Response in the Living and Non-Living

Effect of temperature.—It has been found that when the temperature is raised above a certain point, retinal response shows rapid diminution. On cooling, however, response reappears, with its original intensity. In the response given by the sensitive cell, the same peculiarity is noticed. I give below (fig. 101, a) a set of responsecurves for 20° C. These responses, after showing slight fatigue, became fairly constant. On raising the temperature to 50° C. response practically disappeared (101, 4). But on cooling to the first temperature again, it reappeared, with its original if not slightly greater intensity (fig. 101, c). A curious point is that while in

record (a), before warming, slight fatigue is observed, in (ce), after cooling, the reverse, or staircase effect, appears. me fet es eed the ta 7 aes ae Tomah Uisteae Peer Bi a a EOS Ge a 50°C rane (ies (pean ee ee Ga at Td es VAG tener ba \ \ Z ‘ ‘ ‘ i \ ‘ { \ _—. “ (6) RCH he oleae pe Fic. 101.—InFrivEence or TEMPERATURE ON RESPONSE Illumination 20”, obscurity 40”. In (a) is shown a series of responses at 20° C.—the record exhibits slight fatigue.

(b) is the slight irregular response at 50°C. (c) is the record on re-cooling ; it exhibits ‘staircase’ increase. Effect of increasing length of exposure. —If the intensity of light be kept constant, the magnitude of In (a) the source of light was at a distance of 50cm.; in (0) it was at a distance of 25 cm. response of the sensitive cell increases with length of exposure. increase of duration does not increase magnitude of effect. Too long an exposure may however, owing to fatigue, produce an actual decline.

I give here two sets of curves (fig. 102) illustrating the effect of lengthening exposure. The intensities of light in the two cases are as | to 4. The incandescent burner was in the two cases at distances 50 and 25 em. respectively. It will be observed that beyond eight seconds’ exposure the responses are approximately uniform. Another noticeable fact is that with long exposure there is an after-oscillation, This growing effect with lengthening exposure and attainment of limit is exactly paralleled by responses of retina under similar conditions.

Relation between intensity of light and magnitude of response.—In the responses of retina, it is found that increasing intensity of light produces an increasing effect. But the rate of increase is not uniform: increase of effect does not keep pace with increase of stimulus. Thus a curve giving the relation between stimulus and response is concave to the axis which represents the stimulus. The same is true of the sensation of light. That is to say, within wide limits, intensity of sensation does not increase so rapidly as stimulus.

This particular relation between stimulus and effect is also exhibited in a remarkable manner by the sensitive cell. For a constant source of lght I used an incandescent burner, and graduated the intensity of the incident light by varying its distance from the sensitive cell. The intensity of light incident on the cell, when the incandescent burner is at a distance of 150 cm., has been taken as the arbitrary unit. In order to make allowance for the possible effects of fatigue I took two

Fic. 103.—Responses or Sensitive Cent to various INTENSITIES OF LicuT On the left the responses are for diminishing intensities in the ratios of 7, 5, 3, and 1. On the right they are for the increasing intensities 1, 3, 5, and 7. The thick lines are records during exposures of one minute; the dotted lines represent recoveries for one minute. successive series of responses (fig. 103). In the first, records were taken with intensities diminishing from 7 to 1, and immediately afterwards increasing from | to 7, in the second.

(The intensity of an incandescent gas-burner at a distance of 150 cm. is taken as unit. ) course of the experiment, the deflection in each curve’ was measured from a line joing the beginning of the response to the end of itsrecovery. A mean deflection, corresponding to each intensity, was obtained by taking the average of the descending and ascending readings. The two sets of readings did not, however, vary to any marked extent. The deflections corresponding to the intensities 1, 3, D, 7, are, then, as 9°5 to 18, to 30, to SiS siiee deflections had been strictly proportionate to the inten-

Fic. 104.—Curves GiIvinc THE RELATION BETWEEN INTENSITY oF LIGHT AND In another set of records, with a different cell, I obtained the deflections of 6, 10, 13, 15, corresponding to light intensities of 3, 5, 7, and 9. The two curves in fig. 104, giving the relation between response and stimulus, show that in the case of inorganic substances, as in the retina (Waller), magnitude of response does not increase so rapidly as stimulus. After-oscillation—When the sensitive surface is subjected to the continued action of heht, the E.M.

effect attains a maximum at which it remains constant for some time. If the exposure be maintained after this for a longer period, there will be a decline, as we found to be the case in other instances of continued stimulation. The appearance of this decline, and its rapidity, depends on the particular condition of the substance. When the sensitive element is considerably strained by the action of light, and if that lHght be now cut Fic. 105.—ArrEer-OscILLATION Exposure of one minute followed by obscurity of one minute.

Note the decline during illumination, and after-oscillation in darkness. That is to say, the curve of recovery falls below the zero point, and then slowly oscillates back to the position of equilibrium. We have already seen an instance of this in fig. 102. Above is given a series of records showing the appearance of decline, from too long-continued exposure and recovery, followed by after-oscillation on the cessation Certain visual analogues to this phenomenon will be noticed later.

Abnormal effects—We have already treated of all the normal effects of the stimulus of heht on the retina, and their counterparts in the sensitive cell. But the retina undergoes molecular changes when injured, stale, or in a dying condition, and under these circumstances various complicated modifications are observed in the response. 1. Preliminary negative twitch.—When the light is incident on the froe’s retina, there is sometimes a transitory negative variation, followed by the normal

Fic. 106.—TRanstent Positive AUGMENTATION GIVEN BY THE F'ROG’s RETINA ON THE Cessation oF Licur L (WALLER) positive response. This is frequently observed in the sensitive cell (see fig. 96, 0). 2. Reversal of response.—Again, in a stale retina, owing to molecular modification the response is apt to undergo reversal (Waller). That is to say, it now becomes negative. In working with the same sensitive cell on different days I have found it occasionally exhibiting this reversed response.

3. Transient rise of current on cessation of light.— Another very curious fact observed in the retina by Kuhne and Steiner is that immediately on the stoppage of light there is sometimes a sudden increase in the retinal current, before the usual recovery takes place. This is very well shown in the series of records taken by Waller (fig. 106). It will be noticed that on illumination the response-curve rises, that continued illumination produces a decline, and that on the cessation of light there is a transient rise or current. I give here a series of records which will show the remarkable similarity between the responses of the cell and retina, in respect even of abnormalities so marked as those described

eurious effects, that is to The thick line represents response ralim : amie exposure), and dotted line the Say, the pr eliminary negatl\ e recovery during darkness. Note fwiteh and sudden augmen- — %® ‘*mins! positive fied tation of the current on the cessation of light, have also been noticed by Minchin in photo-electric cells. 4. Decline and reversal.—We have seen that under the continuous action of light, response begins to in any case, under continued light, the deflection falls. (1) The decline may nearly reach zero. If now the

after-oscillation (fig. 108, a) (2) If the light be continued for a longer time, the Fic. 108 —Dxcuine UNDER THE ConTINUED Action or LicuT (a) Decline short of zero; on stoppage of light, rebound downwards to zero ; after-oscillation. (b) Decline below zero; on stoppage of light, rebound towards zero, with preliminary negative twitch. (c) The same, decline further down; negative twitch almost disappearing. response now becomes apparently negative. If, now, the light be stopped, there is a rebound upwards to recovery, with, generally speaking, a slight preliminary twitch downwards (fig. 108, 6, c). This rebound carries it back, not only to the zero position, but sometimes beyond that position. We have here a parallel to the following observation of Dewar and McKendrick:

‘When diffuse light is allowed to impinge on the eye of the frog, after it has arrived at a tolerably stable condition, the natural E.M.F. is in the first place increased, then diminished ; during the continuance of light it is still slowly diminished to a poimt where it remains tolerably constant, and on the removal of light there is a sudden increase of the E.M. power nearly ap to its original position.’ ! (3) I have sometimes obtained the following curious result. On the incidence of light there is a response, say, upward. On the continuation of ight the response declines to zero and remains at the zero position, there being no further action during the continuation of stimulus. But on the cessation or ‘ break’ of light stimulus, there is a response downwards, followed by the usual recovery. This reminds us of a somewhat similar responsive action produced by constant electric current on the muscle. At the moment of ‘make’ there is a responsive twitch, but afterwards the muscle remains quiescent during the passage of the current, but on breaking the current there is seen a second responsive twitch.

Résumé.—So we see that the response of the sensitive inorganic cell, to the stimulus of light, is in every way similar to that of the retina. In both we have, under normal conditions, a positive variation; in both the intensity of response up to a certain limit increases with the duration of illumination ; it is affected, in both alike, by temperature; in both there is comparatively little fatigue ; the increase of response with intensity of stimulus is similar in both; and finally, even in abnormalities—such as reversal of response, preliminary negative twitch on commencement, and terminal positive twitch on cessation of illumination, and decline and reversal under continued action of light—parallel effects are noticed.

We may notice here certain curious relations even in these abnormal responses (fig. 109). If the equilibrium position remain always constant, then it is easy to understand how, when the rising curve has attained its maximum, on the cessation of light, recovery should proceed downwards, towards the equilibrium position (fig. 109, a). One can also understand how, after reversal by the continued action of light, there should be a recovery upwards towards the old equilibrium position (fig. 109, 0). What is curious is that in certain cases we get, on the stoppage of light, a preliminary twitch away from the zero or equilibrium position, upwards as in (c) (compare also fig. 107) and downwards as in (d) (compare also fig. 108 0).

produced by stimulus of light, we find that there is not a single phenomenon in the responses, normal or abnormal, exhibited by the retina which has not its counterpart in the sensitive cell constructed of inorganic material. Effect of light of short duration—A fter-oscillation—Positive and negative after-images—Binocular alternation of vision—Period of alternation modified by physical condition—After-images and their reyival—Un- conscious visual impression.

WE have already referred to the electrical theory of the visual impulse. We have seen how a flash of light causes a transitory electric impulse not only in the retina, but also in its morganic substitute. Light thus produces not only a visual but also an electrical impulse, and it is not improbable that the two may be identical. Again, varying intensities of light give rise to corresponding intensities of current, and the curves which represent the relation between the increasing stimulus and the increasing response have a general agreement with the corresponding curve of visual sensation. In the present chapter we shall see how this electrical theory not only explains in a simple manner ordinary visual phenomena, but is also deeply suggestive with regard to others which are very obscure. =

We have seen in our silver cell that if the molecular conditions of the anterior and posterior surfaces were exactly similar, there would be no current. In practice, however, this is seldom the case. There is, generally speaking, a slight difference, and a feeble current in the circuit. It is thus seen that there may be an existing feeble current, to which the effect of light is. added algebraically. The stimulus of light may thus increase the existing current of darkness (positive variation). On the cessation of light again, the current of response disappears and there remains only the feeble original current.

In the case of the retina, also, it is curious to note that on closing the eye the sensation is not one of absolute darkness, but there is a general feeble sensation of light, known as ‘the intrinsic light of the retina.’ The effect produced by external light is superposed on this intrinsic light, and certain curious results of this algebraical summation will be noticed later. Effect of light of short duration.—If we subject the sensitive cell to a flash of radiation, the effect is not instantaneous but grows with time. It attains a maximum some little time after the incidence of light, and the effect then gradually passes away. Again, as we have seen previously with regard to mechanical strain, the after-effect persists for a slightly longer time when the stimulus is stronger. The same is true of the after-effect of the stimulus of light. Two curves which exhibit this are given below (fig. 110). With regard to the first point—that the maximum effect is attained some time after the cessation of a short exposure—the corresponding experiment on the eye may be made as follows: at the end of a tube is fixed a glass disc coated with lampblack, on which, by scratching with a pin, some words are written in transparent characters.

The length of the tube is so adjusted that the disc is at the distance of most distinct vision from the end of the tube applied to the eye. The blackened disc is turned towards a source of strong light, and a short exposure is given by the release of a photographic shutter interposed between the disc and the eye. On closing the eye, immediately after a short exposure, it will at first be found that there is hardly any well-defined visual sensation; after a short time, however, the writing on

Showing greater persistence of after-effect when the stimulus is strong. (a) Short exposure of 2” to light of intensity 1; (b) short exposure of 2” to light nine times as strong. the blackened disc begins to appear in luminous characters, attains a maximum intensity, and then fades away. In this case the stimulus is of short duration, the light being cut off before the maximum effect is attained. The after-effect here is positive, there beimg no reversal or interval of darkness between the direct image and the after-image, the one being merely the continuation of the other. But we shall see, if light is cut off after a maximum effect is attained by long

exposure, that the immediate after-image would be negative (see below). The relative persistence of after-eflect of lights of different intensities may be shown in the following manner : If a bold design be traced with magnesium powder ona blackened board and fired in a dark room, the observer not being acquainted with the design, the instantaneous flash of light, besides being too quick for detailed observation, is obscured by the accompanying smoke. But if the eyes be closed immediately after the flash, the feebler obscuring sensation of smoke will first disappear, and will leave clear the more persistent aftersensation of the design, which can then be read distinctly. In this manner I have often been able to see distinctly, on closing the eyes, extremely brief phenomena of light which could not otherwise have been observed, owing either to their excessive rapidity or to their dazzling character.1

After-oscillation—In the case of the sensitive silver cell, we have seen (fig. 105), when it has been subjected for some time to strong light, that the current of response attains a maximum, and that on the stoppage of the stimulus there is an immediate rebound towards recovery. In this rebound there may be an over-shooting of the equilibrium position, and an after-oscillation is thus produced. 1 As an instance of this I may mention the experiment which I saw on the quick fusion of metals exhibited at the Royal Institution by Sir William Roberts-Austen (1901), where, owing to the glare and the dense fumes, it was impossible to see what happened in the crucible. But I was able to see every detail on closing the eyes. The effects of the smoke, being of less luminescence, cleared away first, and left the after-image of the molten metal growing clearer on the retina.

If there has been a feeble initial current, this oscillatory after-current, by algebraical summation, will cause the current in the circuit to be alternately weaker and stronger than the initial current. Visual recurrence.— Translated into the visual circuit, this would mean an alternating series of after-images, On the cessation of light of strong intensity and long duration, the immediate effect would be a negative rebound, unlike the positive after-effect which followed on a short exposure.

The next rebound is positive, giving rise to a sensation of brightness. This will go on in a recurrent series. If we look for some time at a very bright object, preferably with one eye, on closing the eye there is an immediate dark sensation followed by a sensation of light. These go on alternating and give rise to the phenomena of recurrent vision. With the eyes closed, the positive or luminous phases are the more prominent. This phenomenon may be observed in a somewhat different manner. After staring at a bright heht we may look towards a well-lighted wall. The dark phases will now become the more noticeable.

If, however, we look towards a dimly leghted wall, both the dark and bright phases will be noticed alternately. The negative effect is usually explained as due to fatigue. That position of the retina affected by hght is supposed to be ‘tired,’ and a negative image to be formed in consequence of exhaustion. By this exhaustion ismeant either the presence of fatigue-stuffs, or the breaking-down of the sensitive element of the tissue, or both of these. In such a case we should expect that this fatigue, with its consequent negative image, would gradually and finally disappear on the restoration of the retina to its normal condition.