Visual functions in congenital night blindness

Visual functions in congenital night blindness R. E. Can; H. Ripps, I. M. Siegel, and R. A. Weale*

It was shown preciously that the physiological basis of congenital night blindness probably involves a defect in neural transmission affecting primarily the scotopic (rod) mechanism. However, spectral sensitivity measurements in the dark-adapted peripheral retina indicate that rod signals, although greatly attenuated, may reach cortical centers. In spite of the decreased sensitivity, measurements of visual threshold as a function of stimulus area showed that the integrative properties of the retina ivere normal. Although toe have been unable to identify the precise nature of the defect, it appears that the abnormality affects both rod and. cone vision and extends also over the rod-free region of the fovea.

rhodopsin, the light-sensitive pigment of the rods. Vision requires, however, that the message initiated by photochemical events be propagated across the retina and transmitted to cortical centers. The night-blind subjects we have examined fail in this regard. Their electroretinal responses indicate that nervous activity is severely depressed in the outer layers of the retina—a region which, in the normal eye, is richly supplied with lateral connections and exhibits a considerable degree of neuronal convergence.5' ° It seemed appropriate, therefore, to determine whether the visual defect in congenital night blindness is the manifestation of a breakdown in the integrative properties of the peripheral retina, thereby giving near equality in the sensitivities of the rod and cone mechanisms. Although spatial integration is readily studied in normal subjects by measuring the area-intensity relationship, in the nyctalope we have no assurance that this method will be successful for studying the scotopic mechanism. The dark-adaptation curves are equivocal in this regard, for nowhere is it obvious that the rods contribute to visual function. Therefore, in the present

t is usually assumed that vision in night-blind subjects is mediated solely by the photopic (cone) system since thresholds, measured during the course of dark adaptation, rarely fall below the cone plateau of the normal curve.f This view probably stems from the unreserved acceptance of the duplicity theory wherein high thresholds are associated with cone vision and low ones, with rod vision. It has been demonstrated, however, that when small test fields are used, thereby depriving the rods of the advantage that areal summation bestows on them, the difference in threshold is reduced if not altogether eliminated.1"1 In the previous paper1 it was shown that the retinas of some congenital nyctalopes contain normal concentrations of From the Department of Ophthalmology, New York University Medical Center, New York, N. Y. This work was supported by Grants NB-05487, B-2589, and 1K3-NB-18/766 from the National Institute of Neurological Diseases and Blindness, United States Public Health Service. "Department of Physiological Optics, Institute of Ophthalmology, Judd Street, London, W. C. 1. fSee Fig. 2 of preceding paper.

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Visual functions in night blindness 509

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had been calibrated at all the wavelengths used. At least 30 minutes of dark adaptation preceded the start of an experimental session which consisted of 4 threshold measurements at each wavelength. The relative energy E x transmitted by the interference filters was determined with a calibrated photomultiplier and microammeter,7 and the reciprocal of log E at absolute threshold, i.e., log Srei, has been plotted in the spectral sensitivity graphs. Area-intensity relationship. This was obtained with the apparatus of Fig. 1 using interchangeable apertures at A, to vary the size of the test field. As before, the subject was fully dark adapted and peripheral thresholds were measured at the same retinal eccentricity of 12 degrees with test field diameters from 6.3 minutes to 3 degrees, 6 minutes of visual angle. All peripheral measurements were obtained for X = 501 m/t (see Table I). The effect of area on threshold was measured in the fovea with light of X = 560 m/t.

experiments we have tested the spectral sensitivity of the peripheral retina to determine whether scotopic function can be demonstrated for some regions of the visible spectrum. On the basis of these findings, we could better interpret the results obtained for the effect of stimulus area on absolute threshold. Method Subjects. The same two subjects were used as in the previous experiments.4 Both were congenital ly night-blind, one (P. M.) as a consequence of dominant inheritance, while the other (R. C.) was probably an example of the recessively transmitted form of the disorder. Spectral sensitivity. The apparatus is shown schematically in Fig. 1. Source S, was a 6 v., 18 A ribbon filament lamp supplied by stabilized direct current, and run at a temperature of 2,900 K. The filament was focused by lens Lt onto stop A, which blocked all but a small circular area of the ribbon. The rays were then collimated by Li and brought to a focus by L3 in the plane of the subject's dilated pupil. Aperture At which was imaged on the subject's retina, limited the angular subtense of the test field to 1 degree, 49 minutes in diameter. The fixation spot FP was positioned so as to center the test field 12 degrees in the temporal retina, the same retinal locus studied previously by dark adaptometry and fundus reflectometry.4 The head position was maintained by a dental impression bite and forehead rest. The thresholds were determined for 12 spectral regions isolated by narrow-band interference filters (Schott, Depal, and Farrand) inserted at I; their transmission characteristics are given in Table I. A rotating sector disc D delivered test flashes of 80 msec, duration at 4 sec. intervals. The intensity of each flash was varied by filters F and wedges W, the densities of which

Table I. Spectral characteristics of interference filters

Farrand Farrand Farrand Farrand Schott Schott Schott Schott Schott Schott Farrand Farrand

420 440 462 484 501 518 540 560 581 598 614 646

FP *

W

O

S2

Fig. 1. Diagram of the apparatus (not to scale). For details see text.

±8 ±6 ±4 ±7 ±7 ±9 ±8 ±8 ±7 ±9 ±7 ±8

28.7 39.3 39.5 43.4 40.1 28.3 31.4 41.3 23.4 33.8 49.7 39.3

510 Can et al

An auxiliary light source S= and stop A3 provided a dimly illuminated annulus within which the test flash appeared. For central measurements, the test fields subtended from 6.3 to 50 minutes of arc at the eye.

Results Spectral sensitivity. Before we consider the results of the present study, an examination of the peripheral spectral sensitivity functions of normal subjects may prove helpful. Fig. 2 illustrates the results obtained by Wald,s redrawn with the maximum of the scotopic curve placed arbitrarily at log Srol = 0. Wald's data are noteworthy in several respects. (1) Photopic, as well as scotopic, sensitivities were determined for a paracentral retinal region comparable with that studied in the present investigation. (2) The photopic function, derived from threshold measurements at the cone plateau of the dark-adaptation curve, deviates from the CIE photopic luminosity function (based on foveal measurements) in exhibiting an increased sensitivity at short wavelengths. Wald's scotopic curve, on the other hand, is essentially the same as the corresponding CIE function. (3) Whereas spectral functions are often plotted relative to their own maxima, i.e., the maxima are equated, Wald adopted the correct procedure of indicating the relative sensitivities of the rod and cone mechanisms for a given retinal locus. As shown by the dashed lines in Fig. 2, the maximum of the photopic curve (Ami,x = 560 m/i) is about 2.64 log units below that of the scotopic curve (Ainnx = 500 m,u). The spectral sensitivity data for subject R. C. are shown in Fig. 3. Although peak sensitivity occurs at approximately A. = 500 IT\H, note that the maximum has been plotted at 3.4 on the scale of ordinates to indicate the nyctalope's sensitivity relative to that of normal subjects. The magnitude of this difference was determined from an auxiliary study of paracentral spectral sensitivity in normal observers, and is confirmed by the elevated thresholds of the dark-adaptation curve (cf. Fig. 2 of Carr and associates'1), and the area-intensity

700

5OO

WAVELENGTH (mg)

Fig. 2. Scotopic and photopic spectral sensitivity functions of the normal eye for a 1 degree test field located 8 degrees about the fovea. After Wald,8 replotted with the maximum of the scotopic curve placed at log Sre\ = 0.

R.C.

S

°5

400

500 WAVELENGTH

600 (m

M

700

)

Fig. 3. Spectral sensitivity data for nyctalope R. C. measured in the temporal retina (12 degrees from the fovea) with a 1 degree 49 minutes test field. The ordinate values at each wavelength are plotted relative to the maximum of the scotopic function shown in Fig. 2. The curves represent the normal scotopic (continuous line) and photopic (dashed line) spectral sensitivities of the peripheral retina. Details in text.

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function (Fig. 5). But despite the severe depression of visual sensitivity, the findings indicate that scotopic responses can be elicited at absolute threshold. However, Wald's scotopic sensitivity function (continuous curve) provides a good description of the nyctalope's data only for wavelengths less than or equal to 560 m^. At longer wavelengths, the experimental points lie above the scotopic curve. The "hump" in the red can perhaps be accounted for by assuming the intrusion of photopic activity at long wavelengths. We have seen that for normal subjects the maximum of the photopic function is about 2.64 log units below the maximum of the scotopic function. For subject R. C, however, the latter is depressed 2.6 log units. Therefore, if photopic vision were unaffected in this subject, the maxima of his rod and cone spectral sensitivities would be nearly the same; the envelope of such overlapping functions clearly could not fit the spectral sensitivity data. However, photopic function has indeed been affected, witness the elevation of the cone branch in the dark-adaptation curve (Fig. 2 of Carr and associates'1), and the foveal data of Fig. 6. In these circumstances, the reduction in cone sensitivity is between 0.6 and 0.7 log unit, respectively. Accordingly, Wald's photopic curve (dashed line) has been lowered by 0.65 log unit relative to its normal position on the scale of ordinates. It provides a very satisfactory fit to R. C.'s data for A greater than or equal to 580 n\fx. Thus both rod and cone activity seem to contribute to R. C.'s spectral threshold responses. The spectral sensitivity data for subject P. M. are shown in Fig. 4. As noted in the previous paper,4 the dark-adaptation curve for this subject was monophasic, there being no evidence of rod function. Similarly, the spectral sensitivity data can be described solely on the basis of photopic activity, even though the data for A less than 540 m/x can be fitted by the scotopic curve. Sensitivity is maximal in the region of 550 m/x and the data are in good


agreement with Wald's photopic sensitivity function (continuous curve). Further evidence that scotopic function could not be elicited subjectively was provided by the subject's ability to identify correctly the color of the threshold test flashes at all wavelengths. Thus, although this observer had a normal concentration of rhodopsin, and although the latter regenerated in the normal manner following photolysis, any rod signals that may have been generated were masked by the responses of the more sensitive cone mechanism. Spatial summation. Fig. 5 shows log relative threshold intensity plotted against log stimulus area for test fields located 12 degrees from the fovea (temporal retina). Nearly monochromatic flashes (A = 501 m/x) were used in determining thresholds. The averaged results for two normal observers are included for comparison with the data of nyctalopes R. C. and P. M. The features of the normal data agree with those reported by Barlow0 for comparable stimulus conditions. A line of slope - 1 , indicating complete spatial summation

P.M.

400

500

600

700

WAVELENGTH (mp)

Fig. 4. Spectral sensitivity data for nyctalope P. M. Stimulus conditions as in the caption to Fig. 3. The continuous curve represents the normal photopic function for the peripheral retina. After Wald.s

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12° T e m p o r a l

Retina

? 3

P.M. R.C.

LOG

AREA

(sq. degrees )

Fig. 5. The effect of stimulus area on the absolute threshold for nyctalopes P. M. (filled squares) and R. C. (filled circles). Averaged results for two nonnal observers (open circles) are shown for comparison. The straight lines of slope -1 indicate the range of stimulus area over which complete summation occurs. Data are for X = 501 m/t with test fields located 12 degrees from fixation (temporal retina).

(Ricco's law), fits the data for areas up to approximately 0.3 sq. degree (log area = 1.5). With larger stimulus fields, summation is less than complete, i.e., the data points describe a line of lesser slope. Now for subject R. C, the range over which Ricco's law is valid is essentially nonnal, in spite of a nearly 400 fold elevation in threshold (the data in this figure being correctly related to one another on the scale of ordinates). But of particular significance is the fact that R. C.'s and the nonnal thresholds do not converge on the left (small area) side of the graph. For if the defect in nyctalopia caused an inability to summate spatially, then clearly the

thresholds for nonnal and nyctalope would converge to a common value as the opportunity for summation was eliminated; i.e., as the stimulus area approached the dimensions of a single receptor. It is apparent that no such defect can be ascribed to R. C.'s scotopic (rod) mechanism. Fig. 6 shows that the same is true for R. C.'s cone mechanism. Foveal thresholds for normal and night-blind nin nearly parallel over the entire range of stimulus areas tested. However, the results again demonstrate the loss of cone sensitivity observed electroretinographically and in the dark-adaptation function4; thresholds

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Fovea

•

R C.

O

Norn

2 LOG

AREA

( sq. d e g r e e s )

Fig. C. Spatial summation in the fovea tested at X = 560 vn/i. Results shown are for normal persons (circles) and nyctalope R. C. (squares); the other night-blind subject was not available for these measurements. Note that complete summation (dashed line of slope -1) does not occur over the same range as for the rod mechanism of the

peripheral retina (Fig. 5). Only those data obtained with test areas less than 0.1 sq. degree can be approximated by the dashed line.

are, on average, about 0.7 log unit above the normal value. In contrast with the results for R. C, the peripheral data for nyctalope P. M. (Fig. 5) follow the line of complete summation only up to 0.1 sq. degree (log area = 1.0.). This finding, however, is not unexpected in view of P. M.'s spectral sensitivity data (Fig. 4) which show the dominance of cone activity in this region of the retina. Indeed, a comparable reduction in spatial integration was observed by Barlow9 for the peripheral cone system of normal eyes tested in increment threshold measurements. Although we cannot test P. M.'s scotopic mechanism subjectively, it seems unlikely that the integrative capacity of her retina is at fault. In the normal eye rod and cone thresholds are comparable for very small test fields.2- 10> 1L Thus, applying the same reasoning as before, a loss of summative ability would cause P. M.'s thresholds to approach those of the normal person with

small area stimuli. And again, this is not observed experimentally. Discussion In congenital night blindness, darkadapted peripheral thresholds are raised more than 2 logarithmic units and this rise bears no relation to the concentration of visual pigment in the retina.'1 But a sensitivity loss of this magnitude is not necessarily incompatible with1 the existence of a normal photochemistry.1- For despite statements to the contrary,13 there is no simple expression relating visual sensitivity and pigment concentration,14'15 and nowhere is the lack of correspondence more apparent than near absolute threshold. Then how is one to account for such an enormous loss of sensitivity? In the analysis of our previous findings, we concluded that a malfunction in the retina, between the receptor outer segments and the bipolar cells, is probably responsible for nyctalopia. Furthermore, the anatomy of this region suggested an obvious basis for the functional loss: a defect of the convergent pathways and, hence, an upset in spatial summation. However, the experimental findings gave no support to this view. The integrative capacity of the retina appeared to be quite normal for both nyctalopes, although in one (P. M.) the scotopic mechanism was so severely depressed that only photopic function could be tested. An alternative to the failure in spatial summation is suggested by the parallelism in the area-intensity functions of normal subjects and nyctalopes (Figs. 5 and 6). It is as if the sole obstacle to normal sensitivity were the interposition of a neutral filter between the incident light and the receptors. But literal interpretation of this view is rendered untenable by the following observations: (1) the differential effect on rod and cone thresholds requires that a different filter density be postulated for each; (2) the measured photosensitivity of rhodopsin would be altered by the hypothetical filter, a possibility that was ruled

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514 Can et al.

out in an auxiliary experiment; and (3) a normal ERG could not be elicited even with stimulus intensities more than 1,000 times that required to evoke a large scotopic response in the normal eye. With a normal biochemistiy of the scotopic visual pigment, spatial summation intact, and the physical filter hypothesis discarded, we are left as before, unable to pinpoint the defect beyond implicating the transmission lines between the receptor outer limbs and the inner nuclear layer. However, we believe that the affection (or affections) involves not only the peripheral rods and cones, but extends also over the rod-free part of the retina. In some respects, therefore, the defect is nonselective, although its effect on the rod system is decidedly more pronounced. The erroneous assumption that the scotopic system of congenital nyctalopes is nonfunctional has probably discouraged many earlier investigators from seeking evidence for the existence of rod vision. To our knowledge, only Dieter10 measured the spectral sensitivity of the peripheral retina in a congenital nyctalope and he found only photopic function. But it is difficult to determine whether Dieter's subject was like our case of dominant inheritance (P. M.) in this regard, or if his experimental technique of brightness matching so light adapted the retina as to mask scotopic function. The fact that the spectral sensitivity of the dark-adapted eye of subject R. C. is determined largely by the normal scotopic sensitivity curve shows that the brain can receive messages from the rods. At the same time, the presence of the shoulder at wavelengths longer than 580 m/x indicates that the advantage the rods have over the cones in the normal eye is greatly reduced in the nyctalope. Indeed, the results for subject P. M. suggest that this advantage may be completely lost in some cases. In these

circumstances, vision throughout the spectrum is determined solely by the photopic system. REFERENCES 1. Craik, K. J. W., and Vernon, M. D.: The nature of dark adaptation, Brit. J. Psychol. 32: 62, 1941. 2. Baumgardt, E.: Les batonnets sont-ils plus sensibles que les cones? Compt. rend. Soc. biol. 143: 786, 1949. 3. Arden, G. B.} and Weale, R. A.: Nervous mechanisms and dark-adaptation, J. Physiol. 125: 417, 1954. 4. Carr, R. E., Ripps, H., Siegel, I. M., and Weale, R. A.: Rhodopsin and the electrical activity of the retina in congenital night blindness, INVEST. OPHTH. 5: 497,

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5. Polyak, S. L.: The Retina, 1941, University of Chicago Press. 6. Vilter, V.: Recherches biometriques sur 1' organisation synaptique de la retine humaine, Compt. rend. Soc. biol. 143: 830, 1949. 7. Ripps, H., and Weale, R. A.: Cone pigments in the normal human fovea, Vision Res. 3: 531, 1963. 8. Wald, G.: Human vision and the spectrum, Science 101: 653, 1945. 9. Barlow, H. B.: Temporal and spatial summation in human vision at different background intensities, J. Physiol. 141: 337, 1958. 10. Weale, R. A.: Retinal summation and human visual thresholds, Nature 181: 154, 1958. 11. Pinegin, N. I.: Independence of wavelength of the threshold number of quanta for peripheral rod and foveal cone vision, Symp. Visual Problems of Colour, Paper No. 33, National Physical Laboratory, Teddington, 1957. 12. Ikeda, H., and Ripps, H.: The electroretinogram of a cone-monochromat, Arch. Ophth. 75: 513, 1966. 13. Rushton, W. A. H.: Rhodopsin measurement and dark-adaptation in a subject deficient in cone vision, J. Physiol. 156: 193, 1961. 14. Wald, G., Brown, P. K., and Gibbons, I. R.: The problem of visual excitation, J. Optic. Soc. America 53: 20, 1963. 15. Weale, R. A.: Relation between dark adaptation and visual pigment regeneration, J. Optic. Soc. America 54: 128, 1964. 16. Dieter, W.: Untersuchungen zur Duplizitatstheorie. III. Die angeborene, familiar-erbliche, stationare (idiopathische) Hemeralopie, Pfliiger's Arch. ges. Physiol. 222: 381, 1929.