Chapter 31
SENSATION
AN INVENTORY OF THE ELEMENTARY SENSATIONS OF THE DIFFERENT SENSES
With reflex action, instinct, emotion and feeling, the list of native mental activities is still incomplete. The senses are provided by nature, and the fundamental use of the senses goes with them. The child does not learn to see or hear, though he learns the meaning of what he sees and hears. He gets sensation as soon as his senses are stimulated, but recognition of objects and facts comes with experience. Hold an orange before his open eyes, and he sees, but the first time he doesn't see _an orange_. The adult sees an object, where the baby gets only sensation. "Pure sensation", free from all recognition, can scarcely occur except in the very young baby, for recognition is about the easiest of the learned accomplishments, and traces of it can be seen in the behavior of babies only a few days old.
Sensation is a response; it does not come to us, but is aroused in us by the stimulus. It is the stimulus that comes to us, and the sensation is our own act, aroused by the stimulus. Sensation means the activity of the receiving organ (or sense organ), of the sensory nerves, and of certain parts of the brain, called the sensory centers. Without the brain response, there is apparently no conscious sensation, so that the activity of the sense organ and sensory nerve is preliminary to the sensation proper. Sensation may be called the first response of the brain to the external stimulus. It is usually only the first in a series of brain {188} responses, the others consisting in the recognition of the object and the utilization of the information so acquired.
Sensation, as we know it in our experience, goes back in the history of the race to the primitive sensitivity (or irritability) of living matter, seen in the protozoa. These minute unicellular creatures, though having no sense organs--any more than they have muscles or digestive organs--respond to a variety of stimuli. They react to mechanical stimuli, as a touch or jar, to chemical stimuli of certain kinds, to thermal stimuli (heat or cold), to electrical stimuli, and to light. There are some forces to which they do not respond: magnetism, X-rays, ultraviolet light; and we ourselves are insensitive to these agents, which are not to be called stimuli, since they arouse no response.
The Sense Organs
In the development of the metazoa, or multicellular animals, specialization has occurred, some parts of the body becoming muscles with the primitive motility much developed, some parts becoming digestive organs, some parts conductors (the nerves) and some parts becoming specialized receptors or sense organs. A sense organ is a portion of the body that has very high sensitivity to some particular kind of stimulus. One sense organ is highly sensitive to one stimulus, and another to another stimulus. The eye responds to very minute amounts of energy in the form of light, but not in other forms; the ear responds to very minute amounts of energy in the form of sound vibrations, the nose to very minute quantities of energy in certain chemical forms.
There is only one thing that a sense organ always and necessarily contains, and that is the _termination of a sensory nerve_. Without that, the sense organ, being isolated, would have no effect on the brain or muscles or any other {189} part of the body, and would be entirely useless. The axons of the sensory nerve divide into fine branches in the sense organ, and thus are more easily aroused by the stimulus.
Besides the sensory axons, two other things are often found in a sense organ--sometimes one of the two, sometimes the other and sometimes both. First, there are special sense cells in a few sense organs; and second, in most sense organs there is accessory apparatus which, without being itself sensitive, assists in bringing the stimulus to the sense cells or sensory nerve ends.
_Sense cells_ are present only in the eye, ear, nose and mouth--always in very sheltered situations. The taste cells are located in little pits opening upon the surface of the tongue. In the sides of these pits can be found little flask-shaped chambers, each containing a number of taste cells. The taste cell has a slender prolongation that protrudes from the chamber into the pit; and it is this slender tip of the cell that is exposed to the chemical stimulus of the {190} tasting substance. The stimulus arouses the taste cell, and this in turn arouses the ending of the sensory axon that twines about the base of the cell at the back of the chamber. The taste cell, or its tip, is extra sensitive to chemical stimuli, and its activity, aroused by the chemical stimulus, in turn arouses the axon and so starts a nerve current to the brain stem and eventually to the cortex.
The olfactory cells, located in a little recess in the upper and back part of the nose, out of the direct air currents going toward the lungs, are rather similar to the taste cells. They have fine tips reaching to the surface of the mucous membrane lining the nasal cavity and exposed to the chemical stimuli of odors. The olfactory cell has also a long slender branch extending from its base through the bone into the skull cavity and connecting there with dendrites of nerve cells. This central branch of the olfactory cell is, in fact, an axon; and it is peculiar in being an axon growing from a sense cell. This is the rule in invertebrates, but in vertebrates the sensory axon is regularly an outgrowth of a {191} nerve cell, and only in the nose do we find sense cells providing their own sensory nerve.
In the eye, the sense cells are the rods and cones of the retina. These are highly sensitive to light, or, it may be, to chemical or electrical stimuli generated in the pigment of the retina by the action of light. The rods are less highly developed than the cones. Both rods and cones connect at their base with neurones that pass the activity along through the optic nerve to the brain.
The internal ear contains sense cells of three rather similar kinds, all being "hair cells", Instead of a single {192} sensitive tip, each cell has a number of fine hair-tips, and it is these that first respond to the physical stimulus. In the cochlea, the part of the inner ear concerned with hearing, the hairs are shaken by sound vibrations that have reached the liquid in which the whole end-organ is immersed. In the "semicircular canals", a part of the inner ear that is concerned not with sound but with rotary movements of the head, we find hair cells again, their hair-tips being matted together and so located as to be bent, like reeds growing on the bottom of a brook, by currents of the liquid filling the canals. In the "vestibule", the central part of the inner ear, the hair-tips of the sense cells are matted together, and in the mat are imbedded little particles of stony matter, called the "otoliths". When the head is inclined in any direction, these heavy particles sag and bend the hairs, so stimulating them; and the same result occurs when a sudden motion up or down or in any direction is given to the head. Around the base of the sense cells, in any of these parts of the internal ear, are twined the fine endings of sensory axons, which are excited by the activity of the sense cells, and pass the activity on to the brain.
Accessory sense-apparatus.
Every sense except the "pain sense" has more or less of this. The hairs of the skin are accessory to the sense of touch. A touch on a hair is so easily felt that we often think of the hairs as sensitive; but really it is the skin that is sensitive, or, rather, it is the sensory axon terminating around the root of the hair in the skin. The tongue can be thought of as accessory apparatus serving the sense of taste, and the breathing apparatus as accessory to the sense of smell, "tasting" being largely a tongue movement that brings the substance to the taste cells, and "smelling" of anything being largely a series of little inspiratory movements that carry the odor-laden air to the olfactory part of the nasal cavity.
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But it is in the eye and the ear that the highest development of accessory sense apparatus has taken place. All of the eye except the retina, and all of the ear except the sense cells and the sensory axons, are accessory.
The eye is an optical instrument, like the camera. In fact, it is a camera, the sensitive plate being the retina, which differs indeed from the ordinary photographic plate in recovering after an exposure so as to be ready for another. Comparing the eye with the camera, we see that the eyeball corresponds to the box, the outer tough coat {194} of the eyeball (the "sclerotic" coat) taking the place of the wood or metal of which the box is built, and the deeply pigmented "choroid" coat, that lines the sclerotic, corresponding to the coating of paint used to blacken the inside of the camera box and prevent stray light from getting in and blurring the picture. At the front of the eye, where light is admitted, the sclerotic is transformed into the transparent "cornea", and the choroid into the contractile "iris", with the hole in its center that we call "the pupil of the eye".
The iris corresponds to the adjustable diaphragm of the camera. Just behind the pupil is the lens of the eye, which also is adjustable by the action of a little muscle, called the "ciliary muscle". This muscle corresponds to the focussing mechanism of the camera; by it the eye is focussed on near or far objects. The eye really {195} has two lenses, for the cornea acts as a lens, but is not adjustable. The "aqueous and vitreous humors" fill the eyeball and keep it in shape, while still, being transparent, they allow the light to pass through them on the way to the retina. The retina is a thin coat, lying inside the choroid at the back of the eyeball, and having the form of a hollow hemisphere. The light, coming through the pupil and traversing the vitreous humor, strikes the retina from the inside of the eyeball. Other accessory apparatus of the eye includes the lids, the tear glands, and the muscles that turn the eyeball in any direction.
The ear is about as complex a piece of mechanism as the eye. We speak of the "outer", "middle" and "inner" ear. The outer, in such an animal as the horse, serves as a movable ear trumpet, catching the sound waves and concentrating them upon the ear drum, or middle ear. The human external ear seems to accomplish little; it can be cut off without noticeably affecting hearing. The most essential part of the external ear is the "meatus" or hole that allows the sound waves to pass through the skin to the tympanic membrane or drum head. The sound waves throw this membrane into vibration, and the vibration is transmitted, by an assembly of three little bones, across the air-filled cavity {196} of the middle ear to an opening leading to the water-filled cavity of the inner ear. This opening from the middle to the inner ear is closed by a membrane in which one end of the assembly of little bones is imbedded, as the other end is imbedded in the tympanic membrane; and thus the vibrations are transmitted from the tympanic membrane to the liquid of the inner ear. Once started in this liquid, the vibrations are propagated through it to the sense cells of the cochlea and stimulate them in the way already suggested.
Further study of the accessory apparatus of the eye and ear can be recommended as very interesting, but the little that has been said will serve as an introduction to the study of sensation.
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Analysis of Sensations
Prominent among the psychological problems regarding sensation is that of analysis. Probably each sense gives comparatively few elementary sensations, and many blends or compounds of these elements. To identify the elements is by no means a simple task, for under ordinary circumstances what we get is a compound, and it is only by carefully controlling the stimulus that we are able to get the elements before us; and even then the question whether these are really elementary sensations can scarcely be settled by direct observation.
Along with the search for elementary sensations goes identification of the stimuli that arouse them, and also a study of the sensations aroused by any combination of stimuli. Our task now will be to ask these questions regarding each of the senses.
The Skin Senses
Rough and smooth, hard and soft, moist and dry, hot and cold, itching, tickling, pricking, stinging, aching are skin sensations; but some of these are almost certainly compounds. The most successful way of isolating the elements out of these compounds is to explore the skin, point by point, with weak stimuli of different kinds. If a blunt metal point, or the point of a lead pencil, a few degrees cooler than the skin, is passed slowly over the skin, at most points no sensation except that of contact arises, but at certain points there is a clear sensation of cold. Within an area an inch square on the back of the hand, several of these _cold spots_ can be found; and when the exploration is carefully made, and the cold spots marked, they will be found to give the same sensation every time. Substitute a metal point a few {198} degrees warmer than the skin, and a few spots will be found that give the sensation of warmth, these being the _warmth spots_. Use a sharp point, like that of a needle or of a sharp bristle, pressing it moderately against the skin, and you get at most points simply the sensation of contact, but at quite a number of points a small, sharp pain sensation arises. These are the _pain spots_. Finally, if the skin is explored with a hair of proper length and thickness, no sensation at all will be felt at most points, because the hair bends so readily when one end of it is pressed against the skin as not to exert sufficient force to arouse a sensation; but a number of points are found where a definite sensation of touch or contact is felt; these are the _touch spots_.
No other varieties of "spots" are found, and the four sensations of touch, warmth, cold and pain are believed to be the only elementary skin sensations. Itch, stinging and aching seem to be the same as pain. Tickle is touch, usually light touch or a succession of light touches. Smooth and rough are successions of touch sensations. Moist is usually a compound of smooth and cold. Hard and soft combine touch and the muscular sensation of resistance.
Hot and cold require more discussion. The elementary sensations are warmth and coolness, rather than hot and cold. Hot and cold are painful, and the fact is that strong temperature stimuli arouse the pain spots as well as the warmth or cold spots. Hot, accordingly, is a sensation compounded of warmth and pain, and cold a sensation composed of coolness and pain. More than this, when a cold spot is touched with a point heated well above the skin temperature (best to a little over 100 Fahrenheit), the curious fact is noted that the cold spot responds with its normal sensation of cold. This is called the "paradoxical cold sensation". From this fact it is probable that a hot object excites the cold sensation, along with those of warmth and {199} pain; so that the sensation of heat is a blend of the three. Another curious fact is that a very cold object produces a burning sensation indistinguishable from that of a hot object; so that the sensation of great cold, like that of heat, is probably a blend of the three elementary sensations of warmth, cold and pain.
The stimulus that arouses the touch sensation is a bending of the skin. That which arouses warmth or cold is of {200} course a temperature stimulus, but, strange as it may seem, the exact nature of the effective stimulus has not been agreed upon. Either it is a warming or cooling of the skin, or it is the existence of a higher or lower temperature in the skin than that to which the skin is at the moment "adapted". This matter will become clearer when we later discuss adaptation. The stimulus that arouses the pain sensation may be mechanical (as a needle prick), or thermal (heat or cold), or chemical (as the drop of acid), or electrical; but in any case it must be strong enough to injure or nearly to injure the skin. In other words, the pain sense organ is not highly sensitive, but requires a fairly strong stimulus; and thus it is fitted to give warning of stimuli that threaten injury.
Several kinds of sensory end-organ are found in the skin. There is the "spherical end-bulb", into which a sensory axon penetrates; it is believed to be the sense organ for cold. There is the rather similar "cylindrical end-bulb" believed to be the sense organ for warmth. There is the "touch corpuscle", found in the skin of the palms and soles, and consisting, like the end-bulbs, of a mass of accessory cells with a sensory axon ramifying inside it; this is an end-organ for the sense of touch. There is the hair end-organ, consisting of a sensory axon coiled about the root of the hair; this, also, is a touch receptor. Finally, there is the "free-branched nerve end", consisting simply of the branching of a sensory axon, with no accessory apparatus whatever; and this is the pain receptor. Perhaps the pain receptor requires no accessory apparatus because it does not need to be extremely sensitive.
Now since we find, in the skin, "spots" responsive to four quite different stimuli, giving four quite different sensations, and apparently provided with different types of end-organs, it has become customary to speak of four skin senses in place of the traditional "sense of touch". We {201} speak of the pain sense, the warmth sense, the cold sense, and the pressure sense, which last is the sense of touch proper.
The Sense of Taste
Analysis has been as successful in the sense of taste as in cutaneous sensation. Ordinarily we speak of an unlimited number of tastes, every article of food having its own characteristic taste. Now the interior of the mouth possesses the four skin senses in addition to taste, and many tastes are in part composed of touch, warmth, cold or pain. A "biting taste" is a compound of pain with taste proper, and a "smooth taste" is partly touch. The consistency of the food, soft, tough, brittle, gummy, also contributes, by way of the muscle sense, to the total "taste". But in addition to all these sensations from the mouth, the flavor of the food consists largely of odor. Food in the mouth stimulates the sense of smell along with that of taste, the odor of the food reaching the olfactory organ by way of the throat and the rear passage to the nose. If the nose is held tightly so as to prevent all circulation of air through it, most of the "tastes" of foods vanish; coffee and quinine then taste alike, the only _taste_ of each being bitter, and apple juice cannot be distinguished from onion juice.
But when the nose is excluded, and when cutaneous and muscular sensations are deducted, there still remain a few genuine tastes. These are sweet, sour, bitter and salty--and apparently no more. These four are the elementary taste sensations, all others being compounds. The papillae of the tongue, with their little "pits" already spoken of, correspond to the "spots" of the skin, with this difference, however, that the papillae do not each give a single sensation. Some of them give only two, some only three of the four tastes; and the bitter taste is aroused principally from {202} the back of the tongue, the sweet from the tip, the sour from the sides, the salty from both tip and sides.
The stimulus to the sense of taste is something of a chemical nature. The tasteable substances must be in solution in order to penetrate the pits and get to the sensitive tips of the taste cells. If the upper surface of the tongue is first dried, a dry lump of sugar or salt laid on it gives no sensation of taste until a little saliva has accumulated and dissolved some of the substance.
Exactly what is the chemical agent that produces a given taste sensation is a problem of some difficulty. Many different substances give the sensation of bitter, and the question is, what there is common to all these substances. The sweet taste is aroused not only by sugar, but by glycerine, saccharine, and even "sugar of lead" (lead acetate). The sour taste is aroused by most acids, but not by all, and also by some substances that are not chemically acids. Thus the chemistry of taste stimuli involves something not as yet understood.
Though there is this uncertainty regarding the stimulus, on the whole the sense of taste affords a fine example of success achieved by experimental methods in the analysis of complex sensations. At the same time it affords a fine example of the fusion of different sensations into characteristic _blends_. The numerous "tastes" of every-day life, though found on analysis to be compounded of taste, smell, touch, pain, temperature and muscle sensations, have the effect of units. The taste of lemonade, for example, compounded of sweet, sour, cold and lemon odor, has the effect of a single characteristic sensation. It can be analyzed, but it ordinarily appears as a unit. This is true generally of blends; indeed, what we mean by blending is that, while the component sensations are still present and can be found by careful attention, they are not simply present together {203} but are compounded into a characteristic total. Each elementary sensation entering into the blend gives up some of its own quality, as, in the case of lemonade, neither the sweet nor the sour is quite so distinct and obtrusive as either would be if present alone. The same is true of the lemon odor, and it is true generally of the odor components that enter into the "tastes" of food. Were the odor components in these tastes as clear and distinct as they are when the same substance is smelled outside the mouth, we could not fail to notice that the "tastes" were largely composed of odor. The obtrusive thing about a blend is the total effect, not the elementary sensations that are blended.
The Sense of Smell
The great variety of odors long resisted every attempt at psychological analysis, largely because the olfactory end-organ is so secluded in position. You cannot apply stimuli to separate parts of it, as you can to the skin or tongue. But, recently, good progress has been made, [Footnote: By Henning.] by assembling almost all possible odors, and becoming thoroughly acquainted with them, not as substances, but simply as odors, and noting their likenesses and differences. It seems possible now to state that there are _six elementary odors_, as follows:
1. Spicy, found in pepper, cloves, nutmeg, etc.
2. Flowery, found in heliotrope, etc.
3. Fruity, found in apple, orange oil, vinegar, etc.
4. Resinous, found in turpentine, pine needles, etc.
5. Foul, found in hydrogen sulphide, etc.
6. Scorched, found in tarry substances.
These being the elements, there are many compound odors. The odor of roasted coffee is a compound of resinous and scorched, peppermint a compound of fruity and spicy.
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Each elementary odor corresponds to a certain characteristic in the chemical constitution of the stimulus.
The sense of smell is extremely delicate, responding to very minute quantities of certain substances diffused in the air. It is extremely useful in warning us against bad air and bad food. It has also considerable esthetic value.
Organic Sensation
The term "organic sensation" is used to cover a variety of sensations from the internal organs, such as hunger, thirst, nausea, suffocation and less definite bodily sensations that color the emotional tone of any moment, contributing to "euphoria" and also to disagreeable states of mind. Hunger is a sensation aroused by the rubbing together of the stomach walls when the stomach, being ready for food, begins its churning movements. Careful studies of sensations from the internal organs reveal astonishingly little of sensation arising there, but there can be little doubt that the sensations just listed really arise where they seem to arise, in the interior of the trunk.
Little has been done to determine the elementary sensations in this field; probably the organic sensations that every one is familiar with are blends rather than elements.
The Sense of Sight
Of the tremendous number and variety of visual sensations, the great majority are certainly compounds. Two sorts of compound sensation can be distinguished here: _blends_ similar to those of taste or smell, and _patterns_ which scarcely occur among sensations of taste and smell, though they are found, along with blends, in cutaneous sensation. Heat, compounded of warmth, cold and pain sensations, is an {205} excellent example of a blend, while the compound sensation aroused by touching the skin simultaneously with two points--or three points, or a ring or square--is to be classed as a pattern. In a pattern, the component parts are spread out in space or time (or in both at once), and for that reason are more easily attended to separately than the elements in a blend. Yet the pattern, like the blend, has the effect of a unit. A spatial pattern has a characteristic shape, and a temporal pattern a characteristic course or movement. A rhythm or a tune is a good example of a temporal pattern.
Visual sensations are spread out spatially, and thus fall into spatial patterns. They also are in constant change and motion, and so fall into temporal patterns, many of which are spatial as well. The visual sensation aroused, let us say in a young baby, by the light entering his eye from a human face, is a spatial pattern; the visual sensation aroused by some one's turning down the light is a pure temporal pattern; while the sensation from a person seen moving across the room is a pattern both spatial and temporal. Finding the elements of a visual pattern would mean finding the smallest possible bits of it, which would probably be the sensations due to the action of single rods and cones, just as the smallest bit of a cutaneous sensation would be due to the exciting of a single touch spot, warmth spot, cold spot or pain spot.
Analyzing a visual blend is quite a different job. Given the color pink, for example, let it be required to discover whether this is a simple sensation or a blend of two or more elementary sensations. Studying it intently, we see that it can be described as a whitish red, and if we are willing to accept this analysis as final, we conclude that pink is a blend of the elementary sensations of white and red. Of the thousands and thousands of distinguishable hues, shades {206} and tints, only a few are elements and the rest are color blends; and our main problem now is to identify the elements. Notice that we are not seeking for the physical elements of light, nor for the primary pigments of the painter's art, but for the elementary _sensations_. Our knowledge of physics and painting, indeed, is likely to lead us astray. Sensations are our responses to the physical stimulus, and the psychological question is, what fundamental responses we make to this class of stimuli.
Suppose, without knowing anything of pigments or of the physics of light, we got together a collection of bits of color of every shade and tint, in order to see what we could discover about visual sensations. Leaving aside the question of elements for the moment, we might first try to _classify_ the bits of color. We could sort out a pile of reds, a pile of blues, a pile of browns, a pile of grays, etc., but the piles would shade off one into another. The salient fact about colors is the gradual transition from one to another. We can arrange them in _series_ better than we can classify them. They can be serially arranged in three different ways, according to brightness or intensity, according to color-tone, and according to saturation.
The _intensity series runs from light to dark_. We can arrange such a series composed entirely of reds or blues or any other one color; or we can arrange the whole collection of bits of color into a single light-dark series. It is not always easy to decide whether a given shade of one color is lighter or darker than a given shade of a different color; but in a rough way, at least, every bit of whatever color would have its place in the single intensity series. An intensity series can, of course, be arranged in any other sense as well as in sight.
The _color-tone series_ is best arranged from a collection consisting entirely of full or saturated colors. Start the {207} series with any color and put next to this the color that most resembles it in color-tone, i.e., in specific color quality; and so continue, adding always the color that most resembles the one preceding. If we started with red, the next in order might be either a yellowish red or a bluish red. If we took the yellowish red and placed it beside the red, then the next in order would be a still more yellowish red, and the series would run on to yellow and then to greenish yellow, green, bluish green, blue, violet, purple, purplish red, and so back to red. The color-tone series returns upon itself. It is a circular series.
A _saturation series_ runs from full-toned or saturated colors to pale or dull. Since we can certainly say of a pale blue that it is less saturated than a vivid red, etc., we could, theoretically, arrange our whole collection of bits of color in a single saturation series, but our judgment would be very uncertain at many points. The most significant saturation series confine themselves to a single color-tone, {208} and also, as far as possible, to a constant brightness, and extend from the most vivid color sensation obtainable with this color-tone and brightness, through a succession of less and less strongly colored sensations of the same tone and brightness, to a dead gray of the same brightness. Any such saturation series terminates in a neutral gray, which is light or dark to match the rest of the particular saturation series.
White, black and gray, which find no place in the color-tone series, give an intensity series of their own, running from white through light gray and darker and darker gray to black, and any gray in this series may be the zero point in a saturation series of any color-tone.
A three-dimensional diagram of the whole system of visual sensations can be built up in the following way. Taking all the colors of the same degree of brightness, we can arrange the most saturated, in the order of their color-tone, around the circumference of a circle, put a gray of the same brightness at the center of this circle, and then arrange a saturation series for each color-tone extending from the most saturated at the circumference to gray at the center. This would be a two-dimensional diagram for colors having the same brightness. For a greater brightness, we could arrange a similar circle and place it above the first, and for a smaller brightness, a similar circle and place it below the first, and we could thus build up a pile of circles, ranging from the greatest brightness at the top to the least at the bottom. But, as the colors all lose saturation when their brightness is much increased, and also when it is much decreased, we should make the circles smaller and smaller toward either the top or the bottom of the pile, so that our three-dimensional diagram would finally take the form of a double cone, with the most intense white, like that of sunlight, at the upper point, with dead black at the lower point, {209} and with the greatest diameter near the middle brightness, where the greatest saturations can be obtained. The axis of the double cone, extending from brightest white to dead black, would give the series of neutral grays. All the thousands of distinguishable colors, shades and tints, would find places in this scheme.
Simpler Forms of the Color Sense
Not every one gets all these sensations. In _color-blindness_, the system is reduced to one or two dimensions, instead of three. There are two principal forms of color-blindness: total, very uncommon; and red-green blindness, fairly {210} common. The totally color-blind individual sees only white, black, and the various shades of gray. His system of visual sensations is reduced to one dimension, corresponding to the axis of our double cone.
_Red-green blindness_, very uncommon in women, is present in three or four percent of men. It is not a disease, not curable, not corrected by training, and not associated with any other defect of the eye, or of the brain. It is simply a native peculiarity of the color sense. Careful study shows that the only color sensations of the red-green blind person are blue and yellow, along with white, black and the grays. His color circle reduces to a straight line with yellow at one end and blue at the other. Instead of the color circle, he has a double saturation series, reaching from saturated yellow through duller yellows to gray and thence through dull blues to saturated blue. What appears to the normal eye as red, orange or grass green appears to him as more or less unsaturated yellow; and what appears to the normal eye as greenish blue, violet and purple appears to him as more or less unsaturated blue. His color system can be represented in two dimensions, one for the double saturation series, yellow-gray-blue, and the other for the intensity series, white-gray-black.
Color-blindness, always interesting and not without some practical importance (since the confusions of the color-blind eye might lead to mistaking signals in navigation or railroading), takes on additional significance when we discover the curious fact that _every one is color-blind_--in certain parts of the retina. The outermost zone of the retina, corresponding to the margin of the field of view, is totally color-blind (or very nearly so), and an intermediate zone, between this and the central area of the retina that sees all the colors, is red-green blind, and delivers only blue and yellow sensations, along with white, black and gray. Take {211} a spot of yellow or blue and move it in from the side of the head into the margin of the field of view and then on towards the center. When it first appears in the margin, it simply appears gray, but when it has come inwards for a certain distance it changes to yellow. If a red or green spot is moved in similarly, it first appears gray, then takes on a faint tinge of yellow, and finally, as it approaches the center of the field of view, appears in its true color. The outer zone gets only black and white, the intermediate zone gets, in addition to these, yellow and blue, and the central area adds red and green (and with them all the colors).
Now as to the question of elements, let us see how far we can go, keeping still to the sensations, without any reference to the stimulus. If a collection of bits of color is presented to a class of students who have not previously studied this matter, with the request that each select those colors that seem to him elementary and not blends, there is practically unanimous agreement on three colors, red, yellow and blue; and there are some votes for green also, but almost none for orange, violet, purple, brown or any other colors. {212} except white and black. That white and black are elementary sensations is made clear by the case of total color-blindness, since in this condition there are no other visual sensations from which white and black could be compounded, and these two differ so completely from each other that it would be impossible to think of white as made up of black, or black of white. Gray, on the other hand, appears like a blend of black and white. In the same way, red-green blindness demonstrates the reality of yellow and blue as elementary sensations, since neither of them could be reduced to a blend of the other with white or black; and there are no other colors present in this form of color vision to serve as possible elements out of which yellow and blue might be compounded. That white, black, yellow and blue are elementary sensations is therefore clear from the study of visual sensations alone; and there are indications that red and green are also elements.
Visual Sensations as Related to the Stimulus
Thus far, we have said nothing of the stimulus that arouses visual sensations. Light, the stimulus, is physically a wave motion, its vibrations succeeding each other at the rate of 500,000000,000000 vibrations, more or less, per second, and moving through space with a speed of 186,000 miles per second. The "wave-length", or distance from the crest of one wave to the crest of the next following, is measured in millionths of a millimeter.
The most important single step ever taken towards a knowledge of the physics of light, and incidentally towards a knowledge of visual sensations, was Newton's analysis of white light into the spectrum. He found that when white light is passed through a prism, it is broken up into all the colors of the rainbow or spectrum. Sunlight consists of a {213} mixture of waves of various lengths. At one end of the spectrum are the long waves (wave-length 760 millionths of a millimeter), at the other end are the short waves (wavelength 390), and in between are waves of every intermediate length, arranged in order from the longest to the shortest. The longest waves give the sensation of red, and the shortest that of violet, a slightly reddish blue.
Outside the limits of the visible spectrum, however, there are waves still longer and shorter, incapable of arousing the retina, though the very long waves, beyond the red, arouse the sensation of warmth from the skin, and the very short waves, beyond the violet, though arousing none of the senses, do effect the photographic plate. Newton distinguished seven colors in the visible spectrum, red, orange, yellow, green, blue, indigo and violet; but there is nothing specially scientific about this list, since physically there are not seven but an unlimited number of wave-lengths included in the spectrum, varying continuously from the longest at the red end to the shortest at the violet; while psychologically the number of distinguishable colors in the spectrum, though not unlimited, is at least much larger than seven. Between red and orange, for instance, there are quite a number of distinguishable orange-reds and reddish oranges.
If now we ask what differences in the stimulus give rise to the three kinds of difference in visual sensation that were spoken of previously, we find that color-tone depends on the wave-length of the light, brightness on the energy of the stimulus, i.e., on the amplitude of the vibration, and saturation on the mixture of long and short wave-lengths in a complex light-stimulus--the more mixture, the less saturation.
These are the general correspondences between the light stimulus and the visual sensation; but the whole relationship is much more complex. Brightness depends, not only on the energy of the stimulus, but also on wave-length. The {214} retina is tuned to waves of medium length, corresponding to the yellow, which arouse much brighter sensation than long or short waves of the same physical energy. Otherwise put, the sensitivity of the retina is greatest for medium wavelengths, and decreases gradually towards the ends of the spectrum, ceasing altogether, as has been said, at wavelengths of 760 at the red end and of 390 at the violet end.
Saturation, depending primarily on amount of mixture of different wave-lengths, depends also on the particular wavelengths acting, and also on their amplitude. So, the red and blue of the spectrum are more saturated than the yellow and green; and very bright or very dim light, however homogeneous, gives a less saturated sensation than a stimulus of medium strength.
Color Mixing
Color-tone depends on the wave-length, as has been said, but this is far from the whole truth; the whole truth, indeed, is one of the most curious and significant facts about color vision. We have said that each color-tone is the response to a particular wave-length. But any color-tone can be got without its particular wave-length being present at all; all that is necessary is that wave-lengths centering about this particular one shall be present. A mixed light, consisting of two wave-lengths, the one longer and the other shorter than the particular wave which when acting alone gives a certain color-tone, will give that same color-tone. For example, the orange color resulting from the isolated action of a wave-length of 650 is given also by the combined action of wave-lengths of 600 and 700, in amounts suitably proportioned to each other.
A point of experimental technique: in _mixing colored lights_ for the purpose of studying the resulting sensations, we do not mix painter's pigments, since the physical {215} conditions then would be far from simple, but we mix the lights themselves by throwing them together either into the eye, or upon a white screen. We can also, on account of a certain lag or hang-over in the response of the retina, mix lights by rapidly alternating them, and get the same effect as if we had made them strike the retina simultaneously.
By mixing a red light with a yellow, in varying proportions, all the color-tones between red and yellow can be got--reddish orange, orange and yellowish orange. By mixing yellow and green lights, we get all the greenish yellow and yellowish green color-tones; and by mixing green and blue lights we get the bluish greens and greenish blues. Finally, by mixing blue and red lights, in varying proportions, we get violet, purple and purplish red. Purple has no place in the spectrum, since it is a sensation which cannot be aroused by the action of any single wave-length, but only by the mixture of long and short waves.
To get all the color-tones, then, we need not employ all the wave-lengths, but can get along with only four. In fact, we can get along with three. Red, green and blue will do the trick. Red and green lights, combined, would give the yellows; green and blue would give the greenish blues; and red and blue would give purple and violet.
The sensation of white results--to go back to Newton--from the combined action of all the wave-lengths. But the stimulus _need_ not contain _all_ the wave-lengths. Four are enough; the three just mentioned would be enough. More surprising still, two are enough, if chosen just right. Mix a pure yellow light with a pure blue, and you will find that you get the sensation of white--or gray, if the lights used are not strong.
[Footnote: When you mix blue and yellow _pigments_, each absorbs part of the wave-lengths of white light, and what is left after this double absorption may be predominantly green. This is absolutely different from the addition of blue to yellow light; addition gives white, not green.]
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Lights, or wave-lengths, which when acting together on the retina give the sensation of white or gray, are said to be _complementary_. Speaking somewhat loosely, we sometimes say that two _colors_ are complementary when they mix to produce white. Strictly, the colors--or at least the color sensations--are not mixed; for when yellow and blue lights are mixed, the resulting sensation is by no means a mixture of blue and yellow sensations, but the sensation of white in which there is no trace of either blue or yellow. Mixing the stimuli which, acting separately, give two complementary colors, arouses the colorless sensation of white.
Blue and yellow, then, are complementary. Suppose we set out to find the complementary of red. Mixing red and yellow lights gives the color-tones intermediate between these two; mixing red and green still gives the intermediate color-tones, but the orange and yellow and yellowish green so got lack saturation, being whitish or grayish. Now mix red with bluish green, and this grayishness is accentuated, and if just the right wave-length of bluish green is used, no trace of orange or yellow or grass green is obtained, but white or gray. Red and bluish green are thus complementary. The complement of orange light is a greenish blue, and that of greenish yellow is violet. The typical green (grass green) has no single wave-length complementary to it, but it does give white when mixed with a compound of long and short waves, which compound by itself gives the sensation of purple; so that we may speak of green and purple as complementary.
What Are the Elementary Visual Sensations?
Returning now to the question of elementary sensations, which we laid aside till we had examined the relationship of the sensations to the stimulus, we need to be on our guard against physics, or at least against being so much impressed with the physics of light as to forget that we are concerned with the _response_ of the organism to physical light--a matter on which physics cannot speak the final word.
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Fig. 36.--(After König.) The color triangle, a map of the laws of color mixture. The spectral colors are arranged in order along the heavy solid line, and the purples along the heavy dotted line. The numbers give the wave-lengths of different parts of the spectrum. Inside the heavy line are located the pale tints of each color, merging from every side into white, which is located at the point W.
Suppose equal amounts of two spectral colors are mixed: to find from the diagram the color of the mixture. Locate the two colors on the heavy line, draw a straight line between these two points, and the middle of this line gives the color-tone and saturation of the mixture. For example, mix red and yellow: then the resulting color is a saturated reddish yellow. Mix red (760) and green (505): the resulting yellow is non-saturated, since the straight line between these two points lies inside the figure. If the straight line joining two points passes through W, the colors located at the two points are complementary.
Spectral colors are themselves not completely saturated. The way to get color sensations of maximum saturation is first to stare at one color, so as to fatigue or adapt the eye for that color, and then to turn the eye upon the complementary color, which, under these conditions, appears fuller and richer than anything otherwise obtainable. The corners, R, G, and B, denote colors of maximum saturation, and the whole of the triangle outside of the heavy line is reserved for super-saturated color sensations.
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Physics tells us of the stimulus, but we are concerned with the response. The facts of color-blindness and color mixing show very clearly that the response does not tally in all respects with the stimulus. Physics, then, is apt to confuse the student at this point and lead him astray. Much impressed with the physical discovery that _white_ light is a mixture of all wave-lengths, he is ready to believe the sensation of white a mixed sensation. He says, "White is the sum of all the colors", meaning that the sensation of white is compounded of the sensations of red, orange, yellow, green, blue and violet--which is simply not true. No one can pretend to get the sensations of red or blue in the sensation of white, and the fact of complementary colors shows that you cannot tell, from the sensation of white, whether the stimulus consists of yellow and blue, or red and bluish green, or red, green and blue, or all the wave-lengths, the response being the same to all these various combinations. Total color-blindness showed us, when we were discussing this matter before, that white was an elementary sensation, and nothing that has been said since changes that conclusion.
Consider _black_, too. Physics says, black is the absence of light; but this must not be twisted to mean that black is the absence of all visual sensation. Absence of visual sensation is simply nothing, and black is far from that. It is a sensation, as positive as any, and undoubtedly elementary.
From the point of view of physics, there is no reason for considering any one color more elementary than any other. Every wave-length is elementary; and if sensation tallied precisely with the stimulus, every spectral color-tone would be an element. But there are obvious objections to such a view, such as: (1) there are not nearly as many {219} distinguishable color-tones as there are wave-lengths; (2) orange, having a single wave-length, certainly appears to be a blend as truly as purple, which has no single wave-length; and (3) we cannot get away from the fact of red-green blindness, in which there are only two color-tones, _yellow_ and _blue_. In this form of color vision (which, we must remember, is normal in the intermediate zone of the retina), there are certainly not as many elementary responses as there are wave-lengths, but only one response to all the longer waves (the sensation of yellow), one response to all the shorter waves (the sensation of blue), one response to the combination of long and short waves (the sensation of white), and one response to the cessation of light (the sensation of black). These four are certainly elementary sensations, and there are probably only a few more.
There must be at least two more, because of the fact that two of the sure elements, yellow and blue, are complementary. For suppose we try to get along with one more, as _red_. Then red, blended with yellow, would give the intervening color-tones, namely, orange with reddish and yellowish orange; and red blended with blue would give violet and purple; but yellow and blue would only give white or gray, and there would be no way of getting green. We must admit _green_ as another element. The particular red selected would be that of the red end of the spectrum, if we follow the general vote; and the green would probably be something very near grass green. We thus arrive at the conclusion that there are six elementary visual responses or sensations: white and black, yellow and blue, red and green.
It is a curious fact that some of these elementary sensations blend with each other, while some refuse to blend. White and black blend to gray, and either white or black or both together will blend with any of the four elementary colors or with any possible blend of these four. Brown, for {220} example, is a grayish orange, that is, a blend of white, black, red and yellow. Red blends with yellow, yellow with green, green with blue, and blue with red. But we cannot get yellow and blue to blend, nor red and green. When we try to get yellow and blue to blend, by combining their appropriate stimuli, both colors disappear, and we get simply the colorless sensation of white or gray. When we try to get red and green to blend, both of them disappear and we get the sensation of yellow.
Theories of Color Vision
Of the most celebrated theories of color vision, the oldest, propounded by the physicists Young and Helmholtz, recognized only three elements, red, green and blue. Yellow they regarded as a blend of red and green, and white as a blend of all three elements. The unsatisfactory nature of this theory is obvious. White as a sensation is certainly not a blend of these three color sensations, but is, precisely, colorless; and no more is the yellow sensation a blend of red and green. Moreover, the theory cannot do justice either to total color-blindness, with its white and black but no colors, or to red-green blindness, with its yellow but no red or green.
The next prominent theory was that of the physiologist Hering. He did justice to white and black by accepting them as elements; and to yellow and blue likewise. The fact that yellow and blue would not blend he accounted for by supposing them to be antagonistic responses of the retina; when, therefore, the stimuli for both acted together on the retina, neither of the two antagonistic responses could occur, and what did occur was simply the more generic response of white. Proceeding along this line, he concluded that red and green were also antagonistic responses; but just here {221} he committed a wholly unnecessary error, in assuming that if red and green were antagonistic responses, the combination of their stimuli must give white, just as with yellow and blue. Accordingly, he was forced to select as his red and green elementary color-tones two that would be complementary; and this meant a purplish (i.e., bluish) red, and a bluish green, with the result that his "elementary" red and green appear to nearly every one as compounds and not elements. It would really have been just as easy for Hering to suppose that the red and green responses, antagonizing each other, left the sensation yellow; and then he could have selected that red and green which we have concluded above to have the best claim.
A third theory, propounded by the psychologist, Dr. Christine Ladd-Franklin, is based on keen criticism of the previous two, and seems to be harmonious with all the facts. She supposes that the color sense is now in the third stage of its evolution. In the first stage the only elements were white and black; the second stage added yellow and blue; and the third stage red and green. The outer zone of the retina is still in the first stage, and the intermediate zone in the second, only the central area having reached the third. In red-green blind individuals, the central area remains in the second stage, and in the totally color-blind the whole retina is still in the first stage.
In the first stage, one response, white, was made to light of whatever wave-length. In the second stage, this single response divided into two, one aroused by the long waves and the other by the short. The response to the long waves was the sensation of yellow, and that to the short waves the sensation of blue. In the third stage, the yellow response divided into one for the longest waves, corresponding to the red, and one for somewhat shorter waves, corresponding to the green. Now, when we try to get a blend of red and green {222} by combining red and green lights, we fail because the two responses simply unite and revert to the more primitive yellow response; and similarly when we try to get the yellow and blue responses together, they revert to the more primitive white response out of which they developed.
But, since no one can pretend to _see_ yellow as a reddish green, nor white as a bluish yellow, it is clear that the just-spoken-of union of the red and green responses, and of the yellow and blue responses, must take place _below the level of conscious sensation_. These unions probably take place within the retina itself. Probably they are purely chemical unions.
The _very first_ response of a rod or cone to light is probably a purely chemical reaction. Dr. Ladd-Franklin, carrying out her theory, supposes that a light-sensitive "mother substance" in the rods and cones is decomposed by the action of light, and gives off cleavage products which arouse the vital activity of the rods and cones, and thus start nerve currents coursing towards the brain.
In the "first stage", she supposes, a _single_ big cleavage product, which we may call W, is split off by the action of {223} light upon the mother substance, and the vital response to W is the sensation of white.
In the second stage, the mother substance is capable of giving off two smaller cleavage products, Y and B. Y is split off by the long waves of light, and B by the short waves, and the vital response to Y is the sensation of yellow, that to B the sensation of blue. But suppose that, chemically, Y + B = W: then, if Y and B are both split off at the same time in the same cone, they immediately unite into W, and the resulting sensation is white, and neither yellow nor blue.
Similarly, in the third stage, the mother substance is capable of giving off _three_ cleavage products, R, G and B; and there are three corresponding vital responses, the sensations of red, green and blue. But, chemically, R + G = Y; and therefore, if R and G are split off at the same time, they unite chemically into Y and give the sensation of yellow. If R, G and B are all split off at the same time, they unite chemically as follows: R + G = Y, and Y + B = W; and therefore the resulting sensation is that of white.
This theory of cleavage products is in good general agreement with chemical principles, and it does justice to all the facts of color vision, as detailed in the preceding pages. It should be added that "for black, the theory supposes that, {224} in the interest of a continuous field of view, objects which reflect no light at all upon the retina have correlated with them a definite non-light sensation--that of black." [Footnote: Quotation from Dr. Ladd-Franklin.]
Adaptation
Sensory adaptation is a change that occurs in other senses also, but it is so much more important in the sense of sight than elsewhere that it may best be considered here. The stimulus continues, the sensation ceases or diminishes--that is the most striking form of sensory adaptation. Continued action of the same stimulus puts the sense into such a condition that it responds differently from at first, and usually more weakly. It is much like fatigue, but it often is more positive and beneficial than fatigue.
The sense of smell is very subject to adaptation. On first entering a room you clearly sense an odor that you can no longer get after staying there for some time. This adaptation to one odor does not prevent your sensing quite different odors. Taste shows less adaptation than smell, but all are familiar with the decline in sweet sensation that comes with continued eating of sweets.
All of the cutaneous senses except that for pain are much subject to adaptation. Continued steady pressure gives a sensation that declines rapidly and after a time ceases altogether. The temperature sense is usually adapted to the temperature of the skin, which therefore feels neither warm nor cool. If the temperature of the skin is raised from its usual level of about 70 degrees Fahrenheit to 80 or 86, this temperature at first gives the sensation of warmth, but after a time it gives no temperature sensation at all; the warmth sense has become adapted to the temperature of 80 degrees; and now a temperature of 70 will give the sensation of cool. {225} Hold one hand in water at 80 and the other in water at 66, and when both have become adapted to these respective temperatures, plunge them together into water at 70; and you will find this last to feel cool to the warm-adapted hand and warm to the cool-adapted. There are limits to this power of adaptation.
The muscle sense seems to become adapted to any fixed position of a limb, so that, after the limb has remained motionless for some time, you cannot tell in what position it is; to find out, you have only to move it the least bit, which will excite both the muscle sense and the cutaneous pressure sense. The sense of head rotation is adaptable, in that a rotation which is keenly sensed at the start ceases to be felt as it continues; but here it is not the sense cells that become adapted, but the back flow that ceases, as will soon be explained.
To come now to the sense of sight, we have _light adaptation, dark adaptation_, and _color adaptation_. Go into a dark room, and at first all seems black, but by degrees--provided there is a little light filtering into the room--you begin to see, for your retina is becoming dark-adapted. Now go out into a bright place, and at first you are "blinded", but you quickly "get used" to the bright illumination and see objects much more distinctly than at first; for your eye has now become light-adapted. Remain for some time in a room illuminated by a colored light (as the yellowish light of most artificial illuminants), and by degrees the color sensation bleaches out so that the light appears nearly white.
Dark adaptation is equivalent to sensitizing the retina for faint light. Photographic plates can be made of more or less sensitiveness for use with different illuminations; but the retina automatically alters its sensitivity to fit the illumination to which it is exposed.
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Rod and Cone Vision
You will notice, in the dark room, that while you see light and shade and the forms of objects, you do not see colors. The same is true out of doors at night. In other words, the kind of vision that we have when the eye is dark-adapted is totally color-blind. Another significant fact is that the fovea is of little use in very dim light. These facts are taken to mean that dim-light vision, or _twilight vision_ as it is sometimes called, is _rod vision_ and not cone vision; or, in other words, that the rods and not the cones have the great sensitiveness to faint light in the dark-adapted eye. The cones perhaps become somewhat dark-adapted, but the rods far outstrip them in this direction. The fovea has no rods and hence is of little use in very faint light. The rods have no differential responsiveness to different wave-lengths, remaining still in the "first stage" in the development of color vision, and consequently no colors are seen in faint light.
Rod vision differs then from cone vision in having only one response to every wave-length, and in adapting itself to much fainter light. No doubt, also, it is the rods that give to peripheral vision its great sensitivity to moving objects.
After-Images
After-images, which might better be called after-sensations, occur in other senses than sight, but nowhere else with such definiteness. The main fact here is that the response outlasts the stimulus. This is true of a muscle, and it is true of a sense organ. It takes a little time to get the muscle, or the sense organ, started, and, once it is in action, it takes a little time for it to stop. If you direct your eyes towards the lamp, holding your hand or a book in front of them as a screen, remove the screen for an {227} instant and then replace it, you will continue for a short time to see the light after the external stimulus has been cut off. This "positive after-image" is like the main sensation, only weaker. There is also a "negative after-image", best got by looking steadily at a black-and-white or colored figure for as long as fifteen or twenty seconds, and then directing the eyes upon a medium gray background. After a moment a sensation develops in which black takes the place of white and white of black, while for each color in the original sensation the complementary color now appears.
This phenomenon of the negative after-image is the same as that of color adaptation. Exposing the retina for some time to light of a certain color adapts the retina to that color, bleaches that color sensation, and, as it were, subtracts that color (or some of it) from the gray at which the eyes are then directed; and gray (or white) minus a color gives the complementary color.
Contrast
Contrast is still another effect that occurs in other senses, but most strikingly in vision. There is considerable in common between the negative after-image and contrast; indeed, {228} the negative after-image effect is also called "successive contrast". After looking at a bright surface, one of medium brightness appears dark, while this same medium brightness would seem bright after looking at a dark surface. This is evidently adaptation again, and is exactly parallel to what was found in regard to the temperature sense. After looking at any color steadily, the complementary color appears more saturated than usual; in fact, this is the way to secure the maximum of saturation in color sensation. These are examples of "successive contrast".
"Simultaneous contrast" is something new, not covered by adaptation, but gives the same effects as successive contrast. If you take two pieces of the same gray paper, and place one on a black background and the other on white, you will find the piece on the black ground to look much brighter than the piece on the white ground. Spots of gray on colored backgrounds are tinged with the complementary colors. The contrast effect is most marked at the margin adjoining the background, and grows less away from this margin. Any two adjacent surfaces produce contrast effects in each other, though we usually do not notice them any more than we usually notice the after-images that occur many times in the course of the day.
The Sense of Hearing
Sound, like light, is physically a wave motion, though the sound vibrations are very different from those of light. They travel 1,100 feet a second, instead of 186,000 miles a second. Their wave-length is measured in feet instead of in millionths of a millimeter, and their vibration frequencies are counted in tens, hundreds and thousands per second, instead of in millions of millions. But sound waves vary among themselves in the same three ways that we {229} noticed in light waves: in amplitude, in wave-length (or vibration rate), and in degree of mixture of different wave-lengths.
Difference of amplitude (or energy) of sound waves produces difference of loudness in auditory sensation, which thus corresponds to brightness in visual sensation. Sounds can be arranged in order of loudness, as visual sensations can be arranged in order of brightness, both being examples of intensity series such as can be arranged in any kind of sensation.
Difference of wave-length of sound waves produces difference in the _pitch_ of auditory sensation, which thus corresponds to color in visual sensation. Pitch ranges from the lowest notes, produced by the longest audible waves, to the highest, produced by the shortest audible waves. It is customary, in the case of sound waves, to speak of vibration rate instead of wave-length, the two quantities being inversely proportional to each other (in the same conducting medium). The lowest audible sound is one of about sixteen vibrations per second, and the highest one of about 30,000 per second, while the waves to which the ear is most sensitive have a vibration rate of about 1,000 to 4,000 per second. The ear begins to lose sensitiveness as early as the age of thirty, and this loss is most noticeable at the upper limit, which declines slowly from this age on.
Middle C of the piano (or any instrument) has a vibration rate of about 260. Go up an octave from this and you double the number of vibrations per second; go down an octave and you halve the number of vibrations. Of any two notes that are an octave apart, the upper has twice the vibration rate of the lower. The whole range of audible notes, from 16 to 30,000 vibrations, thus amounts to about eleven octaves, of which music employs about eight octaves, finding little use for the upper and lower extremes of the {230} pitch series. The smallest step on the piano, called the "semitone", is one-twelfth of an octave; but it must not be supposed that this is the smallest difference that can be perceived. A large proportion of people can observe a difference of four vibrations, and keen ears a difference of less than one vibration; whereas the semitone, at middle C, is a step of about sixteen vibrations.
_Mixture of different wave-lengths_, which in light causes difference of saturation, may be said in sound to cause difference of purity. A "pure tone" is the sensation aroused by a stimulus consisting wholly of waves of the same length. Such a stimulus is almost unobtainable, because every sounding body gives off, along with its fundamental waves, other waves shorter than the fundamental and arousing tone sensations of higher pitch, called "overtones". A piano string which, vibrating as a whole, gives 260 vibrations per second (middle C), also vibrates at the same time in halves, thus giving 520 vibrations per second; in thirds, giving 780 per second; and in other smaller segments. The whole stimulus given off by middle C of the piano is thus a compound of fundamental and overtones; and the sensation aroused by this complex stimulus is not a "pure tone" but a blend of fundamental tone and overtones. By careful attention and training, we can "hear out" the separate overtones from the total blend; but ordinarily we take the blend as a unit (just as we take the taste of lemonade as a unit), and hear it simply as middle C of a particular quality, namely the piano quality. Another instrument will give a somewhat different combination of overtones in the stimulus, and that means a different quality of tone in our sensation. We do not ordinarily analyze these complex blends, but we distinguish one from another perfectly well, and thus can tell whether a piano or a cornet is playing. The difference between different instruments, which we have spoken of as a {231} difference in quality or purity of tone, is technically known as _timbre_; and the timbre of an instrument depends on the admixture of shorter waves with the fundamental vibration which gives the main pitch of a note.
Akin to the timbre of an instrument is the _vowel_ produced by the human mouth in any particular position. Each vowel appears to consist, physically, of certain high notes produced by the resonance of the mouth cavity. In the position for "ah", the cavity gives a certain tone; in the position for "ee" it gives a higher tone. Meanwhile, the pitch of the voice, determined by the vibration of the vocal cords, may remain the same or vary in any way. The vowel tones differ from overtones in remaining the same without regard to the pitch of the fundamental tone that is being sung or spoken, whereas overtones move up or down along with their fundamental. The vowels, as auditory sensations, are excellent examples of blends, in that, though compounds, they usually remain unanalyzed and are taken simply as units. What has been said of the vowels applies also to the semi-vowels and continuing consonants, such as l, m, n, r, f, th, s and sh.
Other consonants are to be classed with the noises. Like a vowel, and like the timbre of an instrument, a noise is a blend of simple tones; but the fundamental tone in a noise-blend is not so preponderant as to give a clear pitch to the total sound, while the other tones present are often too brief or too unsteady to give a tonal effect.
Comparison of Sight and Hearing
The two senses of sight and hearing have many curious differences, and one of the most curious appears in mixing different wave-lengths. Compare the effect of throwing two colored lights together into the eye with the effect of {232} throwing two notes together into the ear. Two notes sounded together may give either a harmonious blend or a discord; now the discord is peculiar to the auditory realm; mixed colors never clash, though colors seen side by side may do so to a certain extent. A discord of tones is characterized by imperfect blending (something unknown in color mixing), and by roughness due to the presence of "beats" (another thing unknown in the sense of sight). Beats are caused by the interference between sound waves of slightly different vibration rate. If you tune two whistles one vibration apart and sound them together, you get a tone that swells once a second; tune them ten vibrations apart and you get ten swellings or beats per second, and the effect is rough and disagreeable.
Aside from discord, a tone blend is really not such a different sort of thing from a color blend. A chord, in which the component notes blend while they can still, by attention and training, be "heard out of the chord", is quite analogous with such color blends as orange, purple or bluish green. At the same time, there is a curious difference here. By analogy with color mixing, you would expect two notes, as C and E, when combined, to give the same sensation as the single intermediate note D. Nothing of the kind! Were it so, music would be very different from what it is, if indeed it were possible at all. But the real difference between the two senses at this point is better expressed by saying that D does not give the effect of a combination of C and E, or, in general, that no one note ever gives the effect of a combination or blend of notes higher and lower than itself. Homogeneous orange light gives the sensation of a blend of red and yellow; but there is nothing like this in the auditory sphere. In light, some wave-lengths give the effect of simple colors, as red and yellow; and other wave-lengths the effect of blends, as greenish yellow or bluish {233} green; but in sound, every wave-length gives a tone which seems just as elementary as any other.
There is nothing in auditory sensation to correspond to white, no simple sensation resulting from the combined action of all wave-lengths. Such a combination gives noise, but nothing that seems particularly simple. There is nothing auditory to correspond with black, for silence seems to be a genuine absence of sensation. There are no complementary tones like the complementary colors, no tones that destroy each other instead of blending. In a word, auditory sensation tallies with its stimulus much more closely than visual sensation does with its; and the main secret of this advantage of the sense of hearing is that it has a much larger number of elementary responses. Against the six elementary visual sensations are to be set auditory elements to the number of hundreds or thousands. From the fact that every distinguishable pitch gives a tone which seems as simple and unblended as any other, the conclusion would seem to be that each was an element; and this would mean thousands of elements. On the other hand, the fact that tones close together in pitch sound almost alike may mean that they have elements in common and are thus themselves compounds; but still there would undoubtedly be hundreds of elements.
Both sight and hearing are served by great armies of sense cells, but the two armies are organized on very different principles. In the retina, the sense cells are spread out in such a way that each is affected by light from one particular direction; and thus the retina gives excellent space information. But each retinal cell is affected by any light that happens to come from its particular direction. Every cone, in the central area of the retina, makes all the elementary visual responses and gives all the possible color sensations; so it is not strange that the number of visual {234} elements is small. On the other hand, the ear, having no sound lens, has no way of keeping separate the sounds from different directions (and accordingly gives only meager indications of the direction of sound); but its sense cells are so spread out as to be affected, some by sound of one wavelength, others by other wave-lengths. The different tones do not all come from the same sense cells. Some of the auditory cells give the low tones, others the medium tones, still others the high tones; and since there are thousands of cells, there may be thousands of elementary responses.
Theory of Hearing
The most famous theory of the action of the inner ear is the "piano theory" of Helmholtz. The foundation of the theory is the fact that the sense cells of the cochlea stand on the "basilar membrane", a long, narrow membrane, stretched between bony attachments at either side, and composed partly of fibers running crosswise, very much as the strings of a piano or harp are stretched between two side bars. If you imagine the strings of a piano to be the warp of a fabric and interwoven with crossing fibers, you have a fair idea of the structure of the basilar membrane, except for the fact that the "strings" of the basilar membrane do not differ in length anywhere like as much as the strings of the piano must differ in order to produce the whole range of notes. Now, a piano string can be thrown into "sympathetic vibration", as when you put on the "loud pedal" (remove the dampers from the strings) and then sing a note into the piano. You will find that the string of the pitch sung has been thrown into vibration by the action of the sound waves sung against it.
Now suppose the strings of the basilar membrane to be tuned to notes of all different pitches, within the range of {235} audible vibrations: then each string would be thrown into sympathetic vibration whenever waves of its own vibration rate reached it by way of the outer and middle ear; and the sense cells standing over the vibrating fibers would be shaken and excited. The theory is very attractive because it would account so nicely for the great number of elementary tone sensations (there are over 20,000 fibers or strings in the basilar membrane), as well as for various other facts of hearing--if we could only believe that the basilar membrane did vibrate in this simple manner, fiber by fiber. But (1) the fabric into which the strings of the membrane are woven would prevent their vibrating as freely and independently as the theory requires; (2) the strings do not differ in length a hundredth part of what they would need to differ in order to be tuned to all notes from the lowest to the highest, and there is no sign of differences in stretch or in loading of the strings to make up for their lack of difference in length; and (3) a little model of the basilar membrane, exposed to sound waves, is seen to be thrown into vibration, indeed, and into different forms of vibration for waves of different length, but not by any means into the simple sort of vibration demanded by the piano theory. This theory is accordingly too simple, but it probably points the way towards some truer, more complex, conception.
The fact that there are many elementary sensations of hearing is the chief reason why the art of tones is so much more elaborate than the art of color; for while painting might dispute with music as to which were the more highly developed art, painting depends on form as well as color, and there is no art of pure color at all comparable with music, which makes use simply of tones (and noises) with their combinations and sequences.
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Senses of Bodily Movement
It is a remarkable fact that some parts of the inner ear are not connected with hearing at all, but with quite another sense, the existence of which was formerly unsuspected. The two groups of sense cells in the vestibule--the otolith organs--were formerly supposed to be the sense organ for noise; but noise now appears to be a compound of tones, and its organ, therefore, the cochlea. The _semicircular canals_, from their arrangement in three planes at right angles to each other, were once supposed to analyze the sound according to the direction from which it came; but no one could give anything but the vaguest idea of how they might do this, and besides the ear is now known to give practically no information regarding the direction of sound, except the one fact whether it comes from the right or left, which is given by the difference in the stimulation received by the two ears, and not by anything that exists in either ear taken alone.
The semicircular canals have been much studied by the physiologists. They found that injury to these structures brought lack of equilibrium and inability to walk, swim or fly in a straight course. If, for example, the horizontal canal in the left ear is destroyed, the animal continually deviates to the left as he advances, and so is forced into a "circus movement". They found that the compensatory movements normally made in reaction to a movement impressed on the animal from without were no longer made when the canals were destroyed. They found that something very much like these compensatory movements could be elicited by direct stimulation of the end-organs in the canals or of the sensory nerves leading from them. And they found that little currents of the liquid filling the canals acted as a stimulus to these end-organs and so aroused the {237} compensatory movements. They were thus led to accept a view that was originally suggested by the position of the canals in space.
Each "semicircular" canal, itself considerably more than a semicircular tube, opens into the vestibule at each end and thus amounts to a complete circle. Therefore rotating the head must, by inertia, produce a back flow of the fluid contents of the canal, and this current, by bending the hairs of the sense cells in the canal, would stimulate them and give a sensation of rotation, or at least a sensory nerve impulse excited by the head rotation.
When a human subject is placed, blindfolded, in a chair that can be rotated without sound or jar, it is found that he can easily tell whenever you start to turn him in either direction. If you keep on turning him at a constant speed, he soon ceases to sense the movement, but if then you stop him, he says you are starting to turn him in the opposite {238} direction. He senses the beginning of the rotary movement because this causes the back flow through his canals; he ceases to sense the uniform movement because friction of the liquid in the slender canal soon abolishes the back flow by causing the liquid to move with the canal; and he senses the stopping of this movement because the liquid, again by inertia, continues to move in the direction it had been moving just before when it was keeping pace with the canal. Thus we see that there are conscious sensations of rotation from the canals, and that these give information of the starting or stopping of a rotation, though not of its steady continuance. Excessive stimulation of the canals gives the sensation of dizziness.
The otolith organs in the vestibule are probably excited, not by rotary movements, but by sudden startings and stoppings of rectilinear motion, as in an elevator; and also by the pull of gravity when the head is held in any position. They give information regarding the position and rectilinear movements of the head, as the canals do of rotary head movements. Both are important in maintaining equilibrium and motor efficiency.
The muscle sense is another sense of bodily movement; it was the "sixth sense", so bitterly fought in the middle of the last century by those who maintained that the five senses that were enough for our fathers ought to be enough for us, too. The question was whether the sense of touch did not account for all sensations of bodily movement. It was shown that there must be something besides the skin sense, because weights were better distinguished when "hefted" in the hand than when simply laid in the motionless palm; and it was shown that loss of skin sensation in an arm or leg interfered much less with the coördinated movements of the limb than did the loss of all the sensory nerves to the limb.
Later, the crucial fact was established {239} that sense organs (the "muscle spindles") existed in the muscles and were connected with sensory nerve fibers; and that other sense organs existed in the tendons and about {240} the joints. This sense accordingly might better be called the "muscle, tendon and joint sense", but the shorter term, "muscle sense", bids fair to stick. The Greek derivative, "kinesthesis", meaning "sense of movement", is sometimes used as an equivalent; and the corresponding adjective, "kinesthetic", is common.
The muscle sense informs us of movements of the joints and of positions of the limbs, as well as of resistance encountered by any movement. Muscular fatigue and soreness are sensed through the same general system of sense organs. This sense is very important in the control of movement, both reflex and voluntary movement. Without it, a person lacks information of where a limb is to start with, and naturally cannot know what movement to make; or, if a movement is in process of being executed, he has no information as to how far the movement has progressed and cannot tell when to stop it. Thus it is less strange than it first appears to learn that "locomotor ataxia", a disease which shows itself in poor control of movement, is primarily a disease affecting not the motor nerves but the sensory nerves that take care of the muscle sense.
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EXERCISES
1. Outline the chapter, rearranging the material somewhat, so as to state, under each sense, (a) what sense cells, if any, are present in the sense organ, (b) what accessory apparatus is present in the sense organ, (c) what stimuli arouse the sense, (d) what are the elementary responses of the sense, (e) peculiar blends occurring within the sense or between this sense and another, (f) what can be said regarding adaptation of the sense, and (g) what can be said regarding after-images of the sense.
2. Classify the senses according as they respond to stimuli (a) internal to the body, (b) directly affecting the surface of the body, (c) coming from a distance.
3. What distinctive _uses_ are made of each sense?
4. Explore a small portion of the skin, as on the back of the hand, for cold spots, and for pain spots.
5. Try to analyze the smooth sensation obtained by laying the finger tip on a sheet of paper, and the rough sensation obtained by laying the finger tip on the surface of a brush, and to describe the difference in terms of the elementary skin sensations.
6. Is the pain sense a highly developed sense, to judge from its sense organ? Is it highly specialized? highly sensitive? How does its peculiarity in these respects fit it for its use?
7. Separation of taste and smell. Compare the taste of foods when the nostrils are held closed with the taste of the same food when the nostrils are opened.
8. Make a complete analysis of the sensations obtained from chocolate ice cream in the mouth.
9. Peripheral vision. (a) Color sense. While your eyes are looking rigidly straight ahead, take a bit of color in the hand and bring it slowly in from the side, noticing what color sensation you get from it when it can first be seen at all, and what changes in color appear as it moves from the extreme periphery to the center of the field of view, (b) Form sense. Use printed letters in the same way, noticing how far out they can be read, (c) Sense of motion. Notice how far out a little movement of the finger can be seen. Sum up what you have learned of the differences between central and peripheral vision. What is the use of peripheral vision?
10. Light and dark adaptation. Go from a dimly lighted place into bright sunlight, and immediately try for an instant to read with the sun shining directly upon the page. Remaining in the sunlight, {242} repeat the attempt every 10 seconds, and notice how long it takes for the eye to become adapted to the bright light. Having become light-adapted, go back into a dimly lighted room, and see whether dark-adaptation takes more or less time than light-adaptation.
11. Color adaptation. Look steadily at a colored surface, and notice whether the color fades as the exposure continues. Try looking at the color with one eye only, and after a minute look at the color with each eye separately, and notice whether the saturation appears the same to the eye that has been exposed to the color, and to the eye that has been shielded.
12. Negative after-images. Look steadily for half a minute at a black cross upon a white surface, and then turn the eyes upon a plain gray surface, and describe what you see. (b) Look steadily for half a minute at a colored spot upon a white or gray background, and then turn the eyes upon a gray background, and note the color of the after-image of the spot. Repeat with a different color, and try to reach a general statement as to the color of the negative after-image.
13. Positive visual after-images. Look in the direction of a bright light, such as an electric light, holding the hand as a screen before the eyes, so that you do not see the light. Withdraw the hand for a second, exposing the eyes to the light, and immediately screen the eyes again, and notice whether the sensation of the light outlasts the stimulus.
14. Tactile after-images. Touch the skin lightly for an instant, and notice whether the sensation ends as soon as the stimulus is removed. If there is any after-image, is it positive or negative?
15. Tactile adaptation. Support two fingers on the edge of a table, and lay on them a match or some other light object. Let this stimulus remain there, motionless, and notice whether the tactile sensation remains steady or dies out. What is the effect of making slight movements of the fingers, and so causing the stimulus to affect fresh parts of the skin?
16. Temperature sense adaptation. Have three bowls of water, one quite warm, one cold, one medium. After holding one hand in the warm water and the other in the cold, transfer both simultaneously to the medium water and compare the temperature sensations got by each hand from this water. State the result in terms of adaptation.
17. Overtones. These can be quite easily heard in the sound of a large bell. What use does the sense of hearing make of overtones?
REFERENCES For a somewhat fuller discussion of the topic of sensation, see Warren's _Human Psychology_, 1919, pp. 151-214; and for a much fuller discussion, see Titchener's _Textbook of Psychology_, 1909, pp. 46-224.
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For a really thorough consideration of the facts and theories of color vision, see J. Herbert Parsons, _An Introduction to the Study of Colour Vision_, 1915.
For a more complete statement of the Ladd-Franklin theory, see the article on "Vision", in Baldwin's _Dictionary of Philosophy and Psychology_, 1902.
For a recent study that has revolutionized the psychology of the sense of smell, see _Der Geruch_, by Hans Henning, 1916, or a review of the same by Professor Gamble in the _American Journal of Psychology_, 1921, Vol. 32, pp. 290-296.
For an extensive discussion of the "Psychology of Sound", sec the book with this title by Henry J. Watt, 1917.
For a full account of taste, see Hollingworth and Poffenberger's _Sense of Taste_, 1917.
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