An analysis of retinal receptor orientation

Transcription

1 An analysis of retinal receptor orientation IV. Center of the entrance pupil and the center of convergence of orientation and directional sensitivity Jay M. Enoch and G. M. Hope In the previous study, we related the peaks of photopic directional sensitivity (Stiles-Crawford) functions determined, across the retina to the center of the dilated entrance pupil. It was found that the peaks of directionality were oriented toward a point in close proximity to the center of the dilated exit pupil of the eye. Here we consider whether this center of convergence of orientation and directional sensitivity (projected to the plane of the entrance pupil) bears an improved relationship to the center of the constricted entrance pupil, on the chance that the dilated and constricted entrance pupils in the same observer have different centers. In the three normal observers tested, the small variations in pupil centration recorded with change in pupil size did not further clarify the result. The constricted pupil when compared to the dilated pupil shifted slightly nasal and/'or upward. I Key words: vision, retina, directional sensitivity of the eye, Stiles-Crawford effect, retinal receptor orientation, mydriasis, miosis, center of the entrance and the exit pupils of the eye, corneal reflex, center of convergence of orientation and directional sensitivity. n a previous paper 1 evidence was presented to suggest that the retinal receptors across the human retina have a carefully ordered distribution of orientation. They are aligned transretinally (across the ret- From the Department of Ophthalmology and the Oscar Johnson Institute, Washington University School of Medicine, 660 S. Euclid Ave., St. Louis, Mo This research has been supported in part by National Eye Institute Grant EY and Career Development Award No. K (to J. M. E.), and in part by ARPA, Department of the Army Grant No. DA-ARO-D G759. Manuscript submitted June 26, 1972; revised manuscript accepted Aug. 30, ina) such that their long axes converge at a point in close proximity to the center of the exit pupil of the eye. This point of convergence of the receptor axes in the exit pupil may be considered as the center of the aperture of the retina. Alignment was evaluated by relating the peak of the Stiles-Crawford function, determined at each of several retinal test points, to the center of the dilated entrance pupil. These peaks fell in a small area within the entrance pupil. Their collective mean position, projected to the plane of the exit pupil, was taken as the estimate of the center of convergence of orientation and directional sensitivity. Note: Since the exit pupil is imaged in the entrance pupil of the eye, it

2 1018 Enoch and Hope Investigative Ophthalmology December 1972 is possible to relate factors in the exit pupil to each other when studying relationships present in the entrance pupil. The data in the paper by Enoch and Hope 1 show that in each subject a small error existed in each case between the defined center of maximum (photopic) sensitivity and the center of the exit pupil of the eye. In that study, for theoretical and practical reasons, the center of the dilated entrance pupil of the eye was taken as the reference origin. There are reports in the literature which suggest that the center of the entrance pupil shifts with contraction. 2 ' 3 In this paper, we simply ask whether the alignment of the receptors was directed toward the center of the constricted exit pupil rather than the center of the dilated exit pupil (or some point between). It was assumed that the centers of the dilated and constricted pupils represent the range of pupil centers which might be encountered in a given normal observer. In a sense, this brief experiment represents a test of the generality of the use of the center of the entrance pupil as a reference in such studies. In the previous study, the projection of the center of convergence of orientation and directional sensitivity was estimated for each of three subjects relative to the geometric center of their dilated entrance pupil. In this study, using foveal fixation, the geometric centra of the dilated and constricted entrance pupils of the same three observers were determined relative to each other. The corneal reflex was used as the common reference point. By this means, it is possible to relate the center of convergence of orientation and directional sensitivity to the geometric center of the constricted entrance or exit pupil. Method The Stiles-Crawford apparatus described by Enoch and Hope 1 was employed. The emmetropic observer was fixed to the instrument by head rest and bite bar. His foveal fixation was directed toward the center of the combined test and background field. The experimenter centered the apertures of both fields on the reticule pattern. The subject's entrance pupil (illuminated by infrared light sources) was focused as sharply as possible on the reticule and, hence, on the cathode of the infrared image converter. Shutter 2 (see Fig. 4 in Enoch and Hope 1 ) was closed in order to facilitate viewing of the superimposed corneal reflexes of the test and background field in the image converter. The focused entrance pupil was translated until the corneal reflex was visible. It was centered on the reticule pattern. The picture seen on the phosphor of the infrared image converter was photographed using a special Burke and James single lens reflex copy camera, f/1, -l.ox lateral magnification. The camera was indexed to the Stiles-Crawford device in order to maintain constant object distance and image position. The eyepiece normally used to view the cathode was removed. Polaroid film was employed. The subject's pupil was dilated with repeated doses of Euphthalmine 5 per cent. The experiment was then repeated on a separate day at which time the pupil was constricted with Pilocarpine HC1 1 per cent. Data on refraction and angle kappa on each emmetropic subject are presented in the previous paper. The corneal reflex caused by the incremental field was not visible in the infrared viewer unless the fixing left eye (and entrance pupil) was translated laterally a bit to the left. A very small vertical translation was also indicated on occasion. The optimized corneal reflex appeared in the reticule pattern at the same position as the image of the apertures of the two fields. This suggests virtually normal incidence and reflection. The latter conclusion was reinforced by the following observation. The entrance pupil was focused on the IR cathode in this experiment. Hence, the corneal reflex was blurred because its image lies behind the plane of the entrance pupil. Focusing down on the corneal reflex (about 1 mm.) caused no detectable translation of the beam relative to the reticule. Thus, changing focus slightly did not cause a meaningful parallactic error. This point is important because the plane of the entrance pupil probably shifts in a fore and aft direction with constriction and dilation. Four photographs each were taken of the constricted and dilated entrance pupils of each subject. The four photographs of the dilated pupils were individually projected onto a reference surface by an opaque projector. The pupil circumference, position of the corneal reflex of the (superimposed) test and background beams, and reflexes of the infrared sources were traced onto gridded paper. Each projection was compared with the other three to provide an estimate of the variability in the technique. Each of the four constricted pupil photographs was then projected and traced onto two of the previously traced dilated pupils for each subject. Precise alignment of the eye in the two projections was assured by

3 Volume 11 Number 12 Analysis of retinal receptor orientation. IV careful superimposition of the corneal reflex from the test and background beams. The laterally placed reflexes of the two infrared sources established a horizontal line providing a means of avoiding rotation of one projection relative to the other. The object-image distance of the projector was maintained constant for all projections. A perfect circle of the average diameter of each dilated or constricted pupil was used as a template for locating the center of the pupils. The template was overlaid on the tracings. It was then adjusted to provide the best match to the pupil circumference, and the point on the pupil tracing corresponding to the center of the template was marked precisely. Repeated matches of the template to the tracings indicated that the technique was quite reliable. The displacement of the center of the constricted pupil relative to that of the dilated pupil was determined in the horizontal and vertical directions and converted to millimeters through the use of a millimeter scale placed at the plane of the entrance pupil, photographed, and projected precisely as for the pupils. Eight comparisons of a dilated to a constricted entrance pupil (of the 16 possible in the 4 by 4 matrix) were made for each subject. Results Fig. 1 presents the results of the analysis described above. The center of the constricted entrance pupil (small crosses) was displaced from that of the dilated entrance pupil (center of X and Y coordinates) for all three subjects. The mean centrum of the Stiles-Crawford curves (from seven retinal loci) for each of these subjects 1 is depicted by Xs. The shift of the constricted pupil was such that it essentially coincided with the mean Stiles-Crawford centrum for subject SH, was generally in the right direction, and the range of variabilities for the two data sets overlapped slightly (compare with Fig. 10, HE, Enoch and Hope 1 ) for subject HE, but was in exactly the opposite direction for subject BL. When the mean data points across all three subjects were considered, the three centers fell very close to each other. All three points could be covered by a circle of radius equal to 0.25 mm. and having an area of less than 0.2 mm. 2. This area comprised less than 6.5 per cent of a constricted entrance pupil (2 mm. diameter) and less than 0.3 per cent of a dilated entrance pupil (9 mm. diameter). MEANS ALL SUBJECTS IOS) lmm 0 lmm lmm 0 lmm -^ NASAl(Rr) TEMPORAL (U)»- X MEAN CENTRUM OF STILES-CRAWFORD CURVES I-J-H MEAN CENTER OF CONSTRICTED ENTRANCE PUPIL (WITH RANGE) Fig. 1. Centers of the dilated and constricted entrance pupils and of the projection in the entrance pupil of the center of convergence of orientation and directional sensitivity. The centers of the dilated entrance pupils are depicted by the intersection of the X and Y coordinates of each part of the figure. The centers of the constricted entrance pupils are indicated by small + symbols, the length of the arms of which indicate the range of variability of eight comparisons. The position of the projection of each center of convergence of orientation and directional sensitivity is indicated by an X. The large circle on each set of coordinates depicts a 2 mm. diameter schematic entrance pupil centered on the objectively determined constricted entrance pupil center. Individual data for three subjects and their mean are shown in the four quadrants of this figure. Another factor considered was the position of the corneal reflex of the superimposed test and background beams relative to the centers of the constricted and dilated pupils (Fig. 2). The corneal reflex maintained a fairly consistent relationship to the center of the dilated entrance pupil across the three subjects, lying slightly less than 0.5 mm. nasal and very slightly above or below (< 0.2 mm.) the pupil center. For subject SH, the center of the constricted entrance pupil and the corneal reflex were in fairly good register. For the other two

4 1020 Enoch and Hope Investigative Ophthalmology December 1972 SH MEANS ALL SUBJECTS HE BL lmm lmm lmm 1mm «r- NASAL (Rr.) TEMPORAL (Lt.) * CORNEAL REFLEX FROM STIMULUS ARRAY 4-* CENTER OF CONSTRICTED PUPIL Fig. 2. Centers of the dilated and constricted entrance pupils compared to the position of the corneal reflex. The corneal reflexes for foveal fixation on the stimulus array are depicted by open circles. Other details are as for Fig. 1. subjects however, this was not the case. If, however, one considered the mean values across the three subjects, the three points were quite tightly grouped. Discussion Here we have compared the centration of the dilated and constricted entrance pupils of the eye with the mean centrum of the Stiles-Crawford functions (the projection in the entrance pupil of the center of convergence of orientation and directional sensitivity) for the individual observers. Taken as an average across the three subjects, the three points were in very good register, suggesting that, in the normal observers included in this sample, the centers of the constricted and dilated exit pupils and the center of convergence of orientation and direction sensitivity are, on the average, probably virtually coincident. A slight shift of the constricted pupil relative to the dilated pupil was seen in all three of the observers. The tendency was nasal and slightly upward. 3 Finally, the relationship of the corneal reflex of the stimulus array maintains a fairly consistent relationship to the center of the dilated entrance pupil in the individual observers. Both the center of the dilated entrance pupil and the corneal reflex have been used as primary references in conducting Stiles- Crawford research (e.g., Safir, Hyams, and Philpot* and Enoch and Hope 1 ). Given the brief time-bound defined in this study, the present comparisons suggested that either is acceptable in this respect as long as the direction of view is toward the stimulus array, or very close to it. With oblique viewing, the corneal reflex changes its position relative to the center of the entrance pupil dramatically. It is, of course, precisely this feature which recommends the use of this reference for.certain types of visual task. In the present research, however, the Stiles-Crawford data were taken over a large span of retinal points and fixation directions. Since the corneal reflex from the stimulus array shifts considerably over such a range,- a comparison of the Stiles-Crawford data and the position of this variable was carefully avoided. Departure from concentricity for the constricted and dilated entrance pupils has been described previously. 2-3 The present data demonstrated small shifts of the constricted pupil in all three of our emmetropic subjects. Comparisons of each projection of a dilated pupil photograph with the other three for each subject indicated that there was no detectable variability in the photographs. This was not the case for the photographs of the constricted pupil. However, this source of variability was incorporated with any experimentor measurement error which may have existed, and was shown in Fig. 1. The fact that the range of variability did not overlap the center of the dilated entrance pupil attests to the reality of the shifts. The constricted pupil centers clearly shifted nasalward in two subjects and upward in all three. The mean data, reflecting the nasalward and upward shift, compares almost perfectly with the 0.28 mm. shift predicted by Gullstrand 3 for spherical

5 Volume 11 Number 12 Analysis of retinal receptor orientation. IV corneas! The corneas of these emmetropic observers were virtually spherical. For foveal fixation, the mean center of the constricted entrance pupil tended to shift toward the location of the corneal reflex. Enoch and Hope 1 suggested that one possible cause of the small eccentricities of the transretinally-determmed Stiles-Crawford centra might be due to a shift of the constricted pupil relative to that of the dilated one. This hypothesis received no definitive support in the present experiment. For only one subject was there any indication that this factor may account for the Stiles-Crawford eccentricity and this case was offset by another observer for whom the pupil shift was in the opposite direction to that required. In the mean data, the Stiles-Crawford centrum was about equidistant from the centers of the dilated and constricted entrance pupils (0.40 and 0.35 mm., respectively). In the exit pupil these values would be 0.36 and 0.32 mm., respectively. In fact, the rather good register of the three points is much more impressive than their separation and is a further illustration of the precision with which the several factors contributing to the directional sensitivity of the system function. 1-5 The reader is reminded that a shift of the Stiles-Crawford peak in the entrance pupil by 1.0 mm. reflects only a 2.5 difference in alignment measured at the retina. One must consider an array of millions of receptors only tens of microns long, remotely placed and oriented relative to the plane of the pupillary aperture, a distance measured in tens of millimeters. This precision of alignment is not a chance occurrence. The amazing thing is the limited magnitude of discrepancy between the transretinally-determined peaks of the Stiles-Crawford functions and the center of the exit pupil, not the fact that small discrepancies exist. As pointed out in previous papers in this series, 1 ' 6 it is clearly necessary to define mechanisms establish ing, influencing, and maintaining appropriate receptor alignment and anomalies related to these processes. REFERENCES 1. Enoch, J. M., and Hope, G. M.: An analysis of retinal receptor orientation: III. Results of initial psychophysical tests, INVEST. OPHTHAL- MOL. 11: 765, Le Grand, Y.: Optique Physiologique I,Ed. 2, Paris, 1952, Editions de la Revue D'Optique, p Gullstrand, A.: in Von Helmholtz, H.: Physiological Optics, Ed. 3, New York, 1962, Dover, Vol. I, pp. 356 and Safir, A., Hyams, L., and Philpot, J.: Movement of the Stiles-Crawford effect, INVEST. OPHTHALMOL. 9: 820, Enoch, J. M.: Retinal receptor orientation and the role of fiber optics in vision, Am. J. Optom. 49: 455, 1972.