Two separate points; direction known

The two-point space threshold varies also under other conditions. Practice reduces it ; for instance, a threshold of 14 millimeters may after a few periods of practice be reduced to 5 or 6 millimeters. Practice on one area also lowers the threshold for other bodily areas; the left forearm has been stimulated and the threshold for the right forearm thereby reduced from 11 to 7 millimeters in six practice periods. On the other hand, fatigue raises the threshold; that is, it decreases the sensitiveness. And drugs, such as atropine, morphine, strychnine, and alcohol, hâve a similar effect With

78 TACTUAL SPACE

référence to température it may be said that if the points are cold when they are applied to the skin the threshold is in gênerai Ipwer than if the stimulus points are warm: the cold points seem to be more sharply loca^ized. Warming the skin before applying the points also increases the sensitiveness, while cooling the skin has an opposite effect In the experiments it is best to exclude simultaneous température sensations. Rotation of one point reduces the threshold. Disease, especially of the nervous system, affects spatial sensitiveness, sometimes increasing, sometimes decreasing it, depending upon the nature of the disturbanoe. In some diseases ail pressure sensations are lost throughout more or less circumscribed skin areas. Very large individual différences are found among différent persons, some being several times more sensitive than others.

5. Simultaneous Compared with Successive Touch. — Measure the two-point space discrimination in the same région and in the same manner as in Exp. 2, exoept that instead of touching the two spots simultaneously you touch each with the same point quickly in succession. To regulate the distance, hold a ruler near the skin as a guide.

When two points are applied successively instead of simultaneously, as in Exp. 5, the threshold for the discrimination of two points is very much lowered, probably by about one half .

Two points may be clearly perceptible as two, without knowledge on the part of the observer of the direction of one from the other. Two successive stimuli were

TACTUAL SPACE 79

correctly perceived as two upon the foreann when 3 millimeters apart, but their directions were known only when the séparation was increased to 7 millimeters. Thèse figures show that the direction thresbold is larger than the différence or two-point threshold. The same is true for simultaneous pressures. An analogous tendency is also common in other sensations; for instance, we can tell that an odor is présent before we can identify it, or, we may know that two tones are unlike in pitch without being able to tell which is the higher one.

Related to the comparison of simultaneous and successive touch is the discrimination for the motion of a point upon the skin. We can detect motion before we can detect the direction of the motion. The discrimination for motion varies greatly with the rate of motion.

6. Method "with Knowledge" Compared with Method "without Knowledge." — ^Repeat Exp. 2, but let O, with his eyes open, apply the compassés. Record as before.

In this experiment we hâve the coopérative effect of the Visual, tactual, and muscular space sensés; the method is "with knowledge" as compared with the method of Exp. 2, which was "procédure without knowledge." The two-point threshold by this method is lower than when obtained by the method "without knowledge."

The most effective method of securing a knowledge of objects by tactual impressions is not to allow them to press against the unmoved skin, but to pass the sensitive skin surface over the objects and thereby gain a wealth

80 TACTUAL SPACE

of information quite impossible by the passive method. This is active pressure. It is keen, quick, and sure. It is the method we most often employ. We do not test the smoothness of a surface by simply laying the hands upon it ; we rub the tips of the fingers upon it. The knife-grinder draws the edge of the blade he is sharpening along the finger ; he does not merely hold the edge of it against the finger. In active pressure the skin really seems to be more sensitive; certainly the efficiency of the sensitive skin in mediating sensations of the space qualities of objects is very much increased, but this is really due to favorable conditions for perception and does not indicate any real increase in sensitiveness af ter ail.

The blind live in a world of tactual, muscular, and auditory space. Their fine space distinctions are made by touch, in which they acquire most extraordinary skill. A blind person can read with his finger-tips almost as fast as we ordinarily read with our eyes, impossible though it may seem. The print for the blind consists in raised points or ridges made by embossing a stiff paper. We expérience difiiculty in detecting even the form of a single letter by touch.*

Yet the superiority of the blind in this capacity does not lie so much in superior sensitivity as in the habit of couching impressions in tactual terms. The two-point threshold is only a trifle lower in the blind than in see-

* The style of the raised print has been improved by experiments on the principles hère studied; for example, on the relative legibility of a line as compared with a dot alphabet, or simultaneous impressions of dots compared with successive impres* sions, most favorable spacing, etc.

TACTUAL SPACE 81

ing persons after brief practice; and the blind person's sensitivity to pressure does not differ much from that of seeing persons under careful expérimental conditions.* But the blind person is a 'Hactile.^' He thinks in terms of touch as we think in terms of sight. Tactual discriminations therefore hâve immédiate meaning to him. The superiority of the blind in this activity is not in sensation but in perception. This is a fundamental but neglected distinction.

When we read we see one word at a time by direct vision, and the words around this shade off gradually in indirect vision. In the same manner the blind person gets a clear image of one letter or word at a time by the left hand and skirmishes with the right so as to get a gênerai impression of what lies ahead. He may also leam to read by word instead of letter units.

* Put a human hair 3 centimeters long on a sheet of smooth glass. Lay over it a sheet of clear writing-paper and locate the hair by touch. Add sheet after sheet and locate as before until you come to such thickness that you cannot locate the hair by touch. The greatest thickness through which the hair can be located correctly is a measure of the sensitivity to pressure. The average person whose hands are not callous should locate the hair under forty to fifty sheets of "20-pound" bond paper. A blind person cannot do very much better.

CHAPTEB VII CUTANEOUS SENSATIONS

For One.* The problem is to explore, identify, and plot charts of cold spots, warmth spots, pain spots, and pressure spots found in the skin.

Outline four blocks, each 10 by 10 millimeters, into 2millimeter squares with pen and ink on the back of the left hand. Outline a copy of each of thèse in the notebook and label them Cold, Warmth, Pain, and Pressure respectively.f

Put six of the nails on crushed ice and the other six into water just below the boiling-point.t The senseorgans in the skin fatigue very quiekly — almost instantaneously — when touched. Therefore, sélect points for stimulation systematically ; follow parallel lines, apply the stimulus with précision and with a constant vertical pressure, remove it within a second, and always trust to first impressions. Make a few preliminary trials for practice on some other skin area.

* Provide a dozen tenpeniiy wîre nails, a smaU cork, a horsehair from the mane, warm water^ and ice.

ÎFreehand ruling is accurate enough for the purpose. For more accurate work a pointed métal tube is kept at a constant température by having warm or cold water at the desired température flow through it. Another way is to put the warm water or crushed ice into a tube and change it as often as necessary.

CUTANEOUS SENSATIONS 83

1. Cold Spots. — With the point of a cold nail, explore a plotted area of the skin systematically and mark on the note-book copy of this plot the location and approximate area of the spots which give rise to clear and localized sensations of cold.*

When the cold nail touched certain portions of the skin it did not feel cold ; but at other places definite and unmistakable sensations of cold were experienced. Thèse spots may perhaps be spoken of more properly as areas.

2. Warmth Spots. — Take a warm nail — as warm as it can be made without making it uncomfortable to hold in the hand — and survey another plotted area systematically, and mark on its copy the location and approximate area of the spots which give rise to clear and localized sensations of warmth.

The spots sensitive to warmtïi are more difficult to find. The warmth spots are . less numerous than th« cold, they are less sharply localized, and they seem to lie deeper down in the skin than the cold spots. The warmth sensations do not foUow so quickly after the application of the stimulus as do the cold sensations; that is, there is a more appréciable latent time. The area of a warmth spot usually seems larger than that of a cold spot.

The kind of stimulus which can best call forth a

* Start at one side and pass by parallel Unes until the whole plot has been covered by point stimulations. Never retrace or verify in the same sitting. Lift the nail vertically up and down and touch with the very point. Change the nails as often as it is neoessary to keep them cold. If the spots are large and taper off, this may be shown by shading on the chart.

84 CUTANEOUS SENSATIONS

certain sensation is known as an adéquate stimulus. Thus, light-waves constitute the adéquate stimulus for sight sensations. The adéquate stimulus for sensations of cold is the cooling of the skin, or the lowering of its température; that for warmth is the warming of the skin, or the raising of its température, in other words the application of radiant energy.

Now, since the skin is not the same température ail over the body, the same object may feel warm to one part and cool to another. The température of a given portion of the skin at a given time is known as its physiological zéro-point. Whether or not the stimulus shall excite the cold or warmth spots dépends in part upon the physiological zéro-point; if the stimulus is slightly warmer than the skin, warmth sensations resuit; if it is slightly cooler, cold sensations resuit.

The nerve-endings in the skin which correspond to the cold spots are probably separate and distinct from those which correspond to the warmth spots. Êach température spot has its own nerve-fiber or group of fibers which responds with its own characteristic sensation only ; that is, it has its own spécifie energy. This means that even though a warm stimulus be applied to a cold spot, a sensation of cold and that only, if any, will resuit. The same principle applies to warmth spots. Ordinarily only warmth spots respond to a warm stimulus and cold spots to a cold stimulus, but they sometimes respond to inadéquate stimuli, so that a neutral stimulus, say a pin, may be used to find ail four kinds of spots called for in this chapter.

Cold and warmth are the two qualities of température

CUTANEOUS SENSATIONS 85

sensations. Introspection lias revealed a third température expérience qualitatively différent from cold and warmth, the perception beat. Heat or hotness is something distinct from mère warmth, and it is not due alone to the excessive stimulation of a warmth nerve. Heat is due, to a fusion of cold and warmth sensations aroused by the simultaneous stimulation of cold and warmth nerves by rather high températures. That is, the same stimulus, by radiation of heat or otherwise, excites both cold and warmth nerves, each of which responds with its spécifie sensation, but the complex expérience is neither that of cold nor of warmth but of beat

The fact of température spots was discovered independently by Blix, Goldscheider, and Donaldson in the years 1884 and 1885. It is thought that the nerveendings of Kuffini are the organs for warmth, and the endings of Krause are the organs for cold.*

3. Pain Spots. — Slit the small end of the cork and insert a horsehair into the slit, so that a squarely eut end of the hair projects from one side about 5 millimeters.t Hold the cork firmly near the slitted end, and, by pressing the hair down vertically until it begins to bend, survey a third plotted area systematically and mark on the copy of it the location of those spots which give rise to clear and localized sensations of pain.

* See Barker, "The Nervous System."

f The bristle should project from the cork about 3 millimeters if the hand is callous, or 10 millimeters if it is very sensitive. Adjust the length of the hair between thèse two limits as required. The pressure exerted should be as uniform as possible.

86 CUTANEOUS SENSATIONS

When the whole plot bas been surveyed, verify about a dozen pain spots by a single touch at each and mark différences in intensity of painfulness on the scale of 1, 2, 3, where 1 represents the faintest and 3 the strongest sensations, by Connecting each spot with a number in the margin, as in correcting printer's proof.

Pain spots are easily located, and the pain sensations are very vivid and very unlike any other cutaneons sensations. The pain sensations usually give rise to distinct reflex movements; indeed the movement is sometimes perceived before the pain. The pain spots are more numerous than even the cold spots ; and, while they themselves are very sensitive, intervening skin areas are usually quite anœsthetic as regards pain, sometimes to such a degree that a needle may be inserted deeply into the skin without pain. The cold and warmth spots are analgésie, i.e., insensitive to pain ; but it is diflScult to stimulate thèse without at the same time stimulating one of the very numerous pain spots. Pain spots are to be found only upon the skin or structures closely connected with it genetically. The f ree nerveendings in the skin are probably the end-organs .of pain.

There is no single adéquate stimulus for pain. Any of the gênerai classes of stimuli — ^mechani^^al, thermal, chemical, electrical — ^will cause pain. This gênerai nature of pain stimuli is necessary f rom the biological point of view. Pain is one of the most valuable endowments of living organisms in enabling them to avoid danger. Pain means disaster ; when an expérience be-

CUTANEOUS SENSATIONS 87

cornes painful it means that in jury may foUow unless there is some protective reaction. The point at which expériences become painful is usually just before injury is likely to resuit. From the point of view of évolution, pain was probably the first expérience to appear, because the most necessary. It is still the best safeguard we ourselves hâve against bodily injury.

That pain is a spécial sensé is a relatively récent discovery. It was formerly thought that pain resulted from the excessive stimulation of any sensory nerve. Such, however, is not the case. The pain we expérience under the influence of a very bright light cornes from real pain-nerves in the eye, not from the overstimulation of the optic nerve. The most bitter sensations are not, in the strict sensé of the word, painful. When we bear in mind the profuse distribution of the pain end-organs and the gênerai nature of pain stimuli, the above fact readily becomes intelligible.

Our English word "pain" is ambiguous. We use it to refer, on the one hand, to pain sensations, as in this chapter ; and, on the other hand, to refer to the f eeling of unpleasantness.* Now, pain sensations are nearly always unpleasant, but not ail unpleasant expériences are painful by any means. It is exceedingly unpleasant to overtum one's cup of coffee at dinner, but it is not necessarily painful unless one happens to be scalded by the liquid. As we hâve used the word pain, it has had

♦The Germans hâve two words, "Schmerz" and "Unlust", to cover the ground of our word pain.

88 CUTANEOUS SENSATIONS

référence alone to a spécial sensé of pain, with central, peripheral, and Connecting nervous apparatus such as every other spécial sensé possesses.

Pain sensations hâve only one quality, pain. The varions kinds of pain we think \ve expérience, such as throbbing, burning, sharp or dull pains, are due to variation in attributes other than quality and to the accompaniment of sensations other than pain. Thus, a prick, a pinch, a toothache, and a headache differ in local sign, areal distribution, intensity, duration, and combination with other sensations, but, as pains, they ail hâve the same quality.

4. Pressure Spots. — With the same finder as in Exp. 3, and in the same manner, survey the plotted area and mark on the copy of it the spots which give rise to clear and localized sensations of pressure. Kecord on the same plot the relative position of the nearest hair to each pressure spot

The fourth spécifie cutaneous sensation is pressure. Pressure spots are more numerous than cold and warmth spots, but less numerous than pain spots. They are distributed over the surface of the whole body and extend some distance into the natural openings of the body cavities. Each minute hair on the body has its pressure spot, usually situated to the windward, but pressure spots also occur on the hairless régions. The pressure nerves, which of course are distinct from the température and pain nerves, hâve a specialized mode of termination in the skin, each nervefiber bearing a little terminal bulb, such as the Pacinian

CUTANEOUS SENSATIONS 89

corpuscle or the spindle of Meissner. The adéquate stimulus for pressure is meehanieal.

Pressure sensations are of one quality only, usually described as feeling like a tiny seed embedded in the skin and pressed downward. Ordinarily this peculiar quality is not noticed, as a number of pressure spots are stimulated simultaneously, and, for the most part, there are accompanying température and pain sensations.

In ail the above cutaneous sensations we assume that the "spot" of a spécifie eharacter is due to the présence of the corresponding spécifie end-organ. The spot becomes an appréciable area when the tant skin surrounding the end-organ communicates the stimulus indirectly.

A few years ago a schoolboy who did not know how many sensés he had would hâve been considered a duU boy. He learned on his father's knee that he had five, the five gâtes to Milton's city of Man-soul, namely, eyegate, ear-gate, nose-gate, tongue-gate, and feel-gate. To-day the professor of psychology does not know with certainty how many sensés he has, but he does know that he has more than five — possibly twice as many as he was supposed to hâve fifty years ago. The newer sensés hâve been discovered and explored by the natural-history method followed in thèse experiments.

How shall we know when we come across a new sensé ? Among the chief criteria are the following: (1) the sensation, as a conscious expérience, is unlike every other known sensé quality; (2) it is objective and localized, and (3) it comes through spécifie end-organs.

90 CUTANEOUS SENSATIONS

Some psychologists and physiologists hold that hiinger and thirst are spécial sensés, but the prevailing opinion is that hunger and thirst; fail on the first.and the third criteria; they ean be reduced to other known sensecomplexes.

CHAPTER Vin

For Tnoo.*

Ip you can just barely perceive a différence of one gram added to twenty grams, what is the smallest différence you can perceive when added to forty grams? Eighty grams ? Two hundred grams ?

Weber's law gives the answer: "Equal différence between sensations means proportional différence between stimuli." The ratio just posited is 1 :20 ; hence 2 grams added to 40 grams, 4 grams added to 80 grams, and 10 grams added to 200 grams would be equally perceptible.

This law applies in varions ways in the différent sensés. Weber first demonstrated it for the perception of weight, and as this field présents the çimplest as well as the most exact conditions for measurement, we shall verify and illustrate the law for lifted weights.

Specifically, this is what we shall try to do: Exp. 1 is a preliminary skirmish to détermine within what range of weight it will be most profitable to make the measurement on the observer ; Exp^ 2 reduces this to an exact measurement; and Exp. 3 makes the same measurement with doubled weight and doubled incre-

* Provide two tumblers, some medium-sized shot or small nails or coins, a wad of cotton, and two small pièces of woolen cloth.

92 WEBEK'S LAW

ment. Now, if Weber's law holds, there should be about the same degree of success in the discrimination in Exps. 2 and 3.

Select two ordinary tumblers of the same kind and weight. A fairly heavy jelly-glass is good, but the sides must slope so that the one fits snugly into the other. Put a wad of cotton or soft cloth into the bottom of one. Make a cushion out of three or four folds of cloth on which to set the glass.

Prépare a differential weight as follows: Take a quantity of small pièces of métal that are uniform in weight (e.g., small coins, nails, or shot) and sélect what you judge to be about one tenth the weight of a glass. Tie thèse into a small cloth so that this bag can be lifted into the glass and out of it quickly and without sound or other disturbance.

Blindfold O and let him be seated at the table with the padded glass before him in such a position that he can lift it conveniently and accurately by a free-arm movement. Vary the weight of the glass by putting the differential weight into it or taking it out for successive trials. Every précaution must be taken to prevent any other means of information than the actual sensé of weight. Use two signais : "Ready" for the first of two liftings in a comparison, and "Now" for the second. Make the time between the two liftings as short as possible. Allow répétition of a trial only when the observer calls for it on the ground of some spécifie disturbance. Any accidentai due obtained from the expérimenter would, of course, be such disturbance.

To détermine the order in a comparison, flip a coin

for each trial and put the differential weight into the first lifting if head is up, or into the second if tail is up.

1. The Approximate Threshold of Discrimination for Weight. — Let O lift the glass twiee in close succession and say whether it was heavier in the first or the second lifting. If O is right in more than 75 per cent of a f ew trials, lessen the differential weight ; if he is right only in much less than 75 per cent of the trials, increase it. Skirmish in this way until you get such an incrément of the weight that O will get about 75 per cent of the judgments right in 20 trials. Between 65 and 85 per cent will do. The incrément thus obtained is the one with which to make the more accurate measurement in the next experiment. Record it in terms of the number of objects (coins, nails, or shot).

The weight of an object is judged most accurately when it is lifted in the most convenient position. The same object appears to be heavier if grasped lightly than if grasped tightly. An object appears heavier if lifted in contact with a small area of skin than if in contact with a larger area. An object appears heavier if lifted with a slow movement than if lifted by a quick, but not jerking, movement. A heavy weight grows heavier and a light weight grows lighter when sustained undisturbed in the hand. Of two equal weights lifted in succession, the second appears to be the heavier. An object of moderate weight appears to be heavier or lighter than it really is according as a lighter or a heavier object has been lifted just before. An object held in one hand appears to be lighter than it really is if the other hand

supports another weight or is otherwise engaged, as in a grip or push. Any déviation from the physiological zéro of température increases the apparent weight of the objeet. An object appears to be heavier or lighter than it really is according as the material of whieh it is made suggests lighter or heavier weight than the actual. Of two objects that are of the same weight but différent size, the larger appears to be the lighter when lif ted.

This formidable array of assertions represents some of the well-established laws of the perception of weight. Thèse laws must be kept clearly in mind in the performance of a weight test in order to avoid introducing errors whicH would resuit from the violation of such laws. Thus, two objects which are to be compared in weight by lifting must be lif ted in the same position; they must be grasped with the same area of pressure, and with a grip proportional to their weights ; they must be lifted at the same rate, to the same height, and be sustained the same length of time ; the order must be alternated fairly; contrast-weight, counter-weight, or division of energy must be avoided ; température must be kept constant ; the weights must appear to be made of the same material and must be of the same size. Violation of thèse principles is certain to resuit in error and often appears in the most astonishing distort ions of weight. We must assume that they are eliminated in the foUowing experiments.*

*The flipping of the coin is another type of précaution. It may seem useless, but it is indeed very essential in order to forestall any tendency in the observer to make some inference, whether true or false, as to what the next order shaU be. Suppose that the expérimenter should détermine the order arbitrarilyi