter, because black does not seem like an interesting color, since it is hard to tell from a dirty old shadow. It actually turned out that these flowers were not visited by bees, these are the flowers that are visited by hummingbirds, and hummingbirds can see the red! Another interesting aspect of the vision of the bee is that bees can apparently tell the direction of the sun by looking at a patch of blue sky, without seeing the sun itself. We cannot easily do this. If we look out the window at the sky and see that it is blue, in which direction is the sun? The bee can tell, because the bee is quite sensitive to the polarization of light, and the scattered light of the sky is polarized, There is still some debate about how this sensitivity operates. Whether it is because the reflections of the light are different in different circumstances, or the bee's eye is directly sensitive, is not yet known. t It is also said that the bee can notice flicker up to 200 oscillations per second while we see it only up to 20. The motions of bees in the hives are very quick the feet move and the wings vibrate, but it is very hard for us to see these motions with our eye. However, if we could see more rapidly we would be able to see the motion. It is probably very important to the bee that its eye has such a rapid response. w The human eye also has a slight sensitivity to the polarization of light, and one can learn to tell the direction of the sun The phenomenon that is involved here is called nger's brush: it is a faint yellowish hourglass-like pattern seen at the center of the field when one looks at a broad, featureless expanse using polarizing glasses. It can also be seen in the blue sky without polarizing glasses if one rotates his head back and forth about the axis of vision t evidence obtained since this lecture was given indicates that the eye is directly sensI 36-7
Now let us discuss the visual acuity we could expect from the bee. The eye of a bee is a compound eye, and it is made of a large number of special cells called ommatidia, which are arranged conically on the surface of a sphere(roughly)on the outside of the bee's head. Figure 36-7 shows a picture of one such ommatidium At the top there is a transparent area, a kind of"lens, "but actually it is more like filter or light pipe to make the light come down along the narrow fiber, which is where the absorption presumably occurs. Out of the other end of it comes the nerve fiber. The central fiber is surrounded on its sides by six cells which, in fact have secreted the fiber. That is enough description for our purposes; the point is it is a conical thing and many can fit next to each other all over the surface of Jow let us discuss the resolution of the eye of the bee. If we draw lines(Fig 36-8)to represent the ommatidia on the surface, which we suppose is a sphere of radius r, we may actually calculate how wide each ommatidium is by using our br that evolution is as clever as we are! If we have a very lar ommatidium we do not have much resolution. That is, one cell gets a piece of information from one direction, and the adjacent cell gets a piece of information from another direction, and so on, and the bee cannot see things in between very well.So the uncertainty of visual acuity in the eye will surely correspond to an angle, the angle of the end of the ommatidium relative to the center of curvature of the eye. (The eye cells, of course, exist only at the surface of the sphere; inside Fig. 36-7. The structure of an om. that is the head of the bee. )This angle, from one ommatidium to the next, is, of matidium (a single cell of a compound course, the diameter of the ommatidia divided by the radius of the eye surface (36.1) So, we may " The finer we make the 8, the more the visual doesn't the bee just use very, very fine ommatidia? " Answer: We know physics to realize that if we are trying to get light down into a cannot see accurately in a given direction because of the diffraction effect. The ight that comes from several directions can enter and, due to diffraction, we will light coming in at angle Fig. 36-8. Schematic view of pack Now we see that if we make the 8 too small, then each ommatidium does not ng of ommatidia in the eye of a bee look in only one direction, because of diffraction! If we make them too big, each one sees in a definite direction, but there are not enough of them to get a good view of the scene. So we adjust the distance d in order to make minimal the total effect of these two. If we add the two together, and find the place where the sum has a minimum(Fig. 36-9), we find that d(△n+△a (36.3) d8 △·/8 which gives us a distance Fig. 36-9. The optimum size for an If we guess that r is about 3 millimeters, take the light that the bee sees as 4000 (3×10-3×4×10-7) The book says the diameter is 30u, so that is rather good agreement! So, apparently, it really works, and we can understand what determines the size of the bee's ey It is also easy to put the above number back in and find out how good the bee's eye actually is in angular resolution; it is very poor relative to our own. We can things that are thirty times smaller in apparent size than the bee; the bee has a rather fuzzy out-of-focus image relative to what we e. Nevertheless it is all right, and it is the best they can do. We might ask why the bees do not develop
a good eye like our own, with a lens and so on. There are several interesting rea sons. In the first place, the bee is too small; if it had an eye like ours, but on his scale, the opening would be about 30 u in size and diffraction would be so impor tant that it would not be able to see very well anyway. The eye is not good if it is too small. Secondly, if it were as big as the bee's head, then the eye would occupy the whole head of the bee. The beauty of the compound eye is that it takes up no space, it is just a very thin layer on the surface of the bee. So when we argue that they should have done it our way, we must remember that they had their own 36-5 Other eyes Besides the bees, many other animals can see color. Fish, butterflies, birds, and reptiles can see color, but it is believed that most mammals cannot. The primates can see color. The birds certainly see color, and that accounts for the colors of birds. There would be no point in having such brilliantly colored males if the females could not notice it! That is, the evolution of the sexual"whatever it is"that the birds have is a result of the female being able to see color. So next time we look at a peacock and think of what a brilliant display of gorgeous color it is and how delicate all the colors are and what a wonderful aesthetic sense it takes to appreciate all that, we should not compliment the peacock, but should compliment the visual acuity and aesthetic sense of the peahen, because that is what has generated the beautiful scene! All invertebrates have poorly developed eyes or compound eyes, but all the vertebrates have eyes very similar to our own, with one exception. If we consider the highest form of animal, we usually say, " Here we are!, but if we take a less prejudiced point of view and restrict ourselves to the invertebrates, so that we cannot include ourselves, and ask what is the highest invertebrate animal, most zoologists agree that the octopus is the highest animal! It is very interesting that, besides the development of its brain and its reactions and so on, which are rather good for an invertebrate, it has also developed, independently, a different eye. is not a compound eye or an eye spot-it has a cornea, it has lids, it has an iris, it has a lens, it has two regions of water, it has a retina behind. It is essentially the same as the eye of the vertebrates! It is a remarkable example of a coincidence in olution where nature has twice discovered the same solution to a problem, with one slight improvement. In the octopus it also turns out, amazingly, that the retina is a piece of the brain that has come out in the same way in its embryonic develop ment as is true for vertebrates but the interesting thing which is different is that the cells which are sensitive to light are on the inside, and the cells which do the cal culation are in back of them, rather than"inside out, "as in our eye. So we see, at least, that there is no good reason for its being inside out. The other time nature tried it, she got it straightened out!(See Fig 36-10. The biggest eyes in the world Fig. 36-10. The eye of an octopus are those of the giant squid; they have been found up to 15 inches in diameter 36-6 Neurology of vision One of the main points of our subject is the interconnection of information from one part of the eye to the other Let us consider the compound eye of the horseshoe crab, on which considerable experimentation has been done. First of all, we must appreciate what kind of information can come along nerves. A nerve carries a kind of disturbance which has an electrical effect that is easy to detect,a kind of wavelike disturbance which runs down the nerve and produces an effect at the other end: a long piece of the nerve cell, called the axon, carries the infor mation along, and a certain kind of impulse, called a"spike, " goes along if it is excited at one end. When one spike goes down the nerve, another cannot immedi- ately follow. All the spikes are of the same size, so it is not that we get higher pikes when the thing is more strongly excited, but that we get more spikes per second. The size of the spike is determined by the fiber. It is important to appreci ate this in order to see what happens next
Fig. 36-11. The compound eye of the horseshoe crab. (a)Normal view. (b)Cross section. Figures 36-7, 11, 12, 13 reprinted with permission from Goldsmith, Sensory Ce Figure 36-11(a) shows the compound eye of the horseshoe crab; it is not very much of an eye, it has only about a thousand ommatidia. Figure 36-11(b)is a cross section through the system; one can see the ommatidia, with the nerve fibers nat run out of them and go into the brain. But note that even in a horseshoe crab there are little interconnections. They are much less elaborate than in the human eye, and it gives us a chance to study a simpler example Let us now look at the experiments which have been done by putting fine electrodes into the optic nerve of the horseshoe crab, and shining light on only one of the ommatidia, which is easy to do with lenses. If we turn a light on at some instant to, and measure the electrical pulses that come out, we find that there is a slight delay and then a rapid series of discharges which gradually slow down to a uniform rate, as shown in Fig. 36-12(a). When the light goes out, the discharge stops. Now it is very interesting that if, while our amplifier is connected to this same nerve fiber, we shine light on a diferent ommatidium nothing happens response the nerve fibers of the eye of the horse hoe crab Now we do another experiment: we shine the light on the nd get the same response, but if we now turn light on another one nearby as well the pulses are interrupted briefly and then run at a much lower rate(Fig. 36-12b) The rate of one is inhibited by the impulses which are coming out of the other In other words, each nerve fiber carries the information from one ommatidium but the amount that it carries is inhibited by the signals from the others. So, for xample, if the whole eye is more or less uniformly illuminated, the information coming from any one ommatidium will be relatively weak, because it is inhibited by so many. In fact the inhibition is additive--if we shine light on several nearby ommatidia the inhibition is very great. The inhibition is greater when the om matidia are closer, and if the ommatidia are far enough away from one another, inhibition is practically zero. So it is additive and depends on the distance; here is a first example of information from different parts of the eye being combined in he eye itself. We can see, perhaps, if we think about it awhile, that this is a device to enhance contrast at the edges of objects, because if a part of the scene is light and a part is black, then the ommatidia in the lighted area give impulses that are