Comparative Electro-Physiology: A Physico-Physiological Study

It follows that, in this critical indifferent region, any condition which tends to exalt the excitability of the tissue, and thus the negativity of the response, will also accentuate the negative tone of sensation, its mixed character thus passing into the distinctly painful. Those conditions, on the other hand, which lower excitability, and therefore responsive negativity, will also act by detracting from the painful element in the indifferent sensation in the critical region, and rendering it to a greater or Jess extent positive, soothing, or pleasurable. In this way, by depressing or raising the excitability at will, a stimulus which had already caused a painful sensation might be made not unpleasant, and an indifferent or positive sensation could be made negative or painful. We shall presently see how, by variations due to the presence of internal or external modifying factors, the positive tone of sensation is actually rendered negative, and vice versa, under such transpositions.

Now we have at our disposal various well-known agencies by which the excitability of the tissue can be raised or lowered. In order to lower excitability, certain anzesthetic agents may be employed, and their effect in the modification of sensation-tones will presently be described. The ideally perfect means, however, not only for the exaltation or depression of excitability at will, but also for the rapid interchange of the one effect with the other, is that of electrotonus.

Under the normal condition of a medium intensity of E.M.F., we know that it is the kathode which enhances excit- ability, and the anode which depresses it. Hence an indifferent sensation, however caused, will be rendered painful: when the excited point is made kathode, or pleasurable when anode. If, again, the stimulus be of sufficient intensity: to render the | resulting sensation moderately painful, kat-electrotonus will make it intensely painful, and an-electronus convert it into soothing, by taking away from it the negative element.

The an- and kat-electrotonic effects referred to here are those which fall under the generalisation made by Pfliiger. I have shown, however, that the application of this law is but limited. Under the action of a feeble E.M.F. these an- and kat-electrotonic effects are exactly reversed, and it is then the anode which renders the tissue excitable, the kathode inducing relative depression. If, then, sensation- changes are dependent, both qualitatively and quantitatively, on antecedent physiological changes, we may expect corre- sponding reversals to take place in sensation, according as the anodic or kathodic applications are feeble or moderate.

And, lastly, we might expect to meet with other charac- teristic changes in responsive sensation, due to the peculiar differences shown in figs. 320 and 401, between the positive and negative effects. We there saw that the positive response was short-lived, whereas the negative was more persistent. Even the negative response itself further was, under moderate stimulation, of briefer duration than under difference in the fusion of sensation. Let us suppose that the frequency of stimulus, for the induction of the indifferent sensation, be adjusted in such a way as to cause a sensation which is so fused as to be almost continuous. Under normal kathode, this response, being converted into more persistent negative, will now become completely fused and _ painful Under normal anode, on the other hand, the individua

responses being converted into short-lived positive, the resultant sensation will become discrete and pleasurable Before proceeding to describe these experiments in detail, it will be well to present these inferences in the form of a tabulated summary : Intensity of E.M.F. Anode Kathode Moderate. Soothing and conspicuously | Painful and continuous. discrete. Feeble. Painful and continuous. . Soothing and conspicuously discrete. These generalisations I shall now demonstrate by means of experiments carried out under, not one, but various forms of stimulation. For experimental adjustment two conditions have to be fulfilled. The first is so to regulate the intensity of stimulus as to: cause the indifferent sensation. The second

is so to adjust its frequency as to induce a fusion of effects all but complete. It may here be pointed out that in general the degree of frequency necessary to bring about this semi- fusion. will be a question of the effective intensity of stimulation employed, and the excitability of the subject. Having first decided on the intensity of stimulus to be employed, it is easy to adjust its frequency by means of an adjustable interrupter. I shall first describe the Sensimeter, by means of which these experiments were carried out. Mechanical stimula- tion, when required, was caused by a tapper, actuated by an electro-magnetic arrangement (fig. 405). When an electrical current is sent round the electro-magnet, the soft iron arma- ture is pulled down against an antagonistic spring. When the current is stopped the armature is released, and the tapper delivers a blow. The height from which the tapper falls determines the intensity of the blow, the former being dependent on the strength of the electro-magnetic pull, which is determined in its turn by the intensity of the

current. This latter is adjusted by means of a carbon rheostat. In order to adjust the frequency of stimulation, a toothed-wheel is fixed on clock-work, the normal period of rotation of the axle being once in four seconds. This speed, however, may be continuously adjusted within a certain range by means of a regulating governor. When the toothed wheel has sixteen teeth, the frequency of stimu- lation is four times in a second. By inserting wheels with different numbers of teeth, and by means of the regulating

governor, it is possible to obtain any desired graduation of frequency. The same experimental arrangement, with slight modi- fications, may be employed to give a series of thermal shocks, For this purpose the electro-magnetic tapping arrangement is removed from the apparatus, and the electro-thermic stimulator attached to the circuit, instead of the tapper. The frequency of the thermal shocks may be regulated in the usual manner, their intensity being dependent on that of the heating current, which is adjusted by the rheostat.

In experimenting on the effect of mechanical stimulation, the intensity and frequency were so adjusted as to bring about a neutral and imperfect fusion. In a_ particular experiment, for example, the frequency of stimulus was four times in a second. The receptive point for this experiment was the very sensitive back of the end joint of the human fore-finger—that is to say, the space immediately adjacent to the quick of the nail. In order to study the electrotonic effect, one electrode was applied by means of a piece of cotton, moistened in normal saline, and placed on the receptive area, the second being on a different finger. After so adjusting the intensity and frequency of stimulus as to sive the required semi-continuous and indifferent sensation, the receptive point was made kathode, the E.M.F. employed being moderate—that is to say, of 1°5 volt. The resulting sensation was distinctly painful and continuous. By now reversing the electrotonic current, the receptive point was made anode, and the resulting sensation was not only | positive or soothing, but also strikingly discontinuous. In order to exhibit the reversal of these effects under feeble ; E.M.F., I employed an E.M.F. of ‘2 volt. The initial indifferent sensation was now found to be converted under anode, to negative and continuous, the kathode inducing a soothing and discontinuous sensation. The effect under a feeble E.M.F. is thus found to be in every way the opposite of that under strong. When frequent experiments are catried out on the same finger, the result is apt to be blurred in consequence of fatigue. It is therefore advisable to carry out the preliminary adjustment with one, and to repeat the experiment on another finger. The normal opposite effects of anode and kathode may in general be easily demonstrated by an E.M.F. of 1°5 volt. But the value of the feeble E.M.F.

which induces the reversal of these, varies with different individuals and with different modes of stimulation. It is, therefore, advisable to start with the lowest possible E.M.F. of the order of about ‘or volt, and increase it gradually, till the reversal of effects is most pronounced. On increasing the E.M.F. still more, the reversed effect passes first into neutral and then back to the normal, characteristic of ‘a moderate E.M.F. Similar effects are also obtained on employing the stimulus of thermal shocks. The electro-thermic stimulator is now inserted in the Sensimeter, in place of the mechanical tapper. The indifferent and incompletely fused sensation becomes. continuous and painful, on making the receptive point kathode, under a moderate E.M.F. of 2 volts. On now making the receptive point anode, the sensation becomes converted into a markedly discrete positive. The reversal of these normal effects under a feeble E.M:F. was obtained in a given experiment by thermal shocks, when the polarising E.M.F. was ‘03 volt.

Turning next to the chemical mode of stimulation, it will be remembered that an experiment has already been described (p. 582) where a continuously irritating sensation was caused by the application of salt on a wounded spot, this moderate sensation of pain being rendered soothing ~when the spot was made normal anode, and intensely painful when normal kathode. Effects precisely opposite resulted from the application of a feeble E.M.F. It will thus be seen that those same conditions which depressed the normal negative response, also acted to obliterate the negative tone from the resulting sensation. Those conditions, on the other hand, which exalted the negativity of the response, would convert the positive tone of sensation into negative or painful. These general facts have been demonstrated by the employment of different forms of stimulation, the neces- sary depression or exaltation of excitability having been brought about by electrotonus. So far we have dealt with the modifications induced in the responsive sensation by the variation of the receptivity of the excited point. We shall next briefly discuss the effects which ensue on the variation of the excitability and conductivity of the nerve by different agents. In order to induce depression, there are various anesthetic agents which might be used.

In investigating the influence of alcohol (p. 493) we saw that three distinct effects were induced by it on the receptivity, responsivity, and conductivity of a tissue respectively, these effects themselves being further modifiable by the duration and intensity of the application. It was shown that, in the first stage of its application, alcohol exalted the power of receptivity. But its effect on conductivity and responsivity, especially after a certain duration of application, was one of great depression. The total effect of alcohol is thus somewhat complex. At an early stage of its applica- tion there is an effect of exaltation. After a while, however, the conducting power becomes increasingly depressed by its action. This means that the passage of the true excitatory or negative wave is progressively impeded or even blocked, the positive alone continuing to be transmitted for a time. At a certain stage of alcoholisation, therefore, the negative tone of an existing sensation, normally painful, will, as it were, be erased. And this withdrawal of the painful element, by causing sudden relief, together with the actual trans- mission of the positive wave, may induce a tone of sensation which might even perhaps be regarded as pleasurable. In any case the abolition of conductivity must eliminate the element of pain, and it was undoubtedly this which, in pre-anzesthetic medicine, caused its employment for certain minor operations. Long and intense alcoholisation will, of course, obliterate all sensation. On the after-effects of so depressing a reagent it is unnecessary to dilate.

Effects, in some respects parallel, may be observed under etherisation, where, at a certain stage in the action of the narcotic, there is a cessation, not of all sensation, but of its painful element alone. Thus, by depression of conductivity, and’ consequent suppression of the negative wave, the positive may be ‘dissociated’ from the negative sensation. Sustained pressure on a nerve is also known to depress conductivity, and it is interesting to note here that pressure.on the ulnar nerve-trunk will abolish painful sensation, the positive, or mere contact-sensibility remaining practically undiminished.

We shall next study the various effects induced by the variation of conductivity. We have already seen that a moderately intense stimulus gives rise to two waves, one positive and the other negative, the former having the greater velocity of the two. Thus these two waves, starting from the receptive and reaching the distant responding point, will give rise to two different responsive effects, separated depending on the lag of one wave behind the other, will be greater the greater the length of the conducting tract. The first to arrive at the perceiving centre being the positive impulse, we shall have there a positive sensation of mere touch or contact. The later-arriving negative wave gives rise, according to the nature of the indicator, to mechanical contraction, galvanometric negativity, or a painful sensation, as the case may be. If the conducting tract be not sufficiently long, the two waves will be superposed, or indistinguishable, one masking the other. But they may be analysed, or separated, by anything which diminishes the conductivity of the intervening tract, and this is rendered possible by the fact that the positive wave is not much affected by changes of conductivity. On the other hand, the velocity and intensity of the negative wave are both diminished by anything that diminishes the conductivity. Hence a diphasic response—the sensation of touch followed by pain—may be expected, wherever the intervening con- ducting tract between receptive and responsive points is sufficiently long. In other cases, where the tract is shorter, a sufficiently strong stimulation will give rise to a single sensation of negative or painful tone. This negative sensation, however, is complex in its character, masking as it does, a contained positive element. If then the conductivity of this transmitting nerve be in any way diminished to a

analysed, the negative being made to lag behind the positive. There will thus be a dissociation of the dual elements of the complex sensation, and the result will be a diphasic response —the positive or sensation of contact, followed by that of pain. With: still greater depression of conductivity, the normally painful sensation, by the blocking of its negative element, will be turned into one of positive tone. We shall now proceed to verify these theoretical inferences. The fact that there are actually two distinct nervous impulses, and that of these the positive travels faster than the negative, is demonstrated by the well-known experiment in which a smart tap is applied on the ball of the foot, with the result that we perceive, first, the sensation of touch, and then, at an appreciable interval afterwards, that of pain. The possibility of this demonstration in a normal nerve is due to the length of the conducting nerve here concerned.

In other cases the same dissociation is seen to be effected under varying degrees of loss of conductivity, in different forms of nervous paralysis. The considerations which I have advanced, will, however, I believe, offer a satisfactory explana- tion of those curious instances of ‘dissociation’ and ‘ delayed with a pin, for instance, is first perceived as mere contact, and then, after an appreciable interval, as the sense of pain. Other cases are known in which, the loss of conductivity being very great; the patient could handle burning coal with- out pain, the resultant perception being entirely positive. From the diverse phenomena described in this and the previous chapters, it is clear that a sensation of positive tone is associated with that particular responsive change in the nerve, which is expressed as expansion and galvanometric positivity. We have also seen that such effects are brought about by a feeble intensity of stimulus, which acts to increase the internal energy without inducing the true excitatory negative effect. Of anything which thus increases the internal energy, it may be said in general that it will induce positive mechanical and electrical expressions with concomi- tant positive sensation, With stronger stimulus, as we have seen, the negative mechanical and electrical responses are induced. But such negative responses contain, as we have

also seen, the masked positive component, and from the point of view of energy, they represent the algebraical summation of income and expenditure, positive and negative, increase and diminution. Considered as sensation, then, this particular effect will be composed of dual elements, a mixture of positive and negative tones. With very strong stimulation, finally, the negative effect will be very great; expenditure will be greater than income; and the sensation will assume a pre- dominantly and persistently negative tone.

The complex negative, moreover, containing a masked positive, is capable of dissociation into its component elements. By the partial or complete block of conductivity, it is exhibited as dual response of ‘dissociation, or by the total suppression of the negative, as positive alone. In this way, a sensation, originally painful, may be rendered not unpleasurable, or even pleasurable. It was further shown, employing electrotonus and the sensimeter, that an indif- ferent and semi-fused tone of sensation can, by merely exalting the excitability of the tissue, be converted at will into one

ability, on the other hand, this indifferent sensation may be made pleasurable and strikingly discrete. Memory an after-effect of stimulus— Persistence dependent on strength of stimulus —Rate of forgetting—Multiple after-effects in retina—Spontaneous revival of after-images— Theories of memory— Latent images and their revival—After- effect of stimulus on excitability and conductivity—Differential effect of diffuse stimulus—Revival of latent image in metal—Revival of latent image on phos- phorescent surface—Negative or reversed memory-image—Psycho-physio- logical version of this experiment—Differential excitation under diffuse stimulus, internal or external—Continuity of this seen in mechanical response of plagiotropic stem and pulvinus of A/cmosa, in the electrical discharge of certain fishes, and in psychic response of memory.

Up to the present we have considered only the immediate effects of stimulus. We know, however, that excitation entails, not only an immediate, but also an after-effect.. We are thus led to the question of the physical aspects of the phenomenon known as memory, which is admittedly a matter of after-effects. Even with the somewhat insensitive apparatus for the detection of nervous changes which is at our disposal—the galvanometer or the Kunchangraph—we find that the after-effect of strong is more persistent than that of feeble stimulus. In the very sensitive neurile apparatus, nervous changes and their after-effects are more distinctly perceived. The psychological retention of an impression follows, in general, the curve of response and recovery. The fact that physiological recovery from the effect . of strong stimulus is less rapid than from feeble, has its correspondence in the period required for the fading of sensory impressions, of which also it may be said that the stronger persist longer than the weak.

for a longer time than that of feeble, may be exhibited in.an interesting manner as follows. A simple design is made with magnesium powder and fired in a dark room. A second observer, unacquainted with the design, observes the flash and closes his eyes. The instantaneous flash, obscured as it is by dense smoke, does not at once produce any definite impression. In the retina, however, the ob- scuring image of the smoke, being of little luminosity, quickly passes off, and the after-effect of the brilliant flash, thus separated from the obscuring smoke, grows into perfect distinctness. In this manner I have often been able, by sub- sequent closure of the eyes, distinctly to observe luminous phenomena of brief duration, which, while the eyes were open, had been indistinguishable. :

We have thus touched upon the question of the time required for the obliteration of a mental impression. Another interesting aspect of this subject lies in the rate at which fading takes place, or, in other words, the rate of molecular recovery. In the curve of recovery we find that this rate is at first very rapid, and becomes increasingly slow with the descent of the curve. This is also the characteristic of the process of forgetting, Ebbinghaus, for instance, found that the forgetting of a series of ‘nonsense syllables, at first quick, became increasingly slower with time.

Certain excitable tissues, such as nerve and cardiac muscle, give responses to strong stimulus, which, as we have seen, are multiple in character. The retina, again, under intense stimulus of light, exhibits multiple after-excitations, which may be detected by a galvanometer (p. 426). This fact explains the multiple after-image often seen on closing the eyes after strong light. Another proof that these multiple after-images are physiological lies in the fact that their periodicity is modified by a previous condition of rest or activity. Thus, early in the morning, when fresh from rest, this period I find to be at its shortest, and later in the day to become gradually longer, owing to growing fatigue. In a given instance, the period at 8 A.M. was 3 seconds,

which had lengthened to 5 seconds by 3 P.M.; and was further prolonged to 6°5 seconds by II P.M. | One way of exhibiting the after-images in the retina is, as we have already seen, by means of a stereoscope con- taining two incised slits inclined to each other, instead of photographs. On looking through this at the bright sky for ten seconds, or longer, a composite image is formed of an inclined cross. The eyes are now closed, and the first. effect noticed is one of darkness, due to the molecular rebound. By reason of the Binocular Alternation of Vision, already referred to, one luminous arm of the inclined cross now projects itself aslant the dark field, and then slowly disap- pears, after which the second, perceived by the other eye, shoots out suddenly in a direction athwart the first. This multiple alternation proceeds for a long time, and produces the curious effect of two luminous blades crossing and re- crossing each other. At first the after-images of the cross, seen with the eyes closed, are very distinct, so distinct that any unevenness in the design at the edges of the slanting cuts can be made out clearly. There is here no doubt of the ‘objective’ nature of the strain impressed on the retina, which, on the cessation of direct stimulus of light, gives rise to after-oscillations with concomitant visual recurrences. This recurrence may be taken as a proof of the existence of physical strain in the retina. The recurrent after-image is very distinct at the beginning, but becomes fainter with each repetition. A time comes when it is difficult to tell whether the image is the objective after-effect due to previous strain, or merely an effect of ‘memory.’ There is, in fact, no hard- and-fast line of demarcation between the two—one merges simply into the other. In connection with this, it is interesting to note that some of the phenomena of memory also are admitted to be recurrent.

Visual impressions and their recurrence often persist for a very long time. It usually happens that, owing to weariness, the recurrent images disappear, but in some in- stances, long after this disappearance, they will spontaneously appear at most unexpected moments. Thus in a given case of the present experiment, performed in the afternoon, the subject perceived this recurrence for some time, after which the alternating impressions seemed to disappear, and were completely forgotten. On retiring at night, however, these recurrent images suddenly reappeared. Thinking the matter to be an effect of light, the observer hastily extinguished his lamp. But the recurrent images now became only the more © intense.

In another case the recurrence was observed in a dream, about three weeks after the original impression was made, and in this case it was seen as the crossing and recrossing of bright swords. These instances of the revival at night of impressions made in the daytime, when the interference o distracting influences is withdrawn, is significant. Since an intense stimulation of nerve is liable to recur spontaneously, ‘without the action of the will, or even in spite of it, it follows. that any single impression, when very intense, may become dominant, and persist in recurring automatically. Examples of this are only too familiar.

We have hitherto dealt with that aspect of memory in which it isa more or less immediate after-effect of sensory stimulation. But we encounter a much more difficult pro- blem when we come to the question of the revival of an image long after it has apparently faded. It has been sug- gested that this process of revival depends upon the existence of some ‘scar, or fixed impression, in the brain, or on a certain persistent disposition or tendency to movement created there. It is perhaps worth while to point out here, however, that though when a blow is recent, the smarting effect will persist for some time, yet; when once healed, no scar could of itself reproduce the original excitation. It is of course recognised that such expressions are merely figurative, and that the entire process is not clearly understood. We are more likely, however, to arrive at a true explanation of the phenomenon if we recognise in it two distinct factors, first, that of molecular change, with concomitant change of

properties ; and, second, the effect of an internal stimulus, delivered as a blow from within, by an impulse of the will, upon the sensitive surface in which the image is latent. _ We shall now first observe in some detail those changes which remain as an after-effect of previous stimulation. The differential effect caused by primary stimulus fixes the latent image, and it is only by the reproduction of the same differen- tial excitation, that the memory-image can subsequently be revived.

We must here recall briefly the results which were estab- lished in Chapter XLII., on the modification of response under cyclic molecular variation. It was there shown that, under the action of stimulus, the molecular condition of a substance undergoes a progressive variation, exhibited in its characteristic curve ; that the forward and return curves do not exactly coincide, because the history behind the two half-cycles has not been the same; and, finally, that on the cessation of stimulus, the original molecular condition is not exactly restored, a certain effect being residual. Owing to this residual effect, the properties of the responding sub- stance are changed. We also saw that, in consequence of this impressed change, the conductivity and excitability of’ the substance might be enhanced. A frequent repetition of a stimulus was thus shown to create a habit or disposition by which the mass of a substance, formerly almost non-con- ducting, might be made a conductor of excitation.