To succeed with the Bates method no knowledge of anatomy or the psychology of vision is necessary. All that is needed is to follow the instructions in Part Two, and you may prefer to skip this and the next two chapters for the time being. However, even a slight acquaintance with the visual process will let you evaluate the reasoning behind the instructions and this in turn is likely to help your progress, for you will see that the method is as logical as it is mild. Understanding this will also, to a certain extent, prepare you for the very considerable astonishment of discovering for yourself that the method works.
The anatomy of the eye
Vision, of course, is the sense that animals have evolved using light to provide information about their surroundings. The simplest animals of all are, like plants, sensitive only to light itself. With increasing complexity, animals become capable of discerning contrast, movement, images, colour, and stereo depth.
Compared with that of the other senses, the potentiality of vision is very great, for it is capable of yielding detailed and highly specific information at a distance as well as near to. This is of profound importance for survival; among those animals whose way of life demands good vision, the evolution of the eye has reached incredible levels of development.
The eye of man is not the most structurally complex in the animal kingdom, but it is certainly one of the most advanced, serving a brain which is the most sophisticated creation evolution has yet produced. The quality of this computer is matched by the quality of its major input devices for external stimuli — the ears and the eyes.
In structure, the human eye is a typical vertebrate eye, of a pattern common to all mammals. It is a hollow sphere (actually a spheroid) filled with fluid under slight pressure. This pressure maintains the shape of the sphere.
The eye may be thought of as being divided into two compartments, front and rear, by the lens, an elastic, convex body about eight millimetres across. A perfectly clear fluid, the aqueous, fills the front compartment, while the larger rear compartment is filled by the more gelatinous vitreous, the third component of the optical media — the transparent contents of the sphere through which light must pass.
In anatomical terms, the sphere itself is made up of three distinct layers. These are the sclera, the uvea and the retina.
Figure 1. Horizontal section of the eye, showing terms mentioned in the text
The sclera is the outermost layer, the “white” of the eye, an extremely tough, fibrous sheath which protects the delicate structures within. At the front of the eye the sclera is modified into the cornea, a transparent, dome-shaped window which allows light to enter the eye.
The uvea consists of three parts: the iris, the ciliary body, and the choroid membrane. The iris lies just behind the cornea and is a muscular ring whose contractions can alter the size of the pupil, the aperture at its centre through which light gains access to the interior of the eye. The iris contains the pigment (brown, green, and so on) which gives the eye its “colour”. Having passed through the pupil, light now passes through the lens, which is attached round its edge by a ligamentous membrane, the zonule, to the muscles of the ciliary body. Movements of these muscles alter the convexity of the lens, changing its focal length. The third and final part of the uvea is the choroid membrane. This is the network of blood-vessels lining much of the sclera, and provides the principal blood-supply for the inside of the eyeball.
The innermost layer of the eye is the retina, an exceedingly complex and delicate membrane of nerve-cells which includes the all-important photoreceptors. The photoreceptors are of two types, the rods and the cones. Rods are sensitive to dim light and register only shades of grey, while cones work in good light and are the source of colour vision.
Organisation of the retina
During the development of the human embryo, the forebrain bulges into buds which are destined to become the optic cups; the retina is actually an outgrowth of the surface of the brain, a kind of outpost where the visual information is not only generated but also receives preliminary processing.
There are some 130 million photoreceptors in each retina but only a million nerve fibres in the optic tract — the “cable” running from the retina to the brain. Thus each fibre must be shared, on average, by about 130 photoreceptors. Part of the work of the retina is to achieve this sharing without loss of picture quality. This feat is performed in the layers of specialised cells found between the photoreceptors and the nerve fibres. It is assisted by the way the photoreceptors are distributed throughout the retina.
The outer edges of the retina contain relatively few photoreceptors, mostly rods, and provide vision which may be compared to that of primitive animals. At the very periphery of the retina, indeed, there is no conscious vision at all, merely an awareness of movement and contrast. When you see something “in the corner of your eye” and automatically turn to see it better, you are responding to signals generated in this portion of the retina.
Further in towards the centre, the photoreceptors become more densely packed and the ratio of cones to rods increases. Occupying the centre is a region about 5.5 millimetres across, the macula lutea (usually abbreviated to “macula”). Towards the centre of the macula is a shallow depression called the fovea centralis or simply “fovea”. The fovea is about 1.9 millimetres in diameter; at its centre, lying precisely on the visual axis, is an area only 0.35 millimetres across, the foveola.
In the fovea and foveola there are no rods, only cones, packed together so tightly that they look like rods. The cones reach their highest density in the foveola: the smallest cones here are less than one thousandth of a millimetre in effective diameter.
Throughout the retina as a whole, rods outnumber cones by about 18:1. It is the cones that are responsible for delivering precisely detailed vision. The importance of the cones is reflected in the generosity with which they are supplied with connections to the optic tract. Some of the cones in the foveola have exclusive use of a single nerve fibre. (In passing it is interesting to note that foveae, although found in certain fishes, lizards, and particularly in birds, do not occur in the lower mammals. Among mammals they appear only with the primates; the eye of the chimpanzee is remarkably similar to our own. Man’s highly developed fovea, with the sharp sight it provides both at a distance and near to, has been one of the chief assets in his career as first a hunter, then a farmer, and now a technologist.)
The photoreceptors contain pigments which are bleached by exposure to light. This chemical change is converted into the electrical stimulus which then passes along the nerve to the brain. Once bleached, the pigments in any given photoreceptor take a little while to be replaced. Exposure to a very bright light will completely bleach a whole area of the retina and for a time its sensitivity will be impaired. This is the reason for the familiar after-images experienced after looking at anything very bright.
The muscles of the eye
Selection and control of the image falling on the retina is carried out by three muscle systems, two located inside the eyeball itself and the third outside.
The first of these systems is the iris. As has already been said, the iris is a muscular ring whose central aperture, the pupil, may be varied in size. As any photographer knows, to get the best from his film he must vary the aperture according to the prevailing light intensity. Controlling the amount of light entering the eye, though, is not the primary function of the iris, for, while the area of the pupil changes over a ratio of only about 16:1, the range of light intensities in which the eye works varies over a ratio of at least 1,000,000:1. The main function of the iris is probably to restrict the incoming light to the macula, except at times (such as dawn or dusk) when maximum sensitivity is needed. The pupil also contracts for near vision, “stopping down” the “camera” of the eye so that depth of focus is enhanced.
The pupil opens and closes automatically in response to the amount of light falling on the retina. In other words, there is feedback from the retina to the iris.
This idea of feedback is encountered several times in the study of vision. It is important in accommodation — the process in which the eye adjusts itself to focus on near or far objects. The feedback in accommodation comes from the part of the brain where perception takes place; if the image is out of focus, orders will automatically be sent to readjust the focusing mechanism.
Now we come to the central controversy of the Bates method: the means whereby accommodation is achieved. The currently accepted belief is that accommodation is attained solely through the action of the second internal muscle system of the eye, the ciliary body.
In this chapter the orthodox theory will be described, although even here there is dispute and uncertainty among ophthalmologists about the exact mode of action of the ciliary muscle and its nerve supply.
For distance vision the lens needs to be relatively flat, but to bring the converging rays from a near object into sharp focus, the lens must become more convex. (More about this is explained at the beginning of the next chapter.) The lens consists of a soft central filling enclosed by an elastic capsule. The wall of the capsule is thinner in some places than others, and its natural tendency is to bulge into a convex shape. Unless tension is applied to the capsule by way of the zonule, therefore, the soft filling will tend to form into a convex shape and so decrease the focal length of the lens.
Looking at Figure 1, it would seem evident that, because the more convex shape is the natural resting state of the lens, an effort has to be made only when distance vision is needed. Surprisingly, however, the opposite is the case. The lens is kept under permanent tension by the zonule, so that the usual shape is flattened and suitable for distance vision. When near vision is required, the ciliary muscle contracts, pulling the ciliary body forward. The diameter of the ciliary body (remember it is shaped like a ring) is thus reduced, tension in the zonule eases, and the capsule and with it the substance of the lens assumes a more convex shape.
More will be said about accommodation later; for the present, we will go back to our consideration of the three muscle systems of the eye.
The third of these systems consists of the six extrinsic muscles which control the movement of each eye in its orbit. The extrinsic muscles are arranged in three pairs, attached to the sclera and working together in such a way that the eye can be turned in various directions.
Most muscles of the body contain one of two types of fibre. Muscles under conscious control (for example, the muscles of the hand) contain striped fibres, but those associated with involuntary functions (such as digestion) contain smooth tissue. The extrinsic muscles of the eye, however, contain a unique mixture of both types. As we shall see next, the extrinsic muscles perform some functions which are automatic and others which are under the control of the will.
Eye movements
Our eyes are supremely well adapted to binocular vision — the rather unusual arrangement whereby both eyes share much the same field of view and, by giving two slightly different images, enable the brain to deduce information about depth. The eyes thus work in unison, as a dual organ, and their extrinsic muscles are perhaps the most delicate and sensitive to be found anywhere in the body.
The extrinsic muscles have at least four functions, which may be summed up as follows:
1 controlling the visual axes;
2 tracking;
3 searching;
4 scanning.
If you look across the room and then refocus on a finger held about 30 centimetres (12 inches) from your nose, you will notice that your eyes have become slightly “crossed”: the two visual axes, instead of being virtually parallel, now converge on your finger. In this way both foveae are brought to bear on a single point.
For successful binocular vision, control of the visual axes must be very precise, and this control must of course also be maintained while the eyes are in motion.
The difference between the next two functions, tracking and searching, may be demonstrated quite simply. If you ask someone to follow a moving object (say your finger) with his eyes, you will notice that the eyes swivel smoothly in their sockets. If however you then ask the subject to perform the same eye movements on his own, without your finger to watch, his eyes will not move smoothly, but in a series of jerks.
Tracking, then, is quite different from searching. When tracking a moving target, a gunner must “lead the target” by aiming slightly ahead of it, the size of the lead being determined in part by the velocity and trajectory of the target and its distance from the gun. A practised shot uses his brain to perform the necessary computation almost instantaneously. It has been discovered that, while tracking, the eye must also lead the target. The eye anticipates the direction of movement by about six milliseconds (six thousandths of a second). The implications of this discovery are very remarkable indeed.
One might be forgiven for assuming that movements of the eyes are controlled, like muscle movements elsewhere in the body, by the brain. And in part, this assumption is correct. The commands directing the eyes to any part of the visual field arise in the brain; but what about the commands which enable the eyes to follow a moving object? The time required for the act of perception — for the light to stimulate the photoreceptors, for the nerve pulses to reach the brain, and for the brain to make sense of the signal — is in the order of 135 milliseconds. This delay alone, without even counting the time required for a return message from brain to eye muscles, is far too great to allow a “leading time” of only six milliseconds. If the commands came from the brain, the eyes would always be behind: they would never be able to focus on a flying bird or a moving tennis ball. Thus the guidance system that controls tracking cannot be located in the brain. It must be in the eye itself, almost certainly in the retina. We have already seen that the retina is in origin part of the surface of the brain. Besides the photoreceptors and their immediately associated cells, there are in the retina millions of other nerve-cells, very like those found in the brain itself, whose functions are still an almost total mystery.
The third type of eye movement, searching, has certain features in common with the fourth, scanning. As the second of our experiments showed, the eye searches the visual field by means of a series of jerks. Once something catches the attention, the jerks, or saccades, become smaller and are restricted to the region of the object being observed. Using an apparatus consisting of a minute mirror attached to a contact lens, researchers can trace these saccadic movements on photosensitive paper. When the subject is asked to fix his eyes on a single point, the tracings reveal that his gaze, while returning again and again to the point, wanders involuntarily over the surrounding area.
Because we see clearly only with the central portion of the retina, saccadic movement is necessary for exploration of the visual field. But its involuntary characteristcs are very like those of the fourth type of eye movement, a continuous, high-frequency tremor which here is called “scanning” — for reasons which will be apparent later.
Scanning is essential to vision. If, instead of a mirror, a miniature projector is attached to the contact lens, stabilising the image on the retina, vision rapidly fades. The subject sees the visual field becoming blurred and grey. Finally even the grey fades and is replaced by blackness. Then something unexpected happens. We are reminded that the brain is involved in vision as well as the eyes: from the blackness there emerge, in stately and inexplicable succession, one replacing another, ghostlike fragments of the original image.
Making sense of the signals
If vision can be likened to an end product whose raw material is light, the eye is no more than a supplier of crude and part-finished work to the main factory — the brain.
As we have already seen, a limited amount of preliminary processing of the raw visual data takes place in the retina, in two layers of cells called respectively the bipolar cells and the ganglion cells. To each bipolar cell are connected many individual photoreceptors, while, in turn, each photoreceptor is connected to a number of other bipolar cells. In a similar way, the bipolar cells are interconnected to the ganglion cells.
Figure 2: The pathway of information from retina to brain
From the ganglion cells, the electrical impulses leave the retina and are conducted along the optic tract towards the brain. The brain is divided into two hemispheres, left and right. Signals from the left-hand side of the left retina are conducted into the left hemisphere, but signals from the right-hand side of the left retina cross over into the right hemisphere. There is a similar cross-over from the left-hand side of the right retina. Thus the left hemisphere receives signals from the left-hand sides of both retinas and the right hemisphere receives signals from the right-hand sides. The point at which the pathways cross is called the optic chiasma. The signals next arrive at the primary visual centre, one in either hemisphere, where they are further processed before being sent on to the area striata, which is the region of the cerebral cortex devoted to vision.
The process of human perception is so intricate that only in recent decades has anything of value been learned about it. Attempts to equip machines with even rudimentary vision have increased our respect for the technical achievement of biological perception — and perception in man is almost certainly the most complex to be found in the living world. As for the way the area striata interacts with the rest of the cerebral cortex and, even more mysterious and fascinating, the way the cortex interacts with deeper regions of the brain, science has as yet uncovered virtually nothing.
The route of the nerve signals from retina to brain is summarised in Figure 2. In neurophysiological terms, analysis of the signals in the retina and also in the primary visual centre takes place by means of inhibition/excitation fields. The net result is that the area striata is fed with a coded version of the original image. The code is presented in terms of straight lilies, movement, and colour.
Any image, however complex, can be resolved into a number of straight lines, however tiny those lines might be. A circle, for instance, can be thought of as an infinitely large number of very short straight lines, each one aligned with its neighbours at a constant and very precise angle. (Owners of home computers who know a program for generating polygons will be familiar with this idea: once the number of sides of the polygon reaches 50 or so, the computer in effect draws a circle on the screen. In human vision, the number of sides of the polygon has to be vastly greater before a smooth circle is perceived, but the principle is the same.)
The incoming code classifies the straight lines according to whether they are “edges”, “strips”, or “slits”. It also classifies the movement, if any, of the image into its component directions. The area striata, by reading the code, translates it into a language of fantastic complexity — the language of vision.
The cerebral cortex, in which the area striata is found, is that part of the brain which, in man, is the seat of sensory perception, feelings, imagination, memory, thought, and indeed of personality itself. Although each area of the cortex is dedicated to one special function — such as hearing, word understanding, taste, vision, and so on — the different areas are interconnected by means of association fibres. This means in practice that our sensory perception, imagination, and so on, form an integrated and unified whole in which all parts of the cerebral cortex play a part.
Once a block of incoming code has been read, the information from either area striata — one dealing with the left-hand portion and the other with the right-hand portion of the original image — is recombined.
Studies of the psychology of vision have shown that, in order to understand the image being received by the eyes, the brain relies heavily on two associated functions of the cerebral cortex, imagination and memory. Seeing is an acquired skill as well as an innate one. Besides being a skill, it is also an art. In a real sense it is a creative process, like so many of the other functions of the cerebral cortex. Our past experience of the world is crucial to our present understanding of it. We learn certain rules (for example, that human beings, houses, trees, and so on tend to be of a certain size) and use them when trying to interpret an unfamiliar image. To demonstrate this, one has only to look at the well known optical illusion shown in Figure 3. Our experience of the world — and of stylised representations of it on paper — is such that we understand and accept the rules of perspective. Looking at the two sloping lines, we automatically assume that the rules of perspective are being invoked by the figure. It follows that the upper of the two horizontal lines must be farther away than the lower one. Therefore, the brain concludes, the upper line must be longer than the lower one, even though both are, of course, exactly the same length.
Figure 3: The Ponzo Illusion
Figure 4: An impossible object
Another set of rules is violated by Figure 4. Looking at the left-hand end of the figure, we interpret the image received as a representation of the tips of three parallel cylinders. But as the eye travels to the right the brain makes a new interpretation, based on the new information received. Both interpretations are “correct”, but each is mutually exclusive, and so the figure becomes an impossible object, even though it is, in reality, no more than a harmless set of lines printed on the page. The artist M. C. Escher’s famous representations of impossible scenes — waterfalls flowing uphill, little men endlessly climbing the same staircase, and so on — rely on this technique.
Figure 5: The Necker Cube
A slightly different example of the same sort of idea is demonstrated by Figure 5. Depending how you look at it (literally speaking), the figure can represent one of several things. It is obviously a cube, but on which face is the circle? One answer is to say that the cube is tilted downwards and that the circle is in the centre of the front face — but the circle could equally well be in the lower left corner of the rear face. Two further possibilities are presented if we regard the cube as being tilted upwards instead. Or again, the circle might be a sphere floating inside the cube, or some way behind or in front of it. Every solution is “correct” in perceptual terms; none is better than the others. The brain, however, insists that only one can be right, for this is how it perceives the world, choosing the best and most likely interpretation of the available data — the best guess. In this case the brain is unable to arrive at a decision and so the cube or the circle/sphere appears to jump back and forth according to the interpretation currently being entertained.
The visual process is, then, a function not only of the eyes and the immediately associated area of the brain, but of the cerebral cortex as a whole. Vision is a matter of memory and imagination as well as light. This will be understood immediately by anyone who has ever been fooled by one of those magazine pictures of familiar objects photographed from an unfamiliar angle, or who, in a contemplative moment, has ever seen faces in the fire. Our perceptual habits and beliefs are deeply influenced by our past experience, by our upbringing and background, and by the dictates of our personality. The way we look at the world is not only a unique expression of those habits: at the same time it confirms them and tends to make them more deeply ingrained.
Without going into the philosophical ideas raised by such a thought, we can say that there is a genuine biological basis for prejudice, for “pigeonholing”, and for a variety of other practices which do no one any good. One unexpected and exciting by-product of visual retraining by the Bates method is a change of outlook for the better, accompanied by a steadily increasing sense of integration with the world at large.