Congenital Achromatopsia: A Report of 19 Cases*

It is well recognized, particularly from the work of Hecht (1937), that measurements of visual function at different levels of illumination generally fall into two categories, a low intensity scotopic portion and a high intensity photopic portion. Thus in the time course of dark-adaptation, and in the curves where log. increment threshold, log. acuity or fficker fusion frequency is plotted against log. field intensity, the results are found to fall upon two distinct branches. The low-intensity branch has the spectral sensitivity of rhodopsin, and it is not seen in measurements made upon the fovea centralis; it is therefore considered to be due to rod function. The other branch has a-different spectral sensitivity, it is found upon the rod-free fovea and hence it is considered to be due to cone function. Now though there are considerable difficulties in distinguishing the functions of the various types of cone, it is easy to separate cone curves from rod curves over the whole intensity range of the cones. For not only may measurements be made upon the fovea where rods are absent, but even elsewhere it turns out that cones have an increment sensitivity and fusion frequency so much higher than rods that the plotted curves generally show the cone branch substantially in its entirety. It is otherwise with rod curves, for not only is there no part of the human retina which contains rods without cones, but the very features which make cone measurements outstanding in a mixed population make rod measurements unavailable. In fact, we can only measure rods at intensities below the cone threshold. This does not mean that rods are necessarily inactive at higher intensities, but their activity cannot be measured by ordinary threshold procedures, and we are left wondering what course the rod branch may take after it disappears behind the cone branch.This question could be answered if we were able to investigate a subject who had no cones but whose rods and rod pathways to the brain were * Instituto Neurologico, Via Celoria 11, Milano. t Present address: Physiological Laboratory, University of Cambridge.12-2

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confidence: 56%

Increment thresholds in a subject deficient in cone vision

Fuortes¹,

Gunkel²,

Rushton³

1961

“…He was able, however, to tolerate the very strong bleaching lights used in these experiments. There was no lasting visual impairment such as that described by Walls & Heath (1954 Hecht, Shlaer, Smith, Haig & Peskin (1948), Sloan (1954Sloan ( , 1958, and Alpern, Falls & Lee (1960). We confirm that it exhibits the spectral sensitivity of rhodopsin and we find no StilesCrawford effect.…”

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confidence: 79%

Dark adaptation and increment threshold in a rod monochromat.

Blakemore¹,

Rushton²

1965

107

The well-known fall in sensitivity of the eye when exposed to light and the subsequent rise in the dark has long been attributed to the bleaching and subsequent regeneration of rhodopsin and of cone visual pigments. This conjecture, held for thirty years without evidence, was shown to be on the right lines when Dowling (1960) demonstrated in rats that, after full bleaching, there was at each stage of regeneration a linear relation between the amount of rhodopsin in the retina and the log e.r.g. threshold at that moment. In man a quite similar relation was found between the log visual threshold measured in the usual way and the rhodopsin concentration measured directly upon the same area of retina by the technique of retinal densitometry. Though this result could be inferred with some confidence from work on normal eyes (Rushton, 1961a) it was displayed more clearly in the eye of a special subject (Rushton, 1961 b) who lacked nearly all cone vision but whose rhodopsin and rod vision appeared quite normal. If the threshold is expressed in units of the fully dark-adapted value, the relation found was that log threshold is proportional to the amount of rhodopsin bleached. But, as was pointed out in the brief Discussion of the previous paper (Rushton, 1961 b), there are two respects in which this simple relation fails.(a) If the brief adapting light bleaches only a small fraction of rhodopsin, visual sensitivity recovers rapidly, and all agree that the process is in some way different from that linked to the slow regeneration of rhodopsin after substantial bleaching. In the present paper, as in the former one, all bleaches are substantial; thus the rapid recovery phenomenon lies outside the conditions here examined.(b) The simple relation found that log threshold was proportional to the amount of free opsin could not be quite right. To be sure, something changes uniquely with the rhodopsin concentration and may affect the threshold in the way described, but it is not the threshold itself that stands in unique relation to pigment concentration. For several workers have shown that threshold depends also upon the kind of test flash used to measure it. In DARK ADAPTATION IN A ROD MONOCHROMAT 613 his last paper Lythgoe (1940) Crawford (1947) and Arden & Weale (1954). Wolf & Zigler (1950) placed an acuity grating in the test flash and found that the shape of the resulting dark adaptation curve depended upon whether the threshold task was to see the flash or to resolve the grating. These results were confirmed and extended by Brown, Graham, Leibowitz & Ranken (1953). There would of course be little difficulty in reconiciling these observations with the unique dependence of dark adaptation upon the regeneration of rhodopsin, if a change in the nature of the test flash did nothing more than shift the log threshold curve bodily up and down. But all investigations have shown that not only the position but the shape of the dark adaptation curve alters and that the range of the log threshold change for a given bleach depends u...

“…Over the years, however, sufficient evidence has been found to cast doubt upon this simple interpretation. This includes the findings that the critical flicker, increment threshold and dark adaptation curves measured in some achromats have scotopic and photopic-like segments (Lewis & Mandelbaum, 1943; Hecht, Shlaer, Smith, Haig & Peskin, 1948;Sloan, 1954Sloan, , 1958Walls & Heath, 1954; K. NORDBY AND L. T. SHARPE Alpern, Falls & Lee, 1960). Such evidence has led some researchers to suggest that the eyes of typical, complete achromats have a second, high-intensity type of photoreceptor, in addition to the normal rods functioning at low intensities.…”

Section: Introductionmentioning

confidence: 99%

“…a rod containing in its outer segment either a very dilute concentration of rhodopsin (Hecht et al 1948) or a cone photopigment (Lewis & Mandelbaum, 1943;Sloan, 1954). However, this theory now hardly seems tenable.…”

Section: Introductionmentioning

confidence: 99%

The directional sensitivity of the photoreceptors in the human achromat.

Nordby

Sharpe

1988

SUMMARY1. The anatomical nature of the retinal photoreceptors in a typical, complete achromat was investigated by measuring their directional sensitivity.2. A small (05 deg), brief (94 ms) test flash was placed at threshold by varying either its intensity (indirect method) or the intensity of a large (5 deg) adapting field (direct method). The dependent variable was the intensity of light required for the detection threshold as a function of the position of entry in the pupil. An infra-red viewing system was used to monitor the achromat's pupil and eye position.3. At both scotopic and mesopic adapting field luminances, the complete achromat's receptors displayed only a small directional sensitivity effect. The effect was not wavelength dependent and could be attributed solely to the rods.4. The results are consistent with other psychophysical evidence indicating that the complete achromat's retinae lack the post-receptoral function of the cone photoreceptors. Therefore they do not confirm previous reports that complete achromats have a second, high-intensity type of photoreceptor.