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Studies on the Physiological Color Blindness of the Human Fovea with the Polarization Method

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Studies on the Physiological Color Blindness of the Human Fovea with the Polarization Method STUDIES ON THE PHYSIOLOGICAL COLOR BLINDNESS OF THE HUMAN FOVEA WITH THE POLARIZATION METHOD KOITI MOTOKAWA, MITSURU EBE, YASUHIRO ARAKAWA AND TOSHIHIKO OIKAWA * Department of Physiology, Tohoku University,Sendai Using a small test field viewed centrally, Konig (1) could match any radi- ation by a suitable mixture of two radiations such as 650mμ and 475mμ, and ar- rived at the conclusion that the center of the fovea was blue-blind. Hartridge (2) observed the confusion of blue with dark grey or dark, and yellow with pale grey or white, when the visual angle of a test object was reduced. Willmer (3) studied the same phenomenon, experimenting with small colored patches painted on cards and using suitable fixation spots and viewing technique, and came to a conclusion similar to Konig's namely that the central fovea was tritanopic. The phenomenon was studied more extensively and quantitatively by Wright (4, 5) in conjunction with Willmer with modern color matching devices. Ebe, Isobe and Motokawa (6) analyzed the retinal processes of color-blind subjects by means of Motokawa's method (7) and found that the data thus ob- tained could not be accounted for on the basis of the three-color theory. In the present experiment, we analyzed, by means of the same technique, the color processes at the fovea of the retina evoked by microillumination, in order to elucidate the mechanism of the physiological color blindness under such experimental conditions. METHOD The eye shows a supernormal sensitivity to an electric stimulus after ex- posure to light. This property was utilized by Motokawa (7, 8) for analyzing retinal processes of color vision. In the present experiment, measurements were carried out with a test patch of reduced visual angle (2minutes of arc), instead of a greater one of 2° used in the experiments by Motokawa. The apparatus for microstimulation was a reducing lens system similar to that used by Hartridge (9, 10). As illustrated schematically in fig. 1, a real image of a small patch of ground glass illuminated from behind, was focused at R by means of a microscope, M, and viewed by the subject from a distance of 90cm. The microscope was placed in such a manner that the ocular (Oc) faced toward the patch. The size of the real image was measured accurately in order to know the reduction rate. A minute light spot was placed at a distance of 30minutes in visual angle from the image, R, so as to serve as a Received for publication June 26, 1951. *本 川 弘 一 ,江 部 充,荒 川 安廣,及 川 俊 彦 50 PHYSIOLOGICAL COLOR BLINDNESS OF FOVEA 51 fixation mark. When it was necessary to stimulate the center of the fovea, two light spots were presented on both sides of the center of the fovea so as to make the subject gaze at the point half-way between the two fixation marks. Fig. 1. Apparatus used for micro- stimulation. F fixation mark, M micro- scope, Ob objective, Oc ocular, P patch of ground glass, R real image of P, S spectroscope, Sc screen. The intensity of the equal energy spectrum used in our experiment was such that only the middle range from about 640mμ to 590mμ was visible; the other parts of the spectrum evoked no light sensation. Never the less, the responses to these subthreshold lights could be determined by our method with the same degree of accuracy as those to the superthreshold lights. The details of the apparatus and the procedure for electrical stimulation of the eye are described in a paper by Motokawa (7). After a preliminary dark adaptation of about 20minutes, the electrical excitability of the eye was measured by applying single constant current pulses of O.1sec. in duration to the eye, the index of excitation being the least perceptible electric phosphene. Stimulating voltages were reduced step by step until the subject could no more distinguish an electric phosphene from the background of intrinsic light of the retina. For raising accuracy of such determinations, it was found absolutely necessary to compare the effect of a weak electric stimulus and that of a con- trol stimulus far below the threshold. Similar measurements were done at vary- ing moments after 2sec. exposure of the eye to spectral light, in order to de- termine the time course of supernormal excitability following the light stimulus. Excitability increases were expressed in percentage of the excitability at the resting level. It is to be noted that in such measurements, more than one threshold values could be obtained for a certain phase of supernormality. This is due to con- current excitation of several receptors with different thresholds, as was eluci- dated by Motokawa (11). In such cases, we adopted the lowest threshold value for construction of an excitability curve which represents the time course of electrical excitability after an illumination. RESULTS 1. Excitability curves obtained from the fovea by means of microstimulation Retinal responses to microstimulation are expressed in fig. 2 in form of ex- citability curves, in which excitability increases are plotted as ordinates against 52 K. MOTOKAWA ET AL. time after termination of the light stimulus as abscissas. The curves generally consist of two elevations, one of which has a maximum at about 1 second and the other at about 2.5 or 3 seconds. These humps are, however, not so con- spicuous in the curve for white and yellow pre-illuminating lights as in the other curves. According to the analysis by Motokawa (7, 8), the excitabilitycurves for red, yellow, green and blue receptors of normal trichromats show a maximum at 1, 1.5, 2.25 and 2.75 or 3 seconds respectively. If the result of Motokawa's Fig. 2. Excitability curves obtained at 20minutes from foveal center. Ordinates: percentage increases of electrical excitability above resting level. Abscissas: time in sec. after end of pre- illumination. Figure by each curve indi- cates wave-length of light used for pre- illumination. Zero-level of each curve is given on left side. analysisis applicableto the data obtained with smaller test fields,then from the excitabilitycurves shown in fig.2, it can be said how the test patch ap- peared to the subject in this experimemt. Since the firstmaximum of these curves liesat 1 second, it is certain that the red receptorwas excited by micro- stimuiation. The second maximum located at about 2.5sec. can be considered as representingan envelope of green and blue processes which have a maximum at 2.25sec. and at 2.75 or 3sec. respectively.Hence, this hump must subserve a sensation of blue-green,and according to whether the firstor the second hump is predominant, the test patch must have appeared red or blue-green. As a matter of fact,Hartridge (9, 10) reported that a small testobject appeared blue-green if it was green, blue-green and indigo, and that red, orange and crimson appeared always red. In case the two humps are obscure as in the curves for white and yellow, sensations must have been neutral,because the two systems, red and blue- PHYSIOLOGICAL COLOR BLINDNESS OF FOVEA 53 green, which are complementary to each other, are considered to have been excited to the same extent in such cases. In fact, Hartridge reported that yellow appeared colorless and that white looked always white. These obser- vations were confirmed by the subjects in our experiments. In the excitability curves for the lights from the violet end of the spectrum, the maximum of the second hump lies at 3 seconds, instead of 2.5 seconds (see the curve for 440mμ), and this finding suggests that the light from this part of the spectrum would appear blue or violet, instead of blue-green. However, the excitability curve for blue or violet light is generally so low that the sen- sation caused by it must be very slight; we have confirmed in a series of ex- periments that the subject could not perceive the test patch unless the intensity of pre-illumination was such that the maximum height of the excitability curve obtained under these conditions surpassed a certain critical level. In most cases, the critical level was found about 7 in terms of percentage increase of electrical excitability. As Hartridge observed, a blue test object appears dark grey or dark when it is sufficiently small in visual angle, and this fact seems to correspond to our finding that the excitability curve for blue or violet is lower than those for lights from the other parts of the spectrum. It is apparent from this experiment that the red and the blue-green are the main processes involved. However, our excitability curves as such give no more information about the exact nature of the processes involved than do any other sensory experiments, because an excitability curve represents an enve- lope of a train of processes developing at different velocities. Therefore, the phenomenon was subjected to further analysis in the manner as described in Motokawa's papers (7, 8). 2. Spectral response curves A series of measurements was carried out, with a fixed interval of 1 second intervening between the end of the pre-illumination and the electric stimulus. By plotting the values so obtained as ordinates against the wave-lengths of the pre-illuminating lights as abscissas,we obtained a curve representing the spectral distribution of the red receptor. The curve R in fig.3 is an example. In a similar manner, the response curves for the yellow, green and blue receptors were obtained and denoted by Y, G and B respectively,where the interval be- tween the light stimulus and the electric stimulus was fixed at 1.5, 2.25 and 3 Fig.
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