Jap. Psychol. Research No. 7, 1959

STUDIES ON THE INDUCTION IN VISUAL PROCESS TAKING ELECTRICAL AS AN INDEX (5) EXPERIMENTS ON THE PROPAGATION OF THE INDUCTION ACROSS THE BLIND SPOT (2)*

TAROW INDOW, TAKAAKI KOYATSU AND TOSHIRO YOSHIDA

Keio University

By means of measuring the enhance- fraction and the velocity of propagation ment of the electrical excitability of the etc. (7, 15, 16, 19). when it is pre-illuminated by In the experiment concerning retinal stimulating patterns, Motokawa has induction, silver electrodes are in con- discovered a number of phenomena tact with the brow and cheek around which are now well known as the the eye and the eye is electrically stim- retinal induction (2, 11, 12, 18, 21). ulated by a single constant current The induction is divided into two types, pulse of 0.1 sec. in duration. The direct and indirect, and it is the latter stimulation gives rise to " phosphene " which especially attracts our interest which is usually described by the sub- because it propagates over the ject as vaguely defined clouds of flash in from one part to another, according to the peripheral field of view. The re- Motokawa, in the form of wave exactly ciprocal of the electrical threshold for in the same way as light does in space. the phosphene is defined to represent In fact, marvellous demonstrations were the excitability of the eye. The ex- made that in the propagation of in- citability of the dark-adapted eye is duction the phenomena of absorption, increased from Eo to E if it is exposed, reflection, refraction and even the prior to the delivery of the electrical Doppler effect do occur in the retina pulse, to a white luminous patch such or somewhere else (15, 16). Besides, as W in Fig. 1 (the temporal sequence Motokawa in his ingenious experiments succeeded in measuring the index of re-

* This research was undertaken, as one in the seriesof investigations,by the group of the Departmentof Psychologyin Keio University who are workingin the field of the retinal in- duction. The group consists,at present,of the authors, also S. Ohinata, T. Kozaki and U. Nagumo, nee Kuno. The authors wish to express their warmest thanks to Prof. Moto- kawa of Tohoku Universityfor his continuing interest throughout the seriesof investigations. Thanksare also to Dr. M. Fish whokindly read through the manuscript and gave helpful Fig. 1. criticisms and advices concerning English. This study was financed by the Ministry of b). This enhancement in excitability Education. will he given hereafter by the index 18 T. INDOW, T. KOYATSU AND T. YOSHIDA

ƒÌ B=(E-E0)(E0•~100. A further en- B Inducer and the propagation of the hancement is observed, however, if a induction from B to W is proved by luminous figure, e.g., the yellow one Y is absence of the induction of Y, i.e.,

presented and then NV is given in its CE=0. vicinity (Fig. 1), and the enhancement Among a number of important dis- observed under this condition (the coveries made by Motokawa taking temporal sequence a) is somewhat occurrence of the neutralization as an larger than :ƒÌB. For simplicity, this index, we find the fact that the induc- will he called hereafter the indirect in- tion does not traverse the blind spot duction of Y at W. The amount of (17). This fact was easily confirmed in induction is given by (ƒÌA-ƒÌB) which our laboratory. Under the conditions Motokawa named the contrast effect that the blind spot intervenes between

(CE). In general, is a function of the retinal images of Y, W and that of the interval of time between the cessa- B in Fig. 1, the amount of CE was tion of W and the delivery of the elec- shown to remain the same with the one trical pulse but the interval was fixed obtained when B was absent (the tem- at 1.5 see. all through this study he- poral sequence a). That means, the cause only the indirect induction of Y induction initiating from B does not to AV was at issue. It is well known reach W "across the blind spot "(4).

that CE is maximum at 1.5 sec. interval It is a remarkable fact that if a stim- in this case. ulus pattern is given in such a way that If the blue luminous figure B (Fig. 1) a part of it falls in the blind spot while is presented in between the presenta- the remaining part stretches well into tion of Y and that of W (the temporal the sensitive region of the retina, then sequence c), however, it is observed it gives rise to perception of the com- that CE vanishes unless the interval of plete pattern without any gap. In time between B and W is too short. other words, the part falling into the This phenomenon is called " neutrali- blind spot is, so to speak, compensated zation " as it is believed that the indirect in perception in accordance with the induction once caused by Y is wiped out whole pattern. For brevity, this fact

by the indirect induction propagated will he called hereafter the phenomenon from B. The two inductions are, of of compensation or perception " across course, complementary in nature. The the blind spot ". In the previous ex- indirect induction of a figure per se can periments we showed that once a white not he detected unless the W is present- figure B1 presented across the blind

ed in its vicinity for the induction

decreases fairly rapidly as the distance

between \V and the figure increases

(10, 13). On the other hand, to test whether neutralization takes place or

not is an extremely convenient device

for detecting the arrival at a point of

the induction that is to start from a

figure at a certain distance because a surprisingly small amount of the induc- Fig. 2. tion propagated is sufficient to give rise

to the neutralization (17). In this spot was perceived as a whole without

sense, in Fig. 1, Y is called Delector and any gap (Fig. 2), the induction initiated Studies on the Induction in Visual Process 19 from B reached W (4). It was also corresponding to the blind spot in the ascertained that this phenomenon same way as in any other retinal region. could not be ascribed to the scattered light from B. Hence, once a kind of APPARATUS AND GENERAL continuity is perceived, so to say, to PROCEDURE "bridge the blind spot" , the blind The apparatus for delivering a single spot seems to lose its blocking effect constant pulse of 0.1 sec. in duration to the upon the propagation of induction and right eye of the subject is essentially the it becomes a traversable medium for same as that described, e.g., in (3). The induction. subject (S 1 was seated in a small dark room. The purpose of the present experi- The distance was 56 cm. from the right eye ments was to investigate the following to the transparent screen through which possibilities. When the blind spot be- the rays were delivered as stimuli, i.e., come functionally equivalent to any Detector, inducer and the white patch etc. other region of the retina in the sense In the present experiments the indirect that the compensation takes place and induction caused by yellow and its neutrali- the blind spot becomes traversable for zation by blue were exclusively dealt with the induction, then the phenomenon since this is the pair of which the neutraliza- of reflection, refraction and also ab- tion can be demonstrated even when the sorption would be demonstrated with wave lengths are not exactly matched (22, the beam of induction even in the region 23). The tolerance for being comple-

Table 1 An example of protocol in determining the threshold (S:In).

The intensity of the electrical pulse is given in terms of the resistance (!?) employed 20 T. INDOW, T. KOYATSU AND T. YOSHIDA mentary was so wide in this pair that the etc., and the average of the intensities transparent light through a sheet of yellow corresponding to the levels underlined was cellophane and that through a sheet of defined to be the threshold. blue cellophane could be used without any At the beginning of the series the intensity trouble. of the pulse was decreased by relatively

Since the method for obtaining the elec- large steps but the steps were gradually trical threshold for the phosphene is now in decreased in size as the series proceeded. dispute (8, 25), it might be well to describe We have made no rigid rule in the way of the procedure we employed in detail. grading the intensity of the pulse and it Some examples are given in Table 1. It was varied somewhat from occasion to was a descending series in the method of occasion. This procedure was rather ad- limits in a modified form. The series was vantageous as it was thus impossible for S begun with presentation of an electrical to anticipate the intensity of the pulse to be pulse which produced a phosphene clearly delivered by counting the number of steps recognizable. Then the intensity of the from the first presentation of the series. If pulse was decreased step by step and S was the induction is present, the series necessarily required to make absolute judgment con- has to be terminated at an intensity of con- cerning the phosphene on each presentation. siderably lower than when it is absent.

The sign ++ denotes the case when the As to the way of grading the intensity, phosphene is quite distinct, + the case therefore, making some adjustment from when S is positive on occurrence of the one case to another seems to be indispensa- phosphene and ? the case when recognition ble. It is absolutely necessary, however, to of the phosphene becomes slightly doubtful. take great caution against the possibility

Once the response ? appeared, S was re- that the subject loses sight of the faint phos- quired to make distinction between delivery phene before the true threshold is reached and non-delivery of the pulse in successive since waxing and waning occur instead of presentations. As a matter of course, the a steady decrease as the intensity of the order of the presentation in a pair was pulse is decreased (1, 11, 14, 20). Hence, rigorously randomized. In these paired the series should not be terminated unless it comparisons, the response of the subject becomes entirely clear that no more phos- were divided into the following categories; phene can be aroused by a pulse of lower o (correct response with confidence), •~ intensity and this is especially true where

(false response though S is confident in his occurrence or non-occurrence of the neu- judgment), s (correct response though S is tralization is the point at issue. Suppose not confident), f (S is not confident and that the tide of vividness in the phosphene response is false) and n (S perceives no fails to rise half way of the series when in- phosphene in either presentation and can- duction is present. This might be mis- not make any distinction at all). The interpreted as occurrence of neutralization. intensity level of the pulse being fixed, the We have made, therefore, the following rule. case was designated S when o occurred in On any occasion, no matter whether in- the first comparison or s's occurred in the duction or neutralization was expected, the first two successive comparisons or s's and 7 intensity of the pulse was decreased by the or o's appeared in three out of four com- steps appropriate for determining the parisons. When none of these criteria was threshold under induction. Induction be- met, the sign F was given to the case. The ing present, the threshold would thus be descending series was terminated when one determined with accuracy of desired degree. of the following patterns occurred ; S-F-F- The accuracy depends in part upon the size

F. S-F-S-F-F, S-F-S-S-F-F and S-F-F-.S-F-F, of the steps at the end of the series. When Studies on the Induction in Visual Process 21 neutralization was taking place, on the other values coincided very closely and the hand, the case labelled S would cease to ap- average was taken as the denominator of pear at an earlier stage of the series. Should and ƒÌA, the index of the enhancement ƒÌBin this occur, the steps of that stage would the excitability caused by pre-illumination be too coarse to determine the threshold of W alone and the index of the enhance- accurately, but we would be assured that ment by W preceded by Y and B etc. there would be no recurrence of the phosphene at lower intensities. Then, RESULTS in order to determine the threshold with It might be well to enumerate here, accuracy, the series was started again with from the discoveries made by Moto- appropriate steps for the threshold under kawa concerning induction, some of the neutralization. In this way, when neutrali- laws which were made use of in carry- zation was to be tested, two kinds of series ing out the present experiments. were always in use in pairs for one measure- 1. The interval of time between ment, one only for probing possible re- cessation of Detector and presentation currence of the phosphene at lower in- of W can be prolonged up to 20 sec. tensities of the pulse and another for deter- without appreciable change in the mining the threshold with accuracy. The amount of CE (12). threshold was determined by the latter 2. A surprisingly small amount of only. If possible, the order of these two the induction propagated from Inducer series was randomized and it should be is sufficient to wipe out the induction emphasized again that there was no way of caused by Detector and the pheno- knowing for S at what level of intensity the menon of neutralization follows prac- pulse to be delivered might stand. Or- tically the all-or-none law (17). dinarily measurements were repeated twice 3. Provided that the induction for each S and four times at the critical caused by Inducer is not too weak, the points. Values obtained are given in the neutralization takes place irrespective table below the figure in which the ave- of the interval of time between Detector rage is plotted. and Inducer (17). Four Ss, In, Y, Kz and Ko, participated 4. The threshold of saturation for in the present experiments. These were causing induction is so low that any the subjects with whom the phenomena of colored figure discriminable from white induction and neutralization could be de- can cause the induction. The occur- monstrated definitely from the beginning. rence or non-occurrence of neutraliza- They had also served as Ss in the other ex- tion again follows the all-or-none law periments of this kind (3, 4, 5, 6). One of (17, 24). the authors, Ko, took part of experimenter 5. In general, the presentation of all through the experiments*. a white figure does not cause its own In each session, S was dark adapted for induction and also it has no effect upon a period of at least 20 min. before the the induction caused by Detector and electrical excitability of the eye (E0) was by Inducer (9, 12). determined as the first step of the experi- 6. The retinal region once illu- mentation of the session. Eo was measured minated by a white light reflects induc- again at the end of the session. These two tion of any kind. Hence, pre-illuminat- ing the retina, prior to presentation of

Inducer by an appropriate pattern con- * When Ko participated as S, U. Nagumo sisting of white contour serves as the took trouble to be experimenter. The authors are grateful to her collaboration. collimator and a beam of induction is 92 T. INDOW, T. KOYATSU AND T. YOSHIDA easily obtained. 7. As far as the yellow Detector and the blue Inducer are concerned, the interval from cessation of W to delivery of the electrical pulse should be kept at 1.5 sec. as mentioned before. By virtue of these laws it has become possible to undertake the experiments described below.

Experiment I According to Motokawa, the beam of induction initiating from the blue Inducer B refracts by the refractive in- dex of 1.95 when it passes through the region once illuminated by the prism- Fig. 4. shaped yellow figure P,* and this finding was verified in our experiment the blind spot. reported in (6). Then Experiment I The stimulus pattern shown in Fig. 3 was designed to test the hypothesis that, was used in the temporal sequence a in should the blind spot become func- Fig. 4.** The figures Y, B, Bl, P and tionally equivalent to any other region Ws are arranged in such a way that, if of the retina in the sense that a con- the induction from B is refracted by the tinuous figure is perceived across it, not region corresponding to P with the in- only would the blind spot become trav- dex of 1.95, the beam should reach ersable for induction but also the point 2 and neutralize the induction of refraction of induction would be ob- Y at this point (CE=0). The center served in the region corresponding to of P fell into the blind spot while both ends stretched well into the sensitive region of the retina. Under these conditions, the complete prism-shaped figure was perceived though occasionally the part of its contour corresponding to the blind spot was seen slightly blurred. The lower sides of the white figure Bl1 and Bl2 were also perceived as con- tinuous. As shown in the previous report, perception of a pipe-shaped path as shown in Fig. 2 is not necessary but that of a continuous line is sufficient Fig. 3. to " bridge the blind spot " (4). It

* Motokawa showed that the refractory index might be worth mentioning that the is independent of the luminosities of P and B. superiority of P over B1 in the com- * * It became clear in the preliminary ex- pensation was so marked that no rivalry periment in accordance with theoretical calcula- was observed. After P and Bl were tion that 2 sec. interval between the cessation of removed and especially at the moment B and the presentation of W was enough for when B was presented, the negative the induction to propagate from B to AV in this stimulus pattern. after-image of P and the positive after- Studies on the Induction in Visual Process 23 image of B1 appeared occasionally. periment was repeated with the stim- The after-image of P was dark blue so ulating pattern in which the white line that its contour was not distinct at all. at the left side of the blind spot (Bl2) However, it is to be noted that no gap was completely concealed. Namely, existed in the portion of the after-image in this case, B1 did not " bridge the that corresponds to the blind spot. A blind spot " and it served only as a gun part of B1 (the chamber of white walls) for sending a sufficiently narrow beam and the whole of B were seen in the of induction. In order to change the periphery. Nevertheless, B was still blind spot to a traversable medium for identifiable as blue though it was con- induction, so far, including the experi- siderably desaturated. It appeared in ments in the previous papers, such a the midst of the after-image of the pattern has been always in use that a chamber but the intervening black continuous line is perceived across the space between B and the walls was not blind spot along which the induction is distinct. to propagate. Hence, the purpose of Two subjects, In and Y, participated this experiment was to test the follow- in this experiment. As shown in Fig. ing hypothesis. To bridge the blind 4, occurrence of the refraction was con- spot in the direction of its propagation vincing when P was present (o in Fig. might not be a necessary condition for 4) while the induction proceeded the induction to traverse the blind spot. straight to the point 1 when P was In other words, the occurrence of the absent ( •~ in Fig. 4). It became also compensation of any kind, e.g., percep- clear that passing through the pipe- tion of the complete prism-shaped shaped path on the right side of the figure P in this case, might he sufficient blind spot (Bl1), the beam of induction to make the blind spot functionally equi- was made narrow enough not to affect valent to other regions of the retina. the neighboring points. Three Ss, In, Y and Kz, participated Encouraged by this finding, the ex- in this experiment and the results are

Fig. 5. 24 T. INDOW, T. KOYATSU AND T. YOSHIDA shown in Fig. 5 by circles. This was reflection of induction would also be Kz's first experience as S in experiments observed in the region corresponding involving the phenomena of refraction to the blind spot. In Fig. 6, R denotes and of propagation across the blind a white bar which was seen without any spot. The results clearly showed that gap across the blind spot. It can be the induction not only traversed the expected from the results of Experi- blind spot but also refracted in it even ment I that, if the compensation is when there was no bridge across the present with R, the blind spot is traver- blind spot in the course of the pro- sable for induction. Should the region pagation. It was also demonstrated corresponding to R reflect the beam that concealing the center of P did not of induction from B, the neutralization alter the results if the gap was not per- would be obtained at point 2. On the ceived as the result of the compensa- contrary, if the beam passes through the tion. region it will reach point 4.* By replacing P by a thin yellow bar Two Ss, In and Ko, participated in standing vertically (0.2 •~ 13 cm.), it the experiment and the results are was ascertained that the neutralization shown in Fig. 7. There seems to be no could be demonstrated at point 1 (. doubt that the reflection takes place in in Fig. 5). The induction proceeds straight under this condition. When no figure, neither the prism nor the bar, was presented and hence there was no compensation, the induction was blocked by the blind spot and the neutralization was not obtained at point 1 (•~ in Fig. 5).

Experiment II

The experiment was undertaken with the pre-illuminating pattern shown in

Fig. 6 to test the hypothesis that the Fig. 7. the region corresponding to the blind spot in the same way as it does in any other region of the retina. Although the part of the contour at which the beam hits the bar was occasionally seen slightly blurred in comparison with the other part of the contour falling in the sensitive region of the retina, the reflec- tion was thus clearly demonstrated.

* In this argument , it is assumed that, passing through the region corresponding to R, the beam does not change its course by refraction. The refractory index of the beam in a retinal region once illuminated by a white light is not known since it is reflection or even total reflec- Fig. 6. tion that occurs in this case (15, 16). Studies on the Induction in Visual Process 25

Experiment III When we discovered in Experiment I that the induction was refracted by the region once illuminated by the prism- shaped figure P, we were, in truth, astonished not only by the phenomenon of refraction itself but also by the fact that the beam of induction could pass through a region of such width. In order to discriminate clearly the re- fracted beam from that not refracted, P has to be of the size indicated in Fig. 3 and the width of the part which is to transmit the beam has to be 0.6 cm. The induction which reached the point W was, however, strong enough to cause the neutralization. This fact Fig. 9. made us wonder whether the pheno- the path of the propagation of induction. mena of absorption might fail to take The width of A was varied by con- place in the region corresponding to the cealing its left side (the side toward Y blind spot or whether the index of and W) in an appropriate way. Be- absorption might at least be smaller than that in the sensitive region of the cause A was dominant in size, the white bar B1 was introduced to prevent the retina. Experiment III was designed indirect induction, if any, of A towards to test this hypothesis. The stimulus W. Two Ss, In and Y, participated pattern shown in Fig. 8 was used in the in this experiment and the results are

shown in Fig. 9, where o indicates

CEs when the blind spot was involved

and •~ CEs when the blind spot was

not involved. The threshold of width

for the traversal of a region once il-

luminated by A was thus determined

with accuracy and it became clear

that the absorption occurred in the

blind spot exactly in the same way as

in any other region of the retina. All

conditions except the shape of the

transmitting figure (e.g., luminosity of

the yellow, that of B, and the distance Fig. 8. from B to W, etc.) were exactly the

temporal sequence indicated in Fig. 9. same as those in Fig. 3. Hence it was When the fixation mark I was in use, no wonder that the beam could pass the part of the yellow absorbing figure through the region corresponding to P A that intercepts the beam fell in the in Experiment I. blind spot, but when the mark 2 was It is to be kept in mind that in ob- fixated, the blind spot did not lie on serving A it is impossible for S to tell 26 T. INDOW, T. KOVATSU AND T. YOSHIDA whether it is above or below the thres- cess underlies the phenomenon and at hold in its width and he has no way of what level of the nervous system the intensionally adjusting his responses so process takes place. In the same way, as to yield the regularities obtained in the facts discovered by Motokawa that Fig. 9. As a matter of course, different the induction propagates, reflects, re- widths were repeated in randomized fracts and is absorbed are undeniable order. though Motokawa provides no hypo- thesis about physiological processes un- DISCUSSION derlying these phenomena. As a mat- The propagation of induction cannot ter of course, we are not in a position traverse the blind spot under the or- to speculate concerning the physiologic- dinary conditions in the dark room, al processes in either case. It may be i.e., when the blind spot is not involved said, however, from the results of the in any way with perception of luminous present experiments that whatever the figures. However, if a continuous figure physiological processes may be, they is perceived across the blind spot, ir- occur at the same level and interact respective of the direction of the figure, with each other. not only does the blind spot lose its It is the retinal region once illuminat- blocking effect upon the propagation ed by a light, and not the excitation in of induction but it also becomes func- the retina under the illumination, that tionally equivalent to other regions of reflects, refracts and absorbs the beam the retina in the sense that the pheno- of induction. For this reason, presenta- mena of refraction, reflection and ab- tion of the white bar or of the prism- sorption are demonstrated in the blind shaped figure or of the absorbing figure spot as well as in other retinal regions. always precedes that of Inducer in the Besides it became clear that the re- experiment concerning induction. It fractory index and the index of absorp- is to be kept in mind, therefore, that tion are respectively the same in both. what was discovered in the present Coincidence in both cases of the titres- experiment is as follows if stated with hold for traversal of induction was de- exactness. Once the blind spot is termined with accuracy of up to 1 mm. involved in perception of a figure in the (Experiment III). However, as it was sense that the compensation takes not of primary concern in Experiment I place, it is in the state of being func- to make precise determination of the tionally equivalent to any other region refractory index, we are not in a posi- of the retina at the moment when the tion to state the accuracy with which induction arrives, though the stimula- the index in the blind spot is compared tion responsible for the compensation with that in other regions of the retina. has already ceased to exist. In the It is still true, however, that in the report of Experiment I we include a blind spot the beam of induction follow- description of the after-image of the ed the course expected from the re- figure perceived across the blind spot. fractory index as determined by Moto- It is our impression, however, that kawa in the sensitive region of the presence or absence of the after-image retina. is rather irrelevant to the matter under The fact that a continuous figure can discussion. be perceived across the blind spot is SUMMARY1 quite conspicuous though nobody knows exactly what physiological pro- . The procedure for determining Studies on the Induction in Visual Process 27 the electrical excitability was discussed electric excitation of the . and it was mentioned that great caution Psychol. Bull., 1953, 50, 73-111. was being taken in our procedure 3. INnow, T., KUNO, U., YOSHIDA,T., & KOZAKI, K. Studies on the induction against the possibility of terminating in visual process taking the electrical the descending series before the true phosphene as an index (1)-Experi- threshold is reached. ments under the conditions involving 2. Concerning the enhancement of shift of fixation (1) yap. J. Psychol., 1958, 29, 29-40. (In Japanese) the excitability when the eye is pre- 4. INnow, T., KUNO, U., & YOSHIDA,T. illuminated by stimulation patterns, the Studies on the induction in visual phenomena discovered by Motokawa, process taking the electrical phosphene i.e., the reflection, the refraction and as an index (2)-Experiments on the propagation of the induction across the the absorption of the beam of induction blind spot. (1) Psychologia, 1958, 1, were shown to occur also in the region 175-181. corresponding to the blind spot under 5. INnow, T., KUNO, U., YOSHIDA,T., & the condition that the reflecting bar or KOZAKI, K. Studies on the induction in visual process taking electrical the prism-shaped figure or the ab- phosphene as an index (3)—Experi- sorbing figure is perceived across the ments under the conditions involving blind spot as a result of the compensa- shift of fixation (2). yap. J. Psychol., tion. 1958, 29, 295-302. (In Japanese) 6. INnow, T., Kusso, U., YOSHIDA,T., & 3. In order to make the blind spot KOZAKt, K. Studies on the induction which blocks propagation of the in- in visual process taking electrical duction under ordinary conditions into phosphene as an index (4)-Experi- a traversable medium for induction, it ments under the conditions involving shift of fixation (3) (to appear in Jap. was discovered sufficient that percep- J. Psychol.). (In Japanese) tion of a continuous figure of any direc- 7. KATAVAMA,S., & ALZAWA,T. The me- tion, not necessarily in the direction chanism of spatial induction in the of the propagation, takes place across retina. Tohoku ,J. exp. Med., 1956, 64, 179-189. the blind spot. 8. KoxATA, T., KOMATSU,M., & Motokawa, 4. It was quantitatively ascertained K. After-effects of electrical test that the refractory index and the index shocks as a factor affecting retinal of absorption remain respectively the color effects. yap. 5. Physiol., 1956, 6, 236-248. same when these are determined in the 9. KUROSAWA,T., & KATAYAMA,S. In- region corresponding to the blind spot teraction of successive photic stimuli as in other regions of the retina. in the human retina. Tohoku 5. exp. 5. It was argued that whatever Med., 1955, 63, 9-17. 10. MICHAELS, R. H. The electrical phos- physiological processes may underlie phene threshold as a measure of retinal the fact that the beam of the induction induction and visual organization.J shows the phenomena of refraction, re- . exp. Psychol., 1957, 54, 21-27. flection and absorption, these would he 11. MOTOKAWA,K. Retinal processes and their role in color vision. 5. 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