JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

Nature of the Transmission of Energy in the Retinal Receptors*t

JAY M. ENOCHDepartmient of Opthalmology and The Oscar Johnson Institute, Washington University Medical School,

640 South Kingshighway Boulevard, St. Louis 10, Missouri(Received February 14, 1961)

Waveguide modal patterns have been observed in retinal receptors of rat, monkey, and human eyes.Phenomena characteristically occurring in dielectric waveguides have been noted. That aspect consideredhere is the appearance of different (or combinations of different) hues when the retina is irradiated withwhite light of a xenon arc and the receptor outer segments are viewed. The distribution varies to somedegree with angle of incidence of the radiant energy, and the phenomenon is present in both rods and cones.It is demonstrated in freshly obtained normal human and monkey central foveal areas and in some peripheralretinal receptors. Some implications of these findings are discussed.

INTRODUCTION

IN 1960 this author presented a brief discussion inwhich the possible roles of waveguide modal patterns

in the visual mechanism were discussed. Subsequent tothe presentation of that article, two papers2 3 have beensubmitted in which the presence of modal patterns inretinal receptors of certain species has been verified.Previous literature bearing on this general topic hasbeen covered in these three previous discussions and,for the sake of brevity, will not be repeated in thispaper. The major observations regarding the wave-guide mode form of transmission in retinal receptors maybe summarized as follows:

1. In the outer segments of all reasonably well-oriented retinal receptors in experimental preparationsof the albino rat, the rhesus macaque monkey, thesquirrel monkey (sairniri-sciureus), and man, waveguidemodal patterns have been observed.

In the previous presentations, only human eyes ex-hibiting pathology had been available for analysis. Allprior conclusions based upon study of this materialprove to be valid when normal human eyes are ob-tained. The following properties refer to observationsmade when viewing the terminations or near termina-tions (pigment epithelium side) of the receptor outersegments:

2. In some retinal receptors waveguide modalpatterns vary as a function of wavelength.

3. In some retinal receptors waveguide modalpatterns vary as a function of the obliquity of incidenceof the radiant energy.

The changes in modal pattern induced by obliqueirradiation result in many instances in the same physical

* This research has been supported in part by Grant B-2168from the National Institute of Neurological Diseases and Blind-ness; National Institutes of Health, Public Health Service,Bethesda, Maryland.

t This paper is dedicated to Dr. Hamilton Hartridge. In sodoing, the author wishes to express his gratitude to Dr. Hartridgein response to his recent and gracious gift of his voluminous andwvell-indexed cardl file. The writer has no doubt that the contentof this paper will prove of considerable interest to that gentlemanin view of his extensive work on closelv related subjects.

J . Enoch, J. Opt. Soc. Am. 50, 1025 (1960).2 J. XL Enoch, Am. J. Ophthalmol. (to be )ublishedl).3 J. M. Enoch, Science (to be published).

distributions of energy which are obtained by increasingwavelength.' 2 '4 '

4. The configuration, physical properties (indexes,diameters), and separation between retinal receptorsinfluence the types of and numbers of different modalpatterns seen.2' 4-6

5. In some retinal receptors modal interactive formsare present.

6. In some retinal receptors modal patterns vary intheir total transmissivity and/or their transmissivityas a function of wavelength.

7. Oblique irradiation of the retina results in a de-crease in total retinal receptor transmissivity (in rodsas well as cones). In some retinal receptors the modalpatterns change their transmissivity as a function ofwavelength when the angle of incidence of the incidentbeam is varied.

8. The modal patterns (singly or in combination)most commonly associated with the outer segments ofrods and cones of the designated species2 are HE1 ,TEo, TMoi) HE2 1, HE12, EH,1, and HE31 . EH21 , HE41 ,TE02 , TMo2, and HE22 are very rarely observed. Thesedesignations are based on observations by the authorcompared with computations by Snitzer.'

Other mode forms are occasionally seen and inter-active forms are common, but the patterns recordedabove are the dominant forms observed.

9. Modal patterns are observed at planes of focuswithin the receptor other than at termination and, insome instances, the modal pattern observed changeswith variation in focal plane within the receptor.

Luminance of the observed distribution of energy isusually greatest at or near receptor termination andless at other points. Wavelength distribution of thisenergy may also vary. Peripheral cones need to beplaced in a separate category in regard to this propertybecause of marked variations which occur in cell diame-ter. In addition to other aspects, this property allowsevaluation of the orientation of the individual receptorsand, in part, allows differentiation of rods from cones(other guides are also present as will be seen below).

E. Snitzer et al., J. Opt. Soc. Am. 49, 1128 (1959).5E. Snitzer, J. Opt. Soc. Am. 51, 491 (1961).6 N. Kapany, J. Opt. Soc. Am. 49, 770 (1959).

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October1961 TRANSMISSION OF ENERGY IN RETINAL RECEPTORS 1123

10. All properties described above are common tosome extent in rods, peripheral cones, and central cones.In so stating, nothing is implied regarding magnitudesor frequencies of occurrence.

In this paper an attempt is made to add to the dis-cussion of that portion of the above summary whichmight be considered under the term transmissivity. Theterm "transmissivity" is used purposefully for thefollowing reason. When a given modal pattern is excited,the wavelength composition of the energy associatedwith the propagation of that pattern in the retinal re-ceptor (acting as a waveguide) may differ from the totaldistribution of energy at the point of incidence. It isimportant to realize that the energy which is not trans-mitted is not necessarily absorbed by the receptor. Itmay be reflected or refracted out of the receptor. It willbe important in the future to learn the total distributionof this energy in some detail, at what points the distri-bution is modified, and under what conditions it varies.The presence of differences in transmissivity will beestablished. Photographs of the outer segments ofcentral and peripheral retinal receptors of human andmonkey eyes irradiated with white light from a xenonarc are presented.

APPARATUS AND PROCEDURE

The apparatus and procedure employed in these ex-periments have been described at some length pre-viously2 and are only reviewed briefly in this discussion.An intense source of light (Osram xenon arc XBO-1001,operated at 42 amp) was imaged by a lens upon a flatpreparation of the retina after first passing through awater bath (infrared filter). This, in essence, created aschematic eye since the retina was oriented as it mighthave been in situ. The retina, which was placed in aspecial chamber, 7 was viewed through a microscope. Inthis study, that image seen through the microscope wasphotographed by a 35-mm camera with a 5-cm-focal-length lens placed at the eyepiece. The incident coneof energy was limited in angle subtended at the retinato 3° 15' of arc, and the angle of incidence of this energycould be varied over a small range. (In the Gullstrandschematic eye, 1 mm in the entrance pupil subtendsapproximately 2.5° at the retina.) One may also intro-duce monochromatic light or multiple beams of mono-chromatic light in the apparatus.

The specimens were immediately obtained from theoperating room and were in position for recordingwith 10 to 15 min after enucleation. The dissectionprocedure employed was a simple one. Marked bleach-ing of photopigments was induced with white light(tungsten filament) during dissection. The anteriorhalf of the eye was dissected away; the macula wasthen located by means of its special pigment and itsrelationship to blood vessels and to the nerve head,and was isolated. The retina (including the macula and

I K. Tansley, Nature 178, 1285 (1956).

some of the peri-macular retinal area) was very care-fully lifted away from its substrate and was mountedin a special cover glass (with a depression slightlygreater in depth than the thickness of the retina) innormal saline with the receptors facing the microscopeobjective. Subsequent histological examination (inevery case reported) revealed that the receptors wereintact and that the ground substance which lies betweenthe receptors remained. Some effects of using normalsaline and postmortem changes have been describedpreviously. 2 An additional major problem facing theexperimenter is that of obtaining proper orientation ofthe receptors in the special glass cells. The orientationof any single receptor or groups of receptors may bereadily determined.

RESULTS

It is important to note that it is very easy to identifythe center of the fovea. This point is demonstrated inFig. 1. This is the fovea of a rhesus macaque monkey,photographed at low power through the system de-scribed above. The central fovea is always marked asan area of higher transmissivity. The Stiles-Crawfordeffect8 may account for this, and certainly the thinningof the retina at this point must play a role. It should bepointed out that in all species mentioned, of course withthe exception of the rat which has no fovea, the pictureis essentially the same in the central fovea. All photo-graphs were taken on Eastman Kodak High SpeedEktachrome film.

Figures 2 and 3 show the central foveal areas of twofreshly obtained normal human retinas. They wereirradiated with the white light of a Xenon-arc and themicroscope was focused on or near the extremities ofthe outer segments. The centered receptors are welloriented and are irradiated by a beam that is incidentat an angle near normal. Notice the difference in theover-all "yellowness" of the two foveal areas. This isdue to the marked differences in macular pigment con-

FIG. 1. The central fovea of a rhesus macaque monkey seen ina flat preparation of the retina. It was photographed at a low levelof magnification.

8 W. S. Stiles and B. H. Crawford, Proc. Roy. Soc. (London)B112, 428 (1933).

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JAY M. ENOCH

centrations noted among the normal human maculasstudied. This variability has been noted previously byseveral authors, e.g., Wald.9 When studying thesephotographs it is important to remember that thediameter of the human central retinal cone outer seg-ment is of the order of 1.0 u.1 Where resolved, note theassociated modal pattern(s).

The patient whose central fovea is shown in Fig. 2(W. B.) is a fifty-nine-year-old white male. His eye wasremoved, along with the remainder of the contents ofthe orbit, because of the presence of a malignancy inthe right maxilla which extended into the orbit. At sometime prior to removal, he had had a cataract extractionand postoperative visual acuity had been correctableto 20/25 in that eye. No visible retinal pathology waspresent.

The patient whose specimen is shown in Fig. 3 (A. C.)is a fifty-two-year-old white female whose eye and or-bital contents were removed because of a malignancyfound growing in and on her conjunctiva. Visual acuityprior to surgery was 20/20 (corrected) and no visibleretinal pathology was observed.

Figure 4 is a comparable picture taken in the centralfovea of a squirrel monkey and Fig. 5 is a photographof the central fovea of a rhesus macaque monkey (sameretina as was presented in Fig. 1). To the best of theknowledge of the writer the eyes of these animals werenormal at the time of enucleation. These are includedfor the purpose of establishing first order validity re-garding their use in future experiments.

Taking these four figures as a group, the followingobservations may be made:

1. A distinct wavelength separation mechanism existsin these retinal receptors.

2. In some instances one modal pattern is seen havinga dominant hue.

3. Where multiple colors are seen in a given receptorunit, these are in some instances due to changes inmodal pattern with wavelength.

4. The appearance of multiple colors, observed whenviewing the outer segments, occurs in some receptorswhere mode interactive effects are present.

5. In some receptors white light (modified by themacular pigment) is emergent. In some instances thisis due to overlap of observed modal patterns, 4 10 and inother instances neither modal pattern nor transmissivityappear to vary with wavelength.

Thus, the white light of the xenon arc is brokendown into components in two or more ways in the re-ceptors. There are differences in the wavelength distri-bution of the energy transmitted to the terminal endof the receptor, and there are differences in the spatialor regional distribution of that energy within the outersegments as a function of wavelength (ue to the

G. Wald, Science 101, 653 (1945).'(' B. O'Brien, Pys. 'Today 13, 52 (1960).

presence of different modal patterns and modeinteractions.

These figures do not have the fineness of resolutionof the material previously presented.2' 3 The reasons forthis are manifold. First, the author was particularlyanxious to demonstrate the presence of the color-sepa-ration mechanism, having already recorded the factthat some modal patterns change with wavelengthvariation. Second, the first two of the four normalhuman maculas obtained were ruined by touching thecover glass cell with a high-power objective; hence, alow-power objective was employed in order to providea safe working distance. The detail of the modalpatterns is resolved only at the resolution limit of thelight microscope; in order to approach this limit, it isnecessary to optimize every factor. The author chosenot to risk these specimens and thus, in some of theresulting figures, while color separation is clearly evi-dent, the individual modal patterns may not be wellresolved.

If one varied the angle of incidence of the energystriking the retina, in no way adjusting the microscopeor the specimen, several features were noted. If thereceptors were well oriented, total transmissivity de-creased, the spectral distribution of the transmittedenergy in many receptors changed, and/or the modalpattern in many receptors changed. On a subjectivebasis it is difficult to evaluate the nature of the over-allchanges in perceived hue because of the change inbrightness of the field due to the directional sensitivityof the receptors. The apparatus has since been modifiedto overcome this limitation. At this time, a first estimateof the over-all nature of the color change was obtainedby roughly equating exposures on the color film. Figures6-8 show photographs taken of the same well-orientedcentral foveal receptors at three angles of obliquity ofincidence of the light. The retina, as seen in Fig. 6, isirradiated at an angle near normal incidence and isessentially identical with Fig. 2 except for film exposuretime. In Fig. 7 the angle of incidence is set at an inter-mediate setting and in Fig. 8 the beam is sufficientlyoblique to require triple the exposure time of Fig. 6.These photographs are of the retina of patient W. B.Subjectively, it appears that increased obliquity resultsin more red and blue being transmitted, or less yellowand green being seen (even though the light passesthrough a somewhat greater thickness of macula pig-ment). A somewhat similar result is observed in thecase of photographic records of the retina of patientA. C. At this time the author is satisfied to note thatsignificant changes occur, without attempting to specifythese changes in detail either in the single receptor orafter integrating over an extended retinal area. Themain weakness in the met hod employed lies in the accu-racy of the transfer function of the color film pigments.The partiCuflar emulsioIn (film) selected, le eulsionbatch obtained, exposure, and processing all influencethe result. When acceptable quantitative data are

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available, it will be important to relate these findingsto the Stiles-Crawford effects.1 ""'12

Figure 9 is a photograph of the rods of patient A. C.irradiated with white light. As a comparison, Figs. 10and 11 show the peripheral retina of a squirrel monkey.Compare the general color of Fig. 9 with that seen inFig. 3. This clearly demonstrates the very high con-centration of macular pigment present in the maculaof that patient. Other records of human rods areavailable, allowing superior resolution of the individualmodal patterns. In the case of the rod distribution inFig. 10, some remnants of adherent overlying pigmentepithelium are visible in the field. The peripherial conesterminate further from the pigment epithelium than therods and hence a different plane of focus is necessary(Fig. 11). The higher luminance of these cones is re-lated to the Stiles-Crawford effect.7 It is evident thateffects very similar to those described above (regardingcentral cones) occur in peripheral rods and cones. Notethat in Fig. 11 modal patterns are still being radiatedby many rods at points not coincident with theirterminations.

DISCUSSION

An attempt has been made to describe some of thephenomena associated with the transmission of energyin retinal receptors. These phenomena are particularlyimportant because they occur early in the visualprocess. It now becomes necessary to determine whichof these factors are significant in the response of thereceptors.

It has been shown that there is a physical separationof wavelength information between and within singleretinal receptors. There is a physical basis for thesedifferences in transmissive properties. Whether thesedifferences are due to small variations in orientation,diameter, cross section, indexes, or morphology is notknown at this time. It is doubtful in a highly packednormal retina if the obliquity of immediately neighbor-ing receptors varies greatly. It is important to determinethe distribution of these properties across the retinaand to determine to what extent these properties aredistributed in a predictable pattern.

Quantitative results are necessary in the descriptionof all factors described in this paper. This laboratorywill direct its effort toward this end in the future.

What role do these phenomena play in vision? Is (are)the distribution (s) of photopigment(s) in any way cor-related with the energy transfer properties of the recep-tors? The better the correlation, the more efficient thereceptor. Are the differences in magnitude and wave-length distribution in transmissivity merely reflected inthe response, or are the photopigments so distributedthat they can make use of the added information presentin the spatial or regional distribution of energy presentin the individual receptors? To what degree do the

1 W. S. Stiles and B. H. Crawford, Nature 139, 246 (1937).12 W. S. Stiles, Proc. Roy. Soc. (London) B123, 90 (1937).

properties of energy transfer in the receptors representthe separation of wavelength information for codingpurposes?

One of the more immediate and serious implicationsof these findings is that single nerve fiber recordingsmay not tell as much about the basic coding mechanismof color vision as had previously been hoped. 3 Con-sidering the figures presented above, it is evident thatthe neurological response must redect both wavelengthtransmission properties of the receptor and photopigmentsensitivity (and distribution).

A feature, which is in keeping with Dr Hartridge'swork,"4 is that one would predict that the color of smallretinal images would be incorrectly named. This wouldfollow whether transmissive properties were or were notcorrelated with pigment properties as long as the wave-length absorption functions of the pigments were nottoo narrow. Although some receptors seem to containall the necessary information, the obvious implicationis that the color percept is the result of the integrationof response over a finite retinal area. Specification ofgiven level(s) at which such integration takes place,and the spatial and temporal properties of this (these)integration(s) are not the subject of this paper.

It is of interest that if sufficient luminous energy ispresent, some of almost every wavelength will be passedin any receptor. This is determined by using varyingamounts of monochromatic light. Since receptor re-sponse includes both the transmissive and absorptiveproperties of the receptor, the response function of asingle receptor or group of receptors may change tosome degree as a function of luminance.' Because somany rods feed into a single ganglion cell, one mayassume that a large portion of the individual receptortransmission differences become smoothed out in thescotopic system. Theories regarding vision, which stipu-late equal response probability of all receptors, mayhave to be modified to some degree when quantitativedata on transmissivity are available. One aspect whichmay pose problems as quantification proceeds is thatpart of the energy propagated in modal pattern form ina dielectric waveguide may not be contained withinthe boundaries of the guide.

There are a few additional interesting points whichmay be learned from these findings. The fact that thetransmissivity changes as a function of angle of inci-dence shows that these effects are not based on a singlefilter phenomenon. In other words, the physical basisfor these effects is the waveguide form of transmission.It is obvious, as is stated above, that these changes intransmissivity must be related to the Stiles-Crawfordeffects.8," Since wavelength transmissivity, as well astotal transmissivity, changes with obliquity of inci-dence, it may be further assumed that receptors which

13R. Granit, Sensory Mechanisms of the Retina (Oxford Uni-versity Press, London, 1947), p. 298.

14 H. Hartridge, Recent Advances in the Physiology of Vision(Churchill, London, 1950), p. 163.

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are not oriented properly in the retina will not respondto stimuli in the same manner as receptors which areoriented properly. In other words, disturbance in orien-tation of retinal receptors should result in anomalouscolor vision. The magnitude and type of this anomalyshould be related to the Stiles-Crawford effects. Further,since disturbance in orientation also changes visualacuity as has been shown by Enoch,15 '7 Fankhauser,Enoch, and Cibis,1 5 and Campbell and Gregory, 19 thereshould be correlation among these phenomena. In thesame sense, variations in physical properties (congenitalor acquired) may cause differences in transmissivity (or

16 J. M. Enoch, Am. J. Optom. 34, 298 (1957).'6J. M. Enoch, Am. J. Optom. 36, 111 (1959).17 J. M. Enoch, Am. J. Opthalmol. 48, 262 (1959).18 F. Fankhauser, J. Enoch, and P. Cibis, Am. J. Ophthalmol.

(to be published).19 F. Campbell and A. Gregory, J. Opt. Soc. Am. 50, 831 (1960).

in regional distribution of energy within the receptor)without disturbed orientation and thus result in anoma-lies in color vision.

ACKNOWLEDGMENTS

The author wishes to express his gratitute in par-ticular to Dr. Elias Snitzer of the American OpticalCompany for his encouragement, advice, and assistancein many phases of this work. Many others includingDr. Katherine Tansley, Dr. George Wald, Dr. PaulBrown, Dr. Marguerite Constant, Mr. William Moor,Mr. Eric Seiler, Jr., Dr. Arthur Stickle, Dr. JosephOgura, Dr. Glenn Johnson, Dr. N. S. Kapany, theNational Physical Laboratory (England), Mrs. VerenaFankhauser, etc., have generously and significantly con-tributed in terms of advice, material or specimens, andassistance in several aspects of this study.

LEGENDS FOR COLOR PLATE

FIG. 2. This is a photograph of the central foveal receptor outersegments of patient W. B. The retina is illuminated at near normalincidence with white light and the receptors are wvell oriented.

FIG. 3. This is a photograph of the central foveal receptor outersegments of patient A. C. The retina is illuminated at near normalincidence with white light and the receptors are well oriented.

FIG. 4. This is a photograph of the central foveal receptor outersegments of a squirrel monkey. The retina is illuminated at nearnormal incidence with white light and the receptors are welloriented.

FIG. 5. This is a photograph of the central foveal receptor outersegments of a rhesus macaque monkey. The retina is illuminatedat near normal incidence with white light and the receptors arewell oriented.

FIG. 6. This figure is the same as is seen in Fig. 2 except that theexposure of the film is adjusted to more closely match Figs. 7 and8. The light striking the retina is incident at near normal incidence.

FIG. 7. This is the same retinal area as is shown in Fig. 6. Theangle of incidence of the radiant energy striking the retina is some-what oblique to the axis of the receptors.

FIG. 8. This is the same retinal area as is seen in Figs. 6 and 7.The angle of incidence of the radiant energy striking the retina ismore oblique than in either of the previous two figures.

FIG. 9. This is a photograph of the peripheral retina of patientA. C. The microscope is focused upon the extremities of the outersegments of the rods.

FIG. 10. Peripheral retina of a squirrel monkey. Some adherentpigment epithelium is visible lying over the extremities of theouter segments of the rods.

FIG. 11. The plane of focus of the microscope is shifted relativeto Fig. 10. The microscope is focused on some of the peripheralcones of the same squirrel monkey.

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