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The changes of air pressure in the neighborhood of your ears are very small. But, if you are to hear them, they must be great enough to make a slight vibration in your eardrums, the delicate membrane at the end of the tube which goes in from the external ear. The drum is linked mechanically to the much smaller "basilar membrane" in the inner ear, immersed in fluid and covered with nerve cells which are connected with the brain. So you hear a sound because the oscillations make your basilar membranes vibrate and the nerve cells on them signal the movement to your brain. ------------------ E. H. Adrian: From E. H. Adrian, "The Human Receiving System," The Languages of Science, Granada Lectures of the British Association for the Advancement of Science ( New York: Basic Books, Inc., Publishers, C) 1963), Chapter 6, 100-114. Reproduced with permission of the author and publisher. ---------------- You see an object because your eyes have lenses which focus its image on the sheet of nerve cells at the back of the eyeball, the retina. The cells there signal the pattern of light and shade to the brain, as the cells of the basilar membrane signal the pattern of sound. The eye and the ear are the two sense organs which deal with nearly all the information telling us what is happening in the world outside. The nose some times helps, and there are sensitive structures all over the body to give information about touch, pressure and pain, and temperature. Many other "senses" are needed to make the body work effectively as an organized whole. Signals from all of them must be sent up to the headquarters of the nervous system, the brain, and they must reach it with a minimum of delay. The science of communication has to deal with the physical transmission of information over thousands of miles, but if the information is to end up in the human brain, there is always this final biological step to consider-the transmission between the sense organ on the surface of the body and the brain inside the head. The transmitting elements are the nerve fibers. The nerve cells are a special variety with long or short filaments running out from the cell body, and there is usually one which is much longer than the rest. This is the nerve fiber. In man a fiber may run the whole five feet from the toes to the head. It is only a few thousandths of a millimeter in diameter, but it is protected by a sheath of fatty material, and as a rule many fibers run side by side to form a nerve trunk. One of the large nerves in the arm or leg may contain many thousands of them, and the optic nerves have a million or more. They are used for sending in all the information from the sense organs to the brain and for sending out all the signals to the muscles. Every kind of rapid communication in the body depends on these long threads of living matter which stretch out from the nerve cells, but the signals they transmit are all in the same form. There is, in fact, only one kind of disturbance that can be made to travel rapidly along a nerve fiber-a momentary surface change which we call the nerve impulse. In some fibers it can travel as fast as 100 meters a second, persisting no longer than a thousandth of a second at any point. A series of these impulses can be sent down the fiber at very short intervals, but it is not possible for the fiber to transmit impulses of different kinds. The nerve impulse, then, is the basis of communication within the body. It is a momentary disturbance which moves along a minute thread of living matter, but fortunately we can follow its movement because it produces a small charge of electrical potential. By recording this, we can reach a fairly clear picture of the impulse as a biophysical event. It depends on a sudden change at some point on the surface membrane of the fiber which makes it more permeable and allows a movement of ions in and out. This is soon brought to an end, but the forces set up induce a similar change in the neighboring surface with a similar movement of ions, and in this way the surface change travels from one end of the fiber to the other. The sequence of events has been studied in detail by taking advantage of the remarkable variety of structure in the animal kingdom. The nerve fibers of vertebrates are too slender to stand much manipulation, but the squid and the cuttlefish have a few very large nerve fibers to carry out the most rapid signaling-tubular structures with a diameter of as much as one millimeter. In these giant fibers electrodes and pipettes can be placed inside as well as outside the fiber without interfering with its power to conduct impulses. In this way Hodgkin and his colleagues have been able to show that, when the surface change takes place, there is first a movement of sodium ions into the fiber and then of potassium ions out of it. The movement takes place because the resting fiber always maintains a lower concentration of sodium and a higher of potassium in side than outside. It is these differences in concentration which provide the store of available energy needed in the transmission process, and they are re-established as soon as the impulse has passed. But very little energy is involved and it is not yet certain how the recharging is done. In fact, we are still very far from a complete picture on a molecular scale of all that is taking place in the nerve fiber. In spite of that, we have a reasonably clear picture of the traveling disturbance, the basis for all nervous signaling. The picture applies to nerve fibers of every kind, though the active region does not travel at the same speed in all. The speed depends on the temperature as well as on the dimensions and structure of the fiber. In mammals, which have a constant temperature, the velocity of the impulse in the smallest nerve fibers may be less than one meter a second, but in some of the larger it reaches 100 meters. Thus, in man an impulse set up in the finger tip can reach the brain in one hundredth of a second. However rapidly it may have traveled, the arrival of a single impulse in a sensory nerve fiber can only show that the ending has been stimulated. It cannot show what has stimulated it or whether the stimulus was weak or strong, for the impulses themselves cannot be made to vary; each is the same brief surface change. But the signals in each fiber are nearly always made up of a group of impulses, and this can convey much more information, because both the number in the group and the intervals between them can be varied. There must always be a short interval, a pause for recovery between one impulse and the next. At the sensitive ending, however, a mechanical or chemical stimulus will set up repeated impulses as long as it remains effective, and the interval between them will depend on its intensity. At the receiving end, therefore, the spacing of the impulses will show whether the stimulus is strong or weak. It will also show how it fluctuates from moment to moment, for the intervals we are concerned with are all very short: the frequency of a train of impulses may be as high as 500 a second. Thus a sequence lasting less than a second will be quite capable of indicating rapid changes in the intensity of the stimulus. The nature of the stimulus cannot be indicated in the message, but it can be inferred from the particular nerve fibers which carry it. The fibers from the different kinds of sense organs run a different course in the central nervous system, and so messages arriving in the optic nerve fibers indicate light and darkness, and those in the auditory nerve indicate sound. Messages in the skin nerves have to indicate touch, temperature, or pain, but the fibers for each kind of sensation are of different size and have different central connections. In fact, all our detailed information about sights and sounds and contacts with the body must al ways depend on the arrangement of the nerve fibers as independent pathways leading to corresponding parts of the brain. A complex pattern of light and shade on the retina produces a corresponding pattern of activity in the sensitive elements, and so in the nerve cells of the brain; a complex sound is analyzed into its component frequencies by the basilar membrane of the ear, and each component is signaled by different groups of nerve fibers, so that the pattern which reaches the brain corresponds to that set up in the vibrating membrane. And, of course, the number of nerve fibers in action will show whether the stimulus is restricted or widespread. Touching the skin with a hair will give impulses in a few fibers, pressure over a large area will give them in a large number. It is not difficult to show that these signals from our various sense organs do arrive in different parts of the human brain, for the activity they set up in it gives rise to changes of electrical potential in the brain surface. These can be re corded in detail if the brain surface is exposed, but they can be detected in nor mal conditions by electrodes fastened to the scalp. The signals from the eye arrive in the occipital lobe of the brain, so the electrical disturbance is limited to the occipital region when a flash of light reaches the eye. Signals from the skin arrive in the parietal lobe; a stimulus to the skin produces an electrical disturbance limited to the scalp over the parietal region. Up to this point the story is fairly clear. Information from the sense organs is communicated to the central nervous system by trains of impulses in the nerve fibers; the spacing of the impulses in each fiber shows the intensity of the stimulus from moment to moment; and its nature can be inferred from the particular connections of the nerve fiber-whether it leads from the ear, the eye, or the skin, etc. In theory, at least, if we could record all the impulse messages entering the central nervous system in a given period and identify the nerve fiber conveying each of them, we ought to be able to extract all the information which reaches the brain from the outside world. Beyond that the story is much less clear. The information enters the central nervous system, which is no more than an elaborate organization of nerve cells and connecting fibers. It is a very delicate structure, protected from injury by being enclosed in bone and cushioned by fluid. The brain in man is by far the largest and most important part of the nervous system, for it is the part which is specially related to intelligent behavior and to mental activity and consciousness. The brain is concerned with general policy, with directing activity in accordance with the external situation as signaled by the sense organs, and with the past experience of the individual, as stored up in memory traces, habits or conditioned reflexes. But the administrative details are managed by the spinal cord and brain stem. This is the more primitive part of the central nervous system, well developed in all vertebrates. It is the route by which information reaches the cerebrum and by which all the executive signals to the muscles are sent out. Much of our knowledge of what goes on in the brain has been based on the analysis by Sir Charles Sherrington of what goes on in the spinal cord and brain stem, for these parts contain most of the nervous organization which makes it possible for an animal to move as a whole, to balance its body in the standing position, to walk or run, and to carry out movements in response to sensory stimulation. Elaborate activities of this kind can go on without the brain, and the movements are smoothly executed because there are many sense organs in muscles and joints to supply the brain stem and cord with a continuous picture of tensions and pressure in the limbs. Whenever a muscle contracts, there is a sensory feedback by trains of impulses focused on the nerve cells which are directing the movement. There are many cross-connections and many kinds of nerve cells to be reckoned with, and at the back of all problems of how the nervous sys tem works there is the problem of how it came to be built. But if we take the structural organization for granted, there should be no difficulty in understanding how this part of the central nervous system co-ordinates all our postures and movements. Understanding what goes on in the brain is much more difficult, because we are dealing now with the headquarters of the nervous system. The reports from the outside world are analyzed there, and the appropriate course of action is decided. The receipt of messages from the sense organs by the brain not only decides our behavior; it affects our thinking as well. Diagrams of impulses playing upon an elaborate organization of nerve cells may be all we need to account for the skilled movements we can make, but our conscious activity seems to be in a different category altogether. Perhaps there is no real difficulty in this; at all events, we have not yet reached the stage where the physiologist has to be concerned with it. We can at least be sure that our conscious picture of the world is very closely related to the stream of information which reaches the brain from the sense organs, and we can think of both together as the product of the impulse messages in the sensory nerve fibers. But a diagram of the sense organs with pathways leading to different regions of the brain surface gives a misleading impression of the way in which our information is collected. It seems to imply that the sense organs are left to them selves to signal the particular events which happen to excite them. In fact, they need constant adjustment by the central nervous system if they are to work effectively, and the adjustment often involves activity in other parts of the body. For instance, the eyes have diaphragms which are opened or closed to admit the amount of light most suitable for the retina, and both eyes are directed to the same point by the external ocular muscles. The eardrum has a small muscle to adjust its tension to suit the noise. To smell, we regulate the air current through the nose by sniffing if the odor is faint and holding our breath if it is too strong. Adjustments of this kind are made automatically by nerve cells controlled by the discharge from the sense organ, but in addition we make active use of our sense organs to explore our surroundings. The eyes and the head are both turned to bring fresh areas into the field of vision; we finger an object to discover its texture; in fact, we are constantly focusing our sense organs on objects which have aroused our interest. That brings us to one of the principal factors determining cerebral activity, the factor described as attention. Many different streams of information come into the brain and reach the level of consciousness, and the impulses which the brain sends out to the muscles may keep several kinds of skilled movement in progress at the same time. When we drive a car, for instance, we can watch the road as we listen to what our passenger is saying, and we can answer him as we keep the car on its course. The brain is a very large organization of nerve cells; networks of connecting filaments spread out into a thick layer over the cerebral hemisphere. There should be room enough in it for several independent streams of activity. But it does not take much introspection to realize that at certain times a particular kind of information or a particular line of action may have complete priority. The sight of a car on the wrong side of a crowded road may make us deaf to what our passenger is saying. All our attention is then given to what we are seeing and to the movements we must make to escape an accident. This kind of selective process is most in evidence in emergencies, but it operates continuously, either by making us quite unaware of events which are signaled to the brain or by suppressing the further process of recalling their associations. Thus, I can see an audience, but as long as I am occupied with giving a lecture I cannot attend to the details of the visual scene or recognize a familiar face. This selective process comes in at the cerebral level and does not apply to the reactions carried out by the brain stem and spinal cord: my preoccupation with my talk has no influence at this level and does not affect the adjustments which keep my balance and co-ordinate my breathing movements as I speak. In fact, as long as I stay awake, my spinal cord and brain stem will continue their automatic control of posture and movement: the signals which are sent out to the muscles will always be adjusted by the inflow of impulses which signal the pressures and tensions. At the cerebral level, the signals which claim attention are of two kinds: the sudden interruption and the message which brings exciting news. A flash of light or a loud bang, anything unexpected, can distract us, for the moment at least, whatever its nature. But more lasting effects are produced by the complex messages which arouse association with a strong emotional coloring-fear or anger or pleasure. We are still a long way from understanding how memories are stored in the brain and how an incoming message excites our memory system. Clearly it must contain elements which fit somehow into the pattern laid down by a past experience, but until we know more about the biophysical processes involved in learning and memory, we cannot tell what sort of patterns they are. And until we know more about the forces which direct our behavior, we can say little about the way in which they are related to emotional experiences. We have some evidence to show how the general level of activity in the brain can be influenced by sensory messages. In the upper part of the brain stem there is a mass of cells and cross-connections known as the "reticular formation." If this is stimulated, either directly by an electric current or by the arrival of trains of impulses from the sense organs, there is an immediate change in the activity of the cerebral hemispheres, shown both by the electrical oscillations which can be detected at the brain surface and by the general behavior of the animal. If there is little to activate the reticular formation, the cerebrum soon lapses into the condition it is in when the animal is falling asleep. There are large electrical oscillations from most of the brain surface with a regular rhythm of ten a second or less, or still larger waves at longer intervals. Stimulating the reticular formation brings an immediate change. The regular oscillations are replaced by much smaller electrical oscillations, occurring irregularly at higher frequencies, and the animal becomes awake and alert. In man the closing and opening of the eyes usually causes a change of the same kind in the electrical activity of the brain; we can remain reasonably alert with our eyes closed, but closing them cuts off the most important source of information to the brain, and in most of us this makes the electrical activity change from the smaller irregular oscillations of potential to the regular ten-a-second rhythm. When we are really fast asleep, this rhythm goes, too, for it seems to represent the state in which most of the cerebral hemispheres are unoccupied but are ready to take part in some line of intelligent activity if the need for it arises. The electrical oscillations at the surface of the brain are much easier to record than to interpret. There are other lines of evidence about the change from sleep to waking, however, and they support the idea that it is controlled by the central regions at the base of the brain, including the reticular formation. Any marked increase in the flow of impulses from the sense organs seems to act there, and its effect is to cause a general increase in the activity of the brain and in its ability to interpret the information it receives. The alarm clock or the shaft of sunlight through the curtains wakes us up because the sudden increase in the sensory input turns up the volume control. The whole cerebral system is made more active, and so we become conscious and aware enough of our surroundings to remember where we are. This does not help us to understand how our attention comes to be concentrated in particular fields when we are wide awake. What makes a message arouse and fix our interest? It may help us to think of the memory system of the brain as a vast number of files of different importance and accessibility. Those relating to all the skilled habits we have learned are seldom referred to, because they have been incorporated in the daily routine of our life. Others may be concerned with trivial events; others, marked "personal and confidential," can be examined only at subconscious levels or with the aid of a psychoanalyst. But the signals which contain the key to important files will open them wide enough to cause a stirring in that part of the memory system, whether it reaches consciousness or not. We suppose, perhaps on rather slender evidence, that this increased activity is signaled back to the central controlling regions, and that they react to it by opening up the channels for this particular line of information. Most of the cerebral apparatus will then be brought to bear on it, and information about other events will be neglected or suppressed. We need not suppose that this will apply only to signals which arrive from the sense organs. A good deal of irregular activity goes on in the cells of the brain. From time to time the random pattern may well contain some elements which arouse interest and start a new train of thought. And we need not look far to explain why, after we have attended to one line of information or one train of thought for some time, we begin to lose interest in it and are more likely to let our attention be caught by something else. There is quite enough evidence on the physiological side to show that living cells in general, and particularly those of the central nervous system, become adapted sooner or later to disturbing forces and cease to react to them as they did initially. The basic truth is that expressed by W. S. Gilbert in Trial by Jury: You cannot eat breakfast all day, Nor is it the act of a sinner, When breakfast is taken away, To turn your attention to dinner. But the function of the central nervous system is to receive and classify an immense amount of information from various sense organs and to arrive at a general line of policy for the individual-a policy for the future as well as for the immediate present. It is the supreme integrative organ, and we cannot expect to learn much about the detailed operation of particular parts by studying the smooth performance of the whole. So let us turn our attention again to the sense organs, particularly to the one which we rely on for most of our information- that is, the eye. By recording the flow of impulses in the optic nerve fibers, we are beginning to learn a good deal about the way in which information in general is passed on, for the retina is really far more than a mosaic of light-sensitive end organs connected to the brain by the optic nerve; in fact, it is itself an outlying part of the central nervous system. The retinal sheet is like the cerebral cortex, in that it has several layers of nerve cells with connections laterally as well as in depth. The impulses it sends to the brain have started not from the end-organs, the rods and cones in the sensitive surface of the retina, but from the deepest layer of nerve cells in it. Many end-organs may be in connection with each of these cells, and the information which is handed on by them is not a total record of the illumination of the end organ at each moment: it has already been condensed and edited on its way through the retinal layers. With other kinds of end-organ, the same kind of editing may well occur at various levels below the cerebral hemisphere. It is editing of a journalistic kind, since it emphasizes the latest news and presents a picture of it with all the contrasts exaggerated. One of the ways in which this is done is peculiar to the eye. It has been found that, when we try to see clearly, our eyeballs are not focused steadily on the object: very small jerking movements go on constantly, so that the image on the retina is always shifting to and fro slightly in relation to the sensitive elements which it covers. If this slight shifting of the image is prevented, it is very soon impossible to see any of the details in it. The simplest way of stopping the movement would be to fix the eyeball. A more convenient one, used by Professor Ditchburn, is to fasten a delicate optical system to the front of the eyeball so that, in spite of its movement, the image is always focused on the same place on the retina. It is not difficult to understand why this prevents us from seeing the details. If we look at a small black patch on a white field, a light to-and-fro movement of the image on the retina will insure that the sensitive elements near the boundary will be repeatedly moving from light to dark and back again. A fresh stimulation is always more effective than one which has been in action for some time. Therefore the regions at the edge of the patch send in repeated signals at high intensity. In this way, as long as the to-and-fro movement of the image can go on its contour is repeatedly emphasized. If the image is always kept to the same place, there is no special signaling to show its outline and we soon cease to be aware of it. There are other ways in which the nervous system of the retina can emphasize particular features in the visual pattern. Contrasts are heightened because regions which are more strongly stimulated inhibit the activity of regions nearby. And there is at least the possibility that certain shapes or certain patterns of movement may be given special prominence in the messages to the brain, owing to the shape and size of the retinal fields which lead to each optic nerve fiber. In fact, the more we explore the discharges in the optic nerve, the more evidence we find of an editing in the retina brought about by the particular arrangement of the nervous pathways in it and by the interactions known to take place between one nerve cell and another. The slight unsteadiness of the eyeball can be an aid to distinct vision, because it emphasizes the outlines of the visual pattern. With other sense organs, the outlines are less important, but if we search for a railway ticket in our pocket, we must move the tactile surface of the fingers over all the other objects there until the familiar signals are aroused. The signals will come from muscles and joints as well as from the finger tips, and the different messages must be brought together at some level in the central nervous system. But the pattern which indicates the oblong cardboard will depend on a particular timing of the messages from the different sense organs as well as on their intensity. The timing is, of course, important in appreciating visual information. With the ear, it is still more important. The different frequencies in a sound excite different parts of the sheet of end-organs in the inner ear, so that impulses in a particular group of nerve fibers signal a particular tone. But a tune or a voice is recognized by the way in which the tone pattern changes from moment to moment, and it is the sequence rather than the exact pitch that is important. In fact, with sound and sight, and probably with smell too, it is the arrangement of the pat tern and the way it changes which stirs up our store of memories. The particular groups of nerve fibers which transmit the signals show the exact position of the image on the retina, but it is the shape of the image, rather than its size and exact position, which makes us recognize a letter of the alphabet or a face. This brings us finally to the headquarters of our internal communication system. What arrives in consciousness is the outcome of a vast number of units which transmit independent signals but have many opportunities of influencing one another in the central nervous system. We can analyze the signaling by recording the impulses sent by each unit, but we must never think of them as active in isolation. Any change in the flow of impulses into or out of the central nervous system will cause readjustments in every part. There are countless internal feedback circuits besides those which co-ordinate our movements. At every level we must think of a constantly shifting activity adjusted to keep the whole organ ism running smoothly and engaged in the line of conduct which suits the circum stances. This line, our general policy, is decided at the highest level of all, that of the cerebrum. There the pattern of activity is shaped by the incoming signals, suitably edited, reacting with the store of past experience laid down in the brain. We do not know exactly where or how it is laid down, but we are all aware that it plays an important part in directing our attention and our behavior. Many of the signals from the sense organs can reach the level of consciousness and cause a brief adjustment of our activity, but those that interest us and direct our thoughts have been reinforced by the habits and images formed by past experience. This gives the impulse message its full meaning and fits it into our mental picture of the outside world. Also in Part 3:
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